JP2016075787A - Display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a display device that can highly accurately implement a threshold voltage compensation without increase in a threshold voltage compensation period.SOLUTION: An organic EL display comprises: a display pixel PX that has an organic EL element OEL, a capacitive element Cs, and a drive transistor Trd in which a gate electrode is connected to a first electrode of the capacitive element Cs, and a source electrode is connected to a second electrode of the capacitive element Cs and an anode electrode of the organic EL element OEL; and a control unit. The control unit is configured to execute the steps of: applying a reference voltage to the first electrode in a state where application of a drive voltage to a drain electrode of the drive transistor Trd is halted, and applying an initialization voltage to the second electrode therein; starting an application of the drive voltage to the drain electrode of the drive transistor Trd in a state where the application of the reference voltage and the initialization voltage is maintained; and halting the application of the initialization voltage to the second electrode in a state where the application of the reference voltage and the drive voltage is maintained.SELECTED DRAWING: Figure 21

Description

  The present invention relates to a display device using an organic electroluminescence (EL) element.

  As a display device using a current-driven light emitting element, an organic EL display using an organic electroluminescence element (hereinafter referred to as an organic EL element) is known (see Patent Document 1). This organic EL display has the advantages of good viewing angle characteristics and low power consumption.

  An organic EL display includes an organic EL panel (display panel) made of a glass substrate on which organic EL elements and wirings are formed, an IC (Integrated Circuit) that drives the organic EL panel, a control unit, and the like. Yes.

  The organic EL panel has a plurality of display pixels arranged in a matrix. The display pixel includes the above-described organic EL element, a capacitor element that accumulates a voltage corresponding to the pixel signal, and a drive transistor that supplies a drive current corresponding to the amount of charge held in the capacitor element to the organic EL element.

  In an active matrix organic EL display, a thin film transistor (TFT) is used as a driving transistor. In the TFT, the threshold voltage varies within the display panel because the film thickness and film quality fluctuate in the surface due to temperature variations during the film formation process, residual solution variations during the etching process, and pattern density variations. . In addition, the threshold voltage shifts with time due to stress such as a gate-source voltage during energization. Then, the initial variation due to the threshold voltage process and the shift over time cause fluctuations in the amount of current supplied to the organic EL, so that it affects the brightness control of the display device and deteriorates the display quality.

  In a conventional organic EL display, threshold voltage compensation for adjusting the voltage of the source electrode of the drive transistor according to the threshold voltage is performed after initialization.

JP 2008-287139 A

  However, the threshold voltage varies depending on the characteristic variation of the drive transistor, the accumulated luminance value, and the like, and therefore varies in the organic EL panel. If the threshold voltage varies, there is a problem that it is necessary to lengthen the period for performing threshold voltage compensation (hereinafter referred to as “threshold voltage compensation period”) in order to perform threshold voltage compensation more reliably for a plurality of organic EL elements. is there.

  Therefore, the present invention provides a display device capable of performing threshold voltage compensation with high accuracy without increasing the threshold voltage compensation period.

  A display device according to one embodiment of the present invention is a display device including a display pixel, in which the display pixel includes a light-emitting element, a capacitor for holding voltage, and a gate electrode serving as a first electrode of the capacitor. A drive transistor in which a source electrode is connected to a second electrode of the capacitive element and an anode of the light emitting element, a signal line for supplying a voltage corresponding to a data signal, and the first of the capacitive element A display pixel having a first switch element that switches between conduction and non-conduction with an electrode; and a control unit that controls driving of the display pixel, wherein the control unit drives the drive at the start of a first initialization period. A reference voltage is applied to the first electrode of the capacitive element in a state where application of a driving voltage for driving the light emitting element to the drain electrode of the transistor is stopped, and the second electrode is applied to the second electrode Maintaining the application of the reference voltage to the first electrode at the start of a second initialization period set after execution of the first initialization step and the first initialization step, In addition, after the execution of the second initialization step, the second initialization step of starting the application of the drive voltage to the drain electrode of the drive transistor while maintaining the application of the initialization voltage to the second electrode At the start of a threshold voltage compensation period for compensating the set threshold voltage of the driving transistor, the application of the reference voltage to the first electrode is maintained and the application of the driving voltage to the drain electrode of the driving transistor is maintained. In this state, a threshold voltage compensation step for stopping the application of the initialization voltage to the second electrode of the capacitive element is executed.

  The display device of the present invention can perform threshold voltage compensation with high accuracy without increasing the threshold voltage compensation period.

FIG. 1 is a block diagram illustrating an example of a configuration of an organic EL display in a comparative example. FIG. 2 is a graph showing signal waveforms of the organic EL display in the comparative example. FIG. 3 shows a change in the threshold voltage compensation period of the voltage of the source electrode of the driving transistor in the comparative example. FIG. 4 is a block diagram showing an example of the configuration of the organic EL display according to the embodiment. FIG. 5 is a graph showing an ideal time change of the voltage Vgs between the gate and the source of the drive transistor Trd−the threshold voltage Vth in the threshold voltage compensation period. FIG. 6 is a graph showing an ideal time change of the current Ids flowing between the source electrode and the drain electrode of the drive transistor Trd in the threshold voltage compensation period. FIG. 7 is a graph showing the time change of the drain-source voltage of the drive transistor Trd during the threshold voltage compensation period. FIG. 8 is a graph showing an actual time change of the current Ids flowing between the source electrode and the drain electrode of the drive transistor Trd in the threshold voltage compensation period. FIG. 9 is a graph showing an actual time change of the gate-source voltage Vgs−threshold voltage Vth of the drive transistor Trd in the threshold voltage compensation period. FIG. 10 is a graph showing transition of Vgs−Vth in the threshold voltage compensation period in the comparative example and the present embodiment. FIG. 11 is a graph showing a difference in Vth detection (threshold voltage compensation value) when the threshold voltage variation ΔVth is 0 V and 2 V in the embodiment. FIG. 12 is a graph showing the time change of the drain-source voltage of the drive transistor Trd in the threshold voltage compensation period in the embodiment. FIG. 13 is a graph showing the result of calculating the value of α required for each drive frequency Freq when the number of scanning lines Vline is 2160. FIG. 14 is a graph showing the result of calculating the value of α required for each scanning line number Vline when the driving frequency Freq is fixed to 120 Hz. FIG. 15 is a graph showing the relationship between the Cs initialization voltage and the threshold voltage Vth in the embodiment. 16, in the embodiment, is a graph showing the value of ∂Vgs / ∂Vth, the correlation between the channel length _{L R} of the second switching element Tr2. FIG. 17 is a graph showing signal waveforms of the organic EL display according to the embodiment. FIG. 18 is a circuit diagram illustrating a state of the display pixel PX in the first initialization period in the embodiment. FIG. 19 is a circuit diagram illustrating a state of the display pixel PX in the second initialization period in the embodiment. FIG. 20 shows changes in the voltage of the source electrode of the driving transistor in the second initialization period and the threshold voltage compensation period in the embodiment. FIG. 21 is a circuit diagram illustrating another example of the resistance unit. FIG. 22 is a circuit diagram illustrating another example of the resistance unit. FIG. 23 is a circuit diagram illustrating another example of the resistance unit. FIG. 24 is a circuit diagram illustrating an example in which one display unit PX shares one resistance unit in the modification. FIG. 25 is a circuit diagram showing another example of the signal waveform of the organic EL display.

(Details of the issue)
Hereinafter, the detail of a subject is demonstrated using FIGS. 1-3.

[Configuration of Organic EL Display in Comparative Example]
FIG. 1 is a block diagram illustrating an example of a configuration of an organic EL display 100 in a comparative example. As shown in FIG. 1, the organic EL display 100 includes an organic EL panel 110, a data line driving circuit 120, a scanning line driving circuit 130, and a control unit 200.

  The organic EL panel 110 has a plurality of display pixels P0 arranged in a matrix. Here, the display pixel P0 is a sub-pixel constituting one color. One pixel is composed of three sub-pixels corresponding to red, green, and blue.

  The display pixel P0 includes an organic EL element OEL, a capacitor element Cs, a drive transistor Trd, a first switch element Tr1, a second switch element Tr20, a third switch element Tr3, and a fourth switch element Tr4. ing.

  The organic EL element OEL is a light emitting element that emits light according to a drive current. The drive current is supplied from the drive transistor Trd. The organic EL element OEL has an anode electrode connected to the source electrode of the drive transistor Trd and a cathode electrode connected to a power supply line VEL (VEL is a ground voltage, for example).

  The capacitive element Cs is a capacitive element that accumulates charges according to the voltage of the data line Data. The capacitive element Cs has a first electrode connected to the gate electrode of the drive transistor and a second electrode connected to the source electrode of the drive transistor Trd.

  The drive transistor Trd supplies the organic EL element OEL with a drive current corresponding to the amount of charge of the capacitive element Cs accumulated according to the voltage of the data line Data. The drive transistor Trd is a thin film transistor, and has a gate electrode connected to the first electrode of the capacitive element Cs, a source electrode connected to the anode electrode of the organic EL element OEL, and a drain electrode connected to the power supply line VTFT.

  The first switch element Tr1 switches between selection and non-selection of the display pixel P0 by switching conduction and non-conduction between the data line Data and the first electrode of the capacitive element Cs according to the voltage of the scanning line Scan. More specifically, the first switch element Tr1 is a thin film transistor, the gate electrode is connected to the scanning line Scan, the source electrode is connected to the data line Data, and the drain electrode is connected to the first electrode of the capacitive element Cs.

  The second switch element Tr20 switches between conduction and non-conduction between the second electrode (node N2) of the capacitive element Cs and the power supply line VINI in accordance with the voltage of the signal line Init.

  The third switch element Tr3 switches between conduction and non-conduction between the first electrode (node N1) of the capacitive element Cs and the power supply line VREF according to the voltage of the signal line Ref.

  The fourth switch element Tr4 switches between conduction and non-conduction between the drain electrode of the drive transistor Trd and the power supply line VTFT according to the voltage of the signal line Enable.

  The data line driving circuit 120 supplies a voltage corresponding to the data signal output from the control unit 200 to the plurality of data lines Data.

  The scanning line driving circuit 130 supplies a voltage corresponding to the driving signal output from the control unit 200 to the plurality of scanning lines Scan, the signal line Init, the signal line, Ref, and the signal line Enable.

  The control unit 200 is a circuit that controls display of images on the organic EL panel 110, and is configured using, for example, a TCON (timing controller). The control unit 200 may be configured using a computer system including a microcontroller, a system LSI (Large Scale Integration), or the like.

[Operation of Organic EL Display in Comparative Example]
FIG. 2 is a graph showing signal waveforms of the organic EL display 100 in the comparative example. In FIG. 2, VA, VB, and VC indicate the voltage of the gate electrode, the voltage of the source electrode, and the voltage of the drain electrode of the drive transistor Trd, respectively.

  As shown in FIG. 2, in the organic EL display 100 in the comparative example, initialization, threshold voltage compensation, writing, and light emission are executed in this order for each frame of the video signal input from the outside. In the following description, an initialization period in which initialization is performed, a threshold voltage compensation period in which threshold voltage compensation is performed, a writing period in which writing is performed, and a light emission period in which the organic EL element OEL emits light will be described. Description is omitted.

(Period T22: Initialization period)
A period T22 from time t1 to time t2 shown in FIG. 2 is an initialization period. In the initialization period, the control unit 200 initializes the capacitive element Cs by setting the first switch element Tr1 and the fourth switch element Tr4 in a non-conductive state and the second switch element Tr20 and the third switch element Tr3 in a conductive state. Turn into. Specifically, the control unit 200 sets the voltage of the scanning line Scan and the signal line Enable to L level, thereby bringing the first switch element Tr1 and the fourth switch element Tr4 into a non-conductive state. Further, the control unit 200 sets the voltage of the signal line Init and the signal line Ref to the H level, thereby bringing the second switch element Tr20 and the third switch element Tr3 into a conductive state.

(Period T24: threshold voltage compensation period)
A period T24 from time t3 to time t4 shown in FIG. 2 is a threshold voltage compensation period for compensating for the influence of fluctuations in the threshold voltage of the drive transistor Trd. In the threshold voltage compensation period, the control unit 200 brings the first switch element Tr1 and the second switch element Tr20 into a non-conducting state and the third switch element Tr3 and the fourth switch element Tr4 into a conducting state. By providing the threshold voltage compensation period, it is possible to reduce the influence of the above-described variation in threshold voltage on luminance control. Specifically, the control unit 200 sets the voltage of the scanning line Scan and the signal line Init to the L level, thereby bringing the first switch element Tr1 and the second switch element Tr20 into a non-conductive state. Further, the control unit 200 sets the voltage of the signal line Ref and the signal line Enable to the H level, thereby bringing the third switch element Tr3 and the fourth switch element Tr4 into a conductive state.

  At this time, since the fourth switch element Tr4 becomes conductive, a drain current flows through the drive transistor Trd, and the voltage value of the second electrode of the capacitive element Cs increases according to the threshold voltage of the drive transistor Trd. Thereby, the threshold voltage can be compensated.

(Period T27: Write period)
A period T27 from time t6 to time t7 illustrated in FIG. 2 is a writing period in which charges corresponding to the voltage of the data line Data are accumulated in the capacitor Cs. In the writing period, the control unit 200 puts the first switch element Tr1 into a conducting state and puts the second switch element Tr20, the third switch element Tr3, and the fourth switch element Tr4 into a non-conducting state. Specifically, the control unit 200 sets the voltage of the scanning line Scan to the H level, thereby bringing the first switch element Tr1 into a conductive state. Further, the control unit 200 sets the voltages of the signal line Init, the signal line Ref, and the signal line Enable to L level, thereby bringing the second switch element Tr20, the third switch element Tr3, and the fourth switch element Tr4 into a non-conductive state. To. Further, the control unit 200 causes the data line driving circuit 120 to apply a voltage corresponding to the luminance value of the selected display pixel P0 to the signal line Data.

  At this time, charges corresponding to the voltage of the signal line Data are accumulated in the first electrode of the capacitive element Cs.

(Period T29: Light emission period)
A period T29 from time t8 to time t9 shown in FIG. 2 is a light emission period in which the organic EL element OEL emits light. In the light emission period, the control unit 200 brings the fourth switch element Tr4 into a conducting state and puts the first switch element Tr1, the second switch element Tr20, and the third switch element Tr3 into a non-conducting state. Specifically, the control unit 200 sets the voltage of the signal line Enable to the H level, thereby bringing the fourth switch element Tr4 into a conductive state. In addition, the control unit 200 sets the voltages of the scanning line Scan, the signal line Init, and the signal line Ref to the L level, thereby bringing the first switch element Tr1, the second switch element Tr20, and the third switch element Tr3 into a non-conductive state. To.

  At this time, a drive current corresponding to the charge accumulated in the first electrode of the capacitive element Cs flows between the drain and source of the drive transistor Trd. Then, when the driving current is supplied to the organic EL element OEL, the organic EL element OEL emits light with luminance corresponding to the amount of the driving current.

[Problems in threshold voltage compensation]
FIG. 3 shows changes in the threshold voltage compensation period of the source electrode voltage of the drive transistor Trd in the comparative example. The graph in FIG. 3 corresponds to the graph of the portion surrounded by the broken line in FIG. Note that FIG. 2 illustrates one element, but FIG. 3 illustrates three cases with different threshold voltages.

  As shown in FIG. 3, a display pixel whose threshold voltage of the drive transistor Trd is Vth1 takes time Tth1 until the voltage value becomes VREF−Vth1. A display pixel in which the threshold voltage of the drive transistor Trd is Vth2 takes Tth2 before the voltage value becomes VREF−Vth2. A display pixel in which the threshold voltage of the drive transistor Trd is Vth3 takes time Tth3 until the voltage value becomes VREF−Vth3. In FIG. 3, Tth1> Tth2> Tth3.

  In recent years, high-definition and large-size organic EL displays have progressed, and one horizontal scanning period (hereinafter referred to as “1H” as appropriate) tends to be shortened. If the blanking period is ignored for simplification, 1H is represented by 1H = 1 sec / Fre1 / Vline using the panel drive frequency Freq and the number of scanning lines Vline. From this equation, it can be seen that 1H becomes shorter as the panel drive frequency Freq and the number of scanning lines Vline increase due to the higher definition of the organic EL display.

  In order to secure a sufficient light emission period, for example, when the threshold voltage compensation period is set to 1H, when one horizontal scanning period is shortened, the threshold voltage compensation period is also shortened. Furthermore, in recent years, the organic EL display has been increased in size, and the number of scanning lines Vline and the number of display pixels per scanning line have increased. As the size increases, the amount of current required for compensating the threshold voltage increases in the entire organic EL display, so that the variation in the period for threshold voltage compensation increases. Moreover, since the distance between elements constituting the organic EL display increases due to the increase in size, the variation in film thickness and film quality due to the process increases, and the variation in threshold voltage of the drive transistor also increases.

  In order to perform threshold voltage compensation with sufficient accuracy, it is necessary to ensure a sufficient threshold voltage compensation period until the voltage of the drive transistor Trd of Vth1 having the longest threshold voltage compensation period converges in FIG. Alternatively, it is important to converge the voltages of all the drive transistors Trd within 1H.

  However, as described above, the threshold voltage compensation period tends to be shortened due to high definition of the organic EL display, and it has become difficult to ensure a sufficient threshold voltage compensation period. Further, since the variation in the period until the drive transistors Trd converge due to the increase in size of the organic EL display, it is difficult to converge the voltages of all the drive transistors Trd within 1H.

  For this reason, in particular, in the driving transistor having the threshold voltage Vth1 shown in FIG. 3, the threshold voltage compensation period ends before the voltage of the source electrode reaches VREF−Vth1, and the threshold voltage is compensated with sufficient accuracy. There is a problem that it may not be possible. In other words, the conventional organic EL display has a problem that accuracy is lowered in threshold voltage compensation due to variations in threshold voltage of the driving transistor.

  For this reason, there is a need for a technique that can sufficiently ensure the accuracy of threshold voltage compensation even when the threshold voltage compensation period is shortened.

  In order to solve such a problem, a display device according to one embodiment of the present invention is a display device including a display pixel, and the display pixel includes a light-emitting element, a capacitor element for holding voltage, A driving transistor having a gate electrode connected to the first electrode of the capacitive element and a source electrode connected to the second electrode of the capacitive element and an anode of the light emitting element; and for supplying a voltage corresponding to the data signal A display pixel having a first switching element that switches between conduction and non-conduction between the signal line and the first electrode of the capacitor; and a control unit that controls driving of the display pixel. At the start of one initialization period, the application of the drive voltage for driving the light emitting element to the drain electrode of the drive transistor is stopped, and the first electrode of the capacitor element is stopped. A first initialization step of applying a reference voltage and applying an initialization voltage to the second electrode; and at the start of a second initialization period set after execution of the first initialization step, A second initialization step of starting application of the drive voltage to the drain electrode of the drive transistor while maintaining application of the reference voltage to the electrode and maintaining application of the initialization voltage to the second electrode And maintaining the application of the reference voltage to the first electrode at the start of a threshold voltage compensation period for compensating the threshold voltage of the drive transistor set after execution of the second initialization step, and the drive transistor A threshold voltage for stopping the application of the initialization voltage to the second electrode of the capacitive element while maintaining the application of the drive voltage to the drain electrode of the capacitor To run and amortization step.

  In the display device having the above structure, the reference voltage is applied to the first electrode of the capacitor and the initialization to the second electrode between the first initialization period (same as the conventional initialization period) and the threshold compensation period. A second initialization step for starting the supply of the drive voltage to the drain electrode of the drive transistor is performed while the application of the voltage is maintained.

  With this configuration, a through current flows through the driving transistor in the second initialization step. The through current is a current that flows from the power supply line that supplies the drive voltage to the drain electrode of the drive transistor to the power supply line that supplies the initialization voltage via the drive transistor. The current value of the through current changes according to the threshold voltage of the driving transistor. Specifically, the current value of the through current decreases as the threshold voltage of the driving transistor increases, and increases as the threshold voltage decreases.

  Here, for example, in a display pixel in which a transistor is provided between a source electrode of a driving transistor and a power supply line that supplies an initialization voltage, a voltage drop corresponding to the through current is generated by the transistor. That is, the voltage of the source electrode of the driving transistor increases.

  When the voltage of the source electrode of the driving transistor increases, the increase width of the voltage of the source electrode becomes small in the subsequent threshold voltage compensation. That is, the period for threshold voltage compensation is shortened. Then, the display device having the above configuration can perform the threshold voltage compensation more reliably without extending the threshold voltage compensation period. Alternatively, the display device having the above configuration can shorten the threshold voltage compensation period. That is, in the display device having the above configuration, the threshold voltage can be compensated with sufficient accuracy.

  For example, the display pixel further includes a second switch element that switches between conduction and non-conduction between a power supply line that supplies the initialization voltage and the second electrode of the capacitor, and the first electrode of the capacitor and A third switch element that switches between conduction and non-conduction between a connection point of the gate electrode of the drive transistor and the power supply line that supplies the reference voltage, and conduction between the drain electrode of the drive transistor and the power supply line that supplies the drive voltage And a fourth switch element for switching non-conduction, and in the first initialization step, the control unit brings the first switch element and the fourth switch element into a non-conduction state, the second switch element and The third switch element is turned on, and the second switch is turned off in the second initialization step. Switch element, the third switch element, and the fourth switch element are turned on, and in the threshold voltage compensation step, the first switch element and the second switch element are turned off, and the third switch element and The fourth switch element may be turned on. Further, the second switch element may be a transistor, and the control unit may operate the second switch element as a resistance unit in the second initialization step, and the second switch element is a transistor. The on-resistance of the second switch element may be higher than the on-resistance of other switch elements.

  The display device having the above configuration includes a second switch element that is a path through which a through current flows, and that switches between conduction and non-conduction between a power supply line that supplies an initialization voltage and the second electrode of the capacitor element. By making the on-resistance of the element larger than that of other switch elements, a voltage drop can be favorably generated.

  For example, the second switch element is a transistor, the W / L ratio of the second switch element may be smaller than the W / L ratio of the other switch elements, and the second switch element is a transistor. The control unit controls the voltage applied to the gate electrode of the second switch element to be lower than the voltage applied to the gate electrode of the other switch element in the second initialization step. Alternatively, the second switch element is a transistor, and the control unit is configured such that, in the second initialization step, the voltage applied to the gate electrode of the second switch element is the first initialization step. The voltage may be controlled to be lower than the voltage applied to the gate electrode of the second switch element.

  In the display device having any of the above-described configurations, the number of components can be reduced if the second switch element is integrally formed using a transistor. As a method of using the second switch element as the resistance portion, specifically, for example, the W / L ratio of the transistors constituting the second switch element is made smaller than the W / L ratio of the other switch elements. Conceivable. Alternatively, the voltage applied to the gate electrode of the second switch element may be controlled to be lower than the voltage applied to the gate electrode of another switch element. Alternatively, in the second initialization step, a voltage applied to the gate electrode of the second switch element is lower than a voltage applied to the gate electrode of the second switch element in the first initialization step. It is conceivable to control it. As a result, a voltage drop can be favorably generated in the second switch element.

  For example, the first initialization period may be longer than the second initialization period.

  In the display device having the above structure, the first initialization period is a period for initializing the charge accumulated in the first electrode of the capacitor, that is, an amount of charge corresponding to the gradation value. Therefore, initialization with high accuracy is required. In other words, it is necessary to charge and discharge the electric charge accumulated in the first electrode of the capacitive element at a very high rate, for example, 99.9%.

  On the other hand, the second initialization period is a period for raising the voltage of the source electrode of the drive transistor and does not require fine accuracy. For example, the source electrode of the drive transistor (the anode electrode of the organic EL element) It suffices to charge and discharge about 60% of the charge of the node to which () is connected. In other words, the time constant of the node to which the source electrode of the driving transistor is connected is considerably smaller than the time constant of the node to which the first electrode of the capacitive element is connected.

  That is, the second initialization period is different from the first initialization period in terms of charge and discharge, and the time constant is small, so the length of the second initialization period is shorter than the length of the first initialization period. (The length of the first initialization period is longer than the length of the second initialization period).

  For example, the control unit may further execute a first step of bringing the second switch into a conductive state at the start of a first period set before the first initialization period.

  In the display device having the above configuration, the voltage at the connection node between the source electrode of the driving transistor and the second electrode of the capacitor element is stabilized at the initialization voltage by executing the first step before the first initialization period. It becomes possible to make it.

  For example, the power supply line that supplies the initialization voltage may be arranged to intersect the power supply line that supplies the drive voltage and the power supply line that supplies the reference voltage.

  For example, the control unit further includes the first switch element, the second switch element, the first switch at the start of a second period set after the light emission period for causing the light emitting element to emit light before the first period. You may perform the 2nd step which makes a 3 switch element and the said 4th switch element a non-conduction state.

  In the display device having the above structure, the voltage of the first electrode and the second electrode of the capacitor can be made close to the voltage in the first period.

  For example, the control unit further performs a third step of bringing the fourth switch element into a non-conductive state at the start of a third period set after the threshold voltage compensation period, and ends the third period At the start of a writing period set later, the first switch element is turned on, and the second switch element, the third switch element, and the fourth switch element are turned off. A writing step of writing a voltage to the storage capacitor may be executed.

  In the display device having the above configuration, the threshold voltage compensation period can be satisfactorily ended.

  For example, the control unit further performs a fourth step of bringing the third switch element into a non-conductive state at the start of a fourth period set after the third period and before the writing period. May be.

In the display device having the above structure, the voltage of the data signal and the reference voltage can be prevented from being applied to the first electrode of the capacitor at the same time between the threshold voltage compensation period and the writing period.
The threshold voltage compensation period can be successfully terminated.

  Hereinafter, embodiments will be specifically described with reference to the drawings.

  It should be noted that each of the embodiments described below shows a comprehensive or specific example. The numerical values, shapes, materials, constituent elements, arrangement positions and connection forms of the constituent elements, steps, order of steps, and the like shown in the following embodiments are merely examples, and are not intended to limit the present invention. In addition, among the constituent elements in the following embodiments, constituent elements that are not described in the independent claims indicating the highest concept are described as optional constituent elements.

(Embodiment)
The organic EL display according to the embodiment will be described with reference to FIGS.

  The organic EL display according to the present embodiment has a second initialization period in which the initialization voltage and the drive voltage are applied simultaneously between the first initialization period and the threshold voltage compensation period. Thereby, in the second initialization period, a voltage can be applied to the source electrode of the drive transistor in accordance with the variation of the threshold voltage.

[1. Configuration of organic EL display]
FIG. 4 is a block diagram showing an example of the configuration of the organic EL display 10 in the present embodiment. As shown in FIG. 4, the organic EL display 10 includes an organic EL panel 11, a data line driving circuit 12, a scanning line driving circuit 13, and a control unit 20.

  As in the organic EL panel 110 of the comparative example, the organic EL panel 11 has a plurality of display pixels PX arranged in a matrix. Note that the display pixel PX is a sub-pixel constituting one color, as in the comparative example. One pixel is composed of three sub-pixels corresponding to red, green, and blue.

  The display pixel PX includes an organic EL element OEL, a capacitive element Cs, a drive transistor Trd, a first switch element Tr1, a second switch element Tr2, a third switch element Tr3, and a fourth switch element Tr4. ing. Among the constituent elements of the display pixel PX, constituent elements other than the second switch element Tr2 have the same configuration as the comparative example.

  The organic EL element OEL is a light emitting element that emits light according to a drive current, like the organic EL element OEL of the comparative example. The drive current is supplied from the drive transistor Trd. The organic EL element OEL has an anode electrode connected to the source electrode of the drive transistor Trd and a cathode electrode connected to the power supply line VEL.

  The capacitive element Cs is a capacitive element that accumulates charges according to the voltage of the data line Data, similarly to the capacitive element Cs of the comparative example. The capacitive element Cs has a first electrode connected to the gate electrode of the drive transistor and a second electrode connected to the source electrode of the drive transistor Trd.

  Similarly to the drive transistor Trd of the comparative example, the drive transistor Trd supplies the organic EL element OEL with a drive current corresponding to the amount of charge of the capacitive element Cs accumulated according to the voltage of the data line Data. The drive transistor Trd is a thin film transistor, with a gate electrode connected to the first electrode of the capacitive element Cs, a source electrode connected to the anode electrode of the organic EL element OEL, and a drain electrode connected to a power supply line VTFT that supplies a drive voltage. Yes. The power supply line VTFT is a power supply line that supplies a drive voltage to the drain electrode of the drive transistor.

  Similar to the first switch element Tr1 of the comparative example, the first switch element Tr1 switches the conduction and non-conduction between the data line Data and the first electrode of the capacitor element Cs according to the voltage of the scanning line Scan, thereby displaying the first switch element Tr1. Switching between selection and non-selection of the pixel PX. More specifically, the first switch element Tr1 is a thin film transistor, the gate electrode is connected to the scanning line Scan, the source electrode is connected to the data line Data, and the drain electrode is connected to the first electrode of the capacitive element Cs.

  The second switch element Tr2 switches between conduction and non-conduction between the second electrode (node N2) of the capacitive element Cs and the power supply line VINI that supplies the initialization voltage in accordance with the voltage of the signal line Init. The power supply line VINI is a power supply line that supplies an initialization voltage to the second electrode of the capacitive element. The second switch element Tr2 is a thin film transistor and has a higher on-resistance than the other switch elements. In the present embodiment, the control unit 20 makes the fourth switch element Tr4 non-conductive in the first initialization period, and only charges and discharges the capacitive element Cs. That is, the second switch element Tr2 functions as a switch instead of a resistor. Further, in the second initialization period, the control unit 20 causes the fourth switch element Tr4 to conduct and causes the second switch element Tr2 to function as a resistor by causing a through current to flow through the second switch element Tr2. That is, in the second initialization period, the second switch element Tr2 functions as the above-described resistance unit. Details of the second switch element Tr2 will be described later.

  Similar to the third switch element Tr3 of the comparative example, the third switch element Tr3 is connected between the first electrode (node N1) of the capacitive element Cs and the power supply line VREF that supplies the reference voltage according to the voltage of the signal line Ref. Switch between conduction and non-conduction.

  Similar to the fourth switch element Tr4 of the comparative example, the fourth switch element Tr4 switches between conduction and non-conduction between the drain electrode of the drive transistor Trd and the power supply line VTFT according to the voltage of the signal line Enable.

  Similar to the comparative example, the data line driving circuit 12 supplies a voltage corresponding to the data signal output from the control unit 20 to the plurality of data lines Data.

  Similar to the comparative example, the scanning line driving circuit 13 supplies a voltage corresponding to the driving signal output from the control unit 20 to the plurality of scanning lines Scan.

  As in the comparative example, the control unit 20 is a circuit that controls the display of video on the organic EL panel 11, and is configured using, for example, a TCON (timing controller). The control unit 20 may be configured using a computer system including a microcontroller, a system LSI (Large Scale Integration), or the like. Alternatively, the control unit 20 may be realized by a program execution unit such as a CPU or a processor reading and executing a software program recorded on a recording medium such as a hard disk or a semiconductor memory.

[1-1. Detailed configuration of second switch element]
A detailed configuration of the second switch element Tr2 will be described with reference to FIGS.

  As described above, the second switch element Tr2 of the present embodiment is a transistor having a higher on-resistance than the other switch elements, and functions as a resistance unit in the second initialization period. In other words, the W / L ratio of the second switch element Tr2 is smaller than other switch elements.

  In order to reduce the decrease in accuracy in threshold voltage compensation due to the variation in threshold voltage described in detail of the problem, it is desirable to appropriately set the resistance value of the second switch element Tr2, and the like.

Here, in order to properly set the resistance value of the second switching element Tr2, sets the range of channel widths W _R and a channel length L _R.

[1-1-1. Deviation of Vth detection in comparative example]
FIG. 5 is a graph showing an ideal time change of the voltage Vgs between the gate and the source of the drive transistor Trd−the threshold voltage Vth in the threshold voltage compensation period. FIG. 6 is a graph showing an ideal time change of the current Ids flowing between the source electrode and the drain electrode of the drive transistor Trd in the threshold voltage compensation period. FIG. 5 and FIG. 6 show three cases where the threshold voltage variation ΔVth is 0V, 1V, and 2V.

  In the threshold voltage compensation period, as shown in FIG. 5, it is desirable that Vgs−Vth converge to the same voltage Vofs irrespective of the threshold voltage variation ΔVth of the drive transistor Trd. Since the threshold voltage compensation period is a finite period, Vgs−Vth does not completely become 0V but converges to Vofs.

Similarly, as shown in FIG. 6, it is desirable that the current Ids converge to substantially the same current Iend during the threshold voltage compensation period. Since the threshold voltage compensation period is a finite period, Ids is not completely converged but converges to approximately Iend. At this time, the current Ids can be obtained by the following Equation 1. The current Iend can be obtained by the following equation 2.

  Note that Vgs = Vofs + Vth and Vth = Vth0 + ΔVth.

  However, actually, the current Ids at the end of the threshold voltage compensation period does not converge to the same value. FIG. 7 is a graph showing the time change of the drain-source voltage of the drive transistor Trd during the threshold voltage compensation period. FIG. 7 shows three cases in which the threshold voltage variation ΔVth is 0 to 2V.

  As shown in FIG. 7, as the threshold voltage variation ΔVth increases, the drain-source voltage Vds at the end of the threshold voltage compensation period increases. Then, the convergence current Iend increases as the threshold voltage variation ΔVth increases.

FIG. 8 is a graph showing an actual time change of the current Ids flowing between the source electrode and the drain electrode of the drive transistor Trd in the threshold voltage compensation period. The current Ids can be obtained by the following Equation 3.

  Ideally, the current Ids at the end of the threshold voltage compensation period is desirably converged to the current Iend even when the threshold voltage varies as shown in FIG. However, actually, as shown in FIG. 8, the current Ids at the end of the threshold voltage compensation period varies according to the Vds variation caused by the threshold voltage variation ΔVth. For this reason, Vgs−Vth at the end of the threshold voltage compensation period also varies.

  FIG. 9 is a graph showing an actual time change of the gate-source voltage Vgs−threshold voltage Vth of the drive transistor Trd in the threshold voltage compensation period. As shown in FIG. 9, the larger the threshold voltage variation ΔVth, the larger the amount of current flowing between the source electrode and the drain electrode, so Vgs−Vth becomes smaller. That is, there is a shift in threshold detection, and it is considered that the accuracy of threshold voltage compensation is reduced.

[1-1-2. Channel width and channel length of second switch element Tr2]
Therefore, in this embodiment, Vgs−Vth is adjusted in the initialization period so that Vgs−Vth at the start of the threshold voltage compensation period increases as the threshold voltage variation ΔVth increases. That is, at the end of the initialization period, the initialization voltage is adjusted so that Vgs−Vth increases as the threshold voltage variation ΔVth increases.

  In order to adjust the initialization voltage, the display pixel PX of the present embodiment, as shown in FIG. 4, starts from a node N2 that is a connection point between the second electrode of the capacitive element Cs and the anode electrode of the organic EL element OEL. The resistor section described above is provided on the current path to the power supply line VINI. In the present embodiment, the resistance portion is realized by making the on-resistance of the second switch element Tr2 (thin film transistor) larger than that of other switch elements. As a result, the voltage of the source electrode of the drive transistor Trd can be adjusted according to the variation ΔVth of the threshold voltage Vth of the drive transistor Trd.

  Hereinafter, a method for adjusting the voltage of the source electrode of the drive transistor Trd will be described.

  When the first switch element Tr1 is turned off and the second switch element Tr2, the third switch element Tr3 and the fourth switch element Tr4 are turned on in the initialization period, the drive transistor Trd and the second switch element are connected from the power line VTFT. A through current Id flows through the power line VINI through Tr2.

  At this time, the voltage of the source electrode of the drive transistor Trd is VTFT− (VINI + Ron × Id). As described above, the VTFT is a voltage of the power supply line VTFT that supplies a drive voltage to the drain electrode of the drive transistor. As described above, VINI is the voltage of the power supply line VINI that supplies the initialization voltage to the second electrode of the capacitive element Cs. As described above, Ron is the resistance value of the resistance unit.

  The through current Id changes according to the threshold voltage Vth of the drive transistor Trd. Specifically, the through current Id decreases as the threshold voltage Vth of the drive transistor Trd increases, and increases as the threshold voltage Vth decreases.

  As a result, the larger the threshold voltage Vth, the smaller Ron × Id (voltage drop amount in the resistance portion), and the voltage of the source electrode of the drive transistor at the end of the initialization period and at the start of the subsequent threshold voltage compensation period is Get smaller.

  That is, by providing the resistance portion, the voltage of the source electrode of the drive transistor Trd can be adjusted according to the variation ΔVth of the threshold voltage Vth of the drive transistor Trd.

The resistance value of the resistance portion (in this embodiment, the resistance value of the second switch element Tr2) is set so as to satisfy the following Expression 4.

  Here, by setting ∂Vgs / ∂Vth to 0.3 times or more, the fluctuation of the threshold voltage can be sufficiently reflected in the voltage of the source electrode. Further, by setting ∂Vgs / ∂Vth to 0.7 times or less, the layout size of the resistor portion exceeds the layout size per pixel (display pixel PX), thereby preventing the layout from becoming impossible. At the same time, it is possible to prevent the amount of voltage drop in the resistance portion from becoming too large, and to give a sufficient voltage difference to the capacitive element Cs as the initial value of the threshold voltage compensation period. The basis for setting the value to 0.3 times or more and 0.7 times or less will be described later.

Here, the through current Id is obtained by the following equation 5 using the channel width W _D , the channel length L _D , the electron mobility μ _D of the drive transistor Trd, and the capacitance Cox per unit area of the gate oxide film. it can. Further, Vgs can be obtained by the following Expression 6.

Substituting equation 6 into equation 5, the following equations 7 and 8 are obtained.

From the above, the following formula 9 can be derived for Vgs.

Substituting equation 9 into equation 4, the following equation 10 is obtained.

Here, since the second switch element Tr2 is a transistor operating in a linear region, the following Expressions 11 to 13 hold.

In Formula 11 Formula 13, _{W R} is the channel width of the second switching element Tr2, is _{L R} channel length of the second switching element Tr2, the mu _R is an electron mobility of the second switching element Tr2. Vgs _R is a gate-source voltage of the second switch element Tr2, Vth _R is a threshold voltage of the second switch element Tr2, Vds _R is a drain-source voltage of the second switch element Tr2, and Vgh _R is a second switch element Tr2. Is the voltage of the gate electrode when In is on (Init voltage). When Expression 11 to Expression 13 are applied to Expression 10, the following Expression 14 is obtained.

So as to satisfy the equation 14, setting the channel width of the second switching element Tr2 _{W R} and a channel length _{L R.}

  FIG. 10 is a graph showing transition of Vgs−Vth in the threshold voltage compensation period in the comparative example and the present embodiment. FIG. 10 shows the case where the threshold voltage variation ΔVth is 2V. FIG. 11 is a graph showing a difference in Vth detection when the threshold voltage variation ΔVth is 0V and 2V in the present embodiment. FIG. 12 is a graph showing the time change of the drain-source voltage of the drive transistor Trd in the threshold voltage compensation period in the present embodiment. FIG. 12 shows when the threshold voltage variation ΔVth is 0V and 2V.

  As shown in FIG. 10, the initial value of the graph of the present embodiment is larger than that of the comparative example. As a result, the threshold detection value at the end of the threshold voltage compensation period approaches Vofs. As shown in FIG. 11, in this embodiment, it can be seen that the variation in threshold detection at the end of the threshold voltage compensation period is reduced compared to the comparative example. As shown in FIG. 12, the difference in Vds when ΔVth is 0V and 2V is 2V or less.

[1-1-3. Setting of Vgs variation amount range for threshold voltage Vth variation]
(1) The setting of the lower limit value (for example, 0.3) of ∂Vgs / ∂Vth in Equation 4 will be described.

First, the time TthMax required for Vth detection in the comparative example will be described. In the comparative example, the gate-source voltage Vinit of the drive transistor Trd at the start of the threshold voltage compensation period is obtained by the following Expression 15.

Here, VthMax is a value of the threshold voltage Vth in the display pixel PX in which the threshold voltage Vth of the drive transistor Trd becomes the maximum value among all the display pixels constituting the organic EL panel 11. VthMargin is a voltage margin necessary for threshold voltage compensation. Next, as can be seen from FIG. 3, when the threshold voltage Vth takes the minimum value, the Vth detection time becomes the longest. Therefore, a time TthMax required for Vth detection when the threshold voltage Vth is the minimum value is obtained. First, the charge amount Qmax charged by Vth detection is obtained by the following equation (16).

Here, VthMin is the value of the threshold voltage Vth in the display pixel PX in which the threshold voltage Vth of the drive transistor Trd becomes the minimum value among all the display pixels constituting the organic EL panel 11. A time TthMax necessary for charging the charge amount Qmax is obtained by the following Expression 17.

Here, Id is an average value of the current flowing through the capacitive element Cs via the drive transistor Trd during the threshold voltage compensation period. As described in detail of the problem, when the threshold voltage compensation period is one horizontal scanning period (1H), it is desirable that TthMax is 1H or less. Substituting Equations 16 and 15 into Equation 17 yields Equation 18 below.

Next, the time TthMax required for Vth detection in the present embodiment will be described. As described above, in this embodiment, Vinit changes with a coefficient of α (= ∂Vgs / ∂Vth) with respect to Vth. Therefore, in the display pixel PX in which the threshold voltage of the drive transistor Trd is VthMax, the following Expression 19 is established.

From Expression 18 and Expression 19, the following Expression 20 is established.

  As can be seen from Equations 18 and 20, in the organic EL display according to the present embodiment, TthMax is smaller by the coefficient (1-α) than the comparative example, and the threshold voltage compensation period is set to one horizontal scanning period (1H ) It becomes easy to do the following.

  The present inventors have verified an organic EL panel 11 of 40 type 4K2K (for example, 3480 × 2160 display pixels). As the organic EL panel 11, a panel in which the in-plane variation of the threshold voltage Vth of the drive transistor Trd is 0 to 2 V, VthMargin is 2.5 V, and the capacitive element Cs is 0.5 pF is used. In this case, the average value Id of the current flowing through the capacitive element Cs via the drive transistor Trd during the threshold voltage compensation period was 0.25 μA.

  FIG. 13 is a graph showing the result of calculating the value of α necessary to satisfy Expression 20 for each drive frequency Freq when the number of scanning lines Vline is 2160. From FIG. 13, for example, when the drive frequency is 120 Hz, it is understood that α is preferably 0.3 or more.

  FIG. 14 is a graph showing the result of calculating the value of α necessary to satisfy Expression 20 for each scanning line number Vline when the drive frequency Freq is fixed to 120 Hz. As shown in FIG. 14, when the number of scanning lines Vline is 2160, it is understood that α is preferably 0.3 or more.

  As described above, in the panel having the number of scanning lines Vline ≧ 2160 and the driving frequency Freq ≧ 120 Hz, α ≧ 0.3 is desirable to converge the voltage of the source electrode of the driving transistor Trd within the threshold voltage compensation period. I understand that.

  In the present embodiment, the 40-inch 4K2K panel has been described. However, the organic EL panel 11 having different conditions such as Vline being smaller than 2160 may have a different lower limit value of α. It is desirable to set according to the restrictions of the organic EL panel 11 to be designed.

  (2) The setting of the upper limit value (for example, 0.7) of ∂Vgs / ∂Vth in Equation 4 will be described.

  FIG. 15 is a graph showing the relationship between the Cs initialization voltage (the voltage applied between the first electrode and the second electrode of the capacitive element Cs at the end of the initialization period) and the threshold voltage Vth. The slope of the straight line in the graph corresponds to α = ∂Vgs / ∂Vth.

In Figure 15, 2V voltage of the power supply line VREF, -4 V the voltage of the power supply line VINI, and the voltage Vgh _R of the gate electrode at the time of the second switching element Tr2 in the ON state and 20V. In FIG. 15, it is assumed that the threshold voltage Vth of the drive transistor Trd and the threshold voltage Vth _R of the second switch element Tr2 are substantially the same.

As shown in FIG. 15, the ratio (W _R / L _R ) of the channel width and channel length of the second switch element Tr2 to the ratio (W _D / L _D ) of the channel width and channel length of the drive transistor Trd, That is, when (W _R / L _R ) / (W _D / L _D ) is 1/20, α = ∂Vgs / ∂Vth = 0.67. When the ratio (W _R / L _R ) / (W _D / L _D ) is 1/10, α = ∂Vgs / ∂Vth = 0.55. When the ratio (W _R / L _R ) / (W _D / L _D ) is 1/4, α = ∂Vgs / ∂Vth = 0.38.

Figure 16 is a graph showing the value of ∂Vgs / ∂Vth, the correlation between the channel length of the second switching element Tr2 _{L R.} As shown in FIG. 16, as ほ ど Vgs / ∂Vth increases, the accuracy of threshold voltage compensation increases, and the variation in time required for Vth detection decreases. However, the larger the ∂Vgs / ∂Vth, channel length _{L R} of the second switching element Tr2 increases.

Consider a 40-inch 4K2K panel. In this case, the size of one pixel is about 230 μm, for example. In this case, the channel length _{L R} of the second switching element Tr2 is desirably less 230 .mu.m. For Figure 16, ∂Vgs / ∂Vth is at 0.7 or less, channel lengths _{L R} of the second switching element Tr2 is equal to or less than 230 .mu.m. Therefore, when designing a fine panel having a channel length _LR of 230 μm or less, it is desirable that ∂Vgs / ∂Vth ≦ 0.7. As a result, it is possible to prevent the resistance portion from exceeding the layout size per pixel (display pixel PX) and becoming impossible to layout.

  Further, as described above, the voltage of the source electrode of the driving transistor Trd at the start of threshold voltage compensation is expressed by VTFT− (VINI + Ron × Id), but the second voltage is such that ∂Vgs / ∂Vth> 0.7. In the case of the two-switch element Tr2, the on-resistance Ron is very large. In this case, the voltage of the source electrode becomes too large, and there is a possibility that a gate-source voltage sufficiently large for threshold voltage compensation cannot be secured. Also from this viewpoint, it is desirable that ∂Vgs / ∂Vth ≦ 0.7.

In the case where constraints required of the channel length L _R are different, the upper limit of ∂Vgs / ∂Vth is may be a value other than 0.7. It is desirable to set according to the restrictions of the organic EL panel 11 to be designed.

[2. Operation of organic EL display]
FIG. 17 is a graph showing signal waveforms of the organic EL display 10 according to the present embodiment.

  In the organic EL display 10 of the present embodiment, two initialization periods, a first initialization period and a second initialization period, are provided. In the organic EL display 10 of the present embodiment, the threshold voltage compensation period, the writing period, and the light emission period are set in this order after the initialization period has elapsed.

  It is assumed that all the switch elements are in the OFF state until time t0.

  Further, in the first initialization period T22, the second switch element Tr2 is operated as a switch, and in the second initialization period T23, the second switch element Tr2 is operated as a resistance unit. The H level voltage of each of the signal line Scan, the signal line Ref, and the signal line Enable is regarded as a switch for each of the first switch element Tr1, the third switch element Tr3, and the fourth switch element Tr4 according to the characteristics of each switch element. The voltage is set such that it can be operated with a sufficiently low resistance.

(Period T21: First period)
A period T21 from time t0 to time t1 illustrated in FIG. 17 is a period for stabilizing the voltage of the node N2.

  Specifically, at the start of the period T21, the control unit 20 executes the first step of setting the second switch element Tr2 to the conductive state and the other switch elements to the non-conductive state. Thereby, in the period T21, the voltage of the node N2 is stabilized at the voltage of the power supply line VINI.

  The scanning line driving circuit 13 maintains the voltages of the signal line Scan, the signal line Ref, and the signal line Enable at the L level, thereby turning off the first switch element Tr1, the third switch element Tr3, and the fourth switch element Tr4. To maintain.

  In addition, the scanning line driving circuit 13 sets the voltage of the signal line Init from the L level to the H level at the start of the period T21, thereby causing the second switch element Tr2 to transition from the off state to the on state.

  By providing the period T21, the voltage of the node N2 can be set to the voltage of the power supply line VINI in a short time. Further, the voltage of the node N1 is also reduced to (the voltage of the power supply line VINI + the voltage Vgs between the gate and source of the driving transistor Trd at the time of light emission in the previous frame) due to the capacitive element Cs.

  The reason for providing this period T21 is as follows. When the size of the organic EL panel 11 or the size per pixel (display pixel PX) is large, the capacitance Coled of the organic EL element OEL increases, and the wiring time constant of the power supply line VINI increases. For this reason, the larger the size of the organic EL panel 11 or the size per pixel, the longer it takes to set the voltage at the node N2 to the voltage of the power supply line VINI. Therefore, by providing the period T21 during which the second switch element Tr2 is turned on and the voltage of the power supply line VINI is applied to the node N2, the voltage of the node N2 can be set to the voltage of the power supply line VINI in a shorter period of time. . In other words, by providing the period T21, the voltage of the power supply line VINI can be written into the wiring capacitance of the organic EL element OEL and the power supply line VINI in a shorter period of time.

  Similarly, it takes time to apply the voltage of the power supply line VREF to the node N1. However, the target for charging / discharging the voltage of the power supply line VREF is the capacitance of the capacitive element Cs and the power supply line VREF. Here, the wiring time constant of the power supply line VREF and the wiring time constant of the power supply line VINI are substantially equal. However, the capacitance of the organic EL element OEL> the capacitance of the capacitance element Cs, and the capacitance ratio of the organic EL element OEL to the capacitance element Cs (organic EL element OEL / capacitance element Cs) is 1.3 to 9 times. That is, the time taken to charge the organic EL element OEL is longer than the time taken to charge the capacitive element Cs. In other words, the time taken to set the voltage of the node N2 to the voltage of the power supply line VINI is longer than the time taken to set the voltage of the node N1 to the voltage of the power supply line VREF. In other words, the time taken to write the voltage of the power supply line VINI to the organic EL element OEL is longer than the time taken to write the voltage of the power supply line VREF to the capacitive element Cs.

  In addition, by providing the period T21, the voltage of the node N2 is set to the voltage of the power supply line VINI, so that the load on the power supply line VREF can be reduced. That is, by providing the period T21, the voltage of the node N1 can be set to a low voltage, and the power supply line VREF only needs to supply a current (voltage) for charging the display pixel PX. In other words, in the period T21, since the voltage of the power supply line VREF is not used as a voltage for charging the organic EL element OEL, there is an advantage that the load of the power supply line VREF is reduced.

(Period T22: First initialization period)
A period T22 from time t1 to time t2 shown in FIG. 17 is a first initialization period in which a voltage necessary for flowing a drain current to perform threshold voltage compensation of the drive transistor Trd is applied between the gate and source of the drive transistor Trd. It is.

  In other words, in the first initialization period, the control unit 20 stops the application of the drive voltage for driving the organic EL element OEL to the drain electrode of the drive transistor Trd, and the first of the capacitive element Cs. By applying a reference voltage to the electrode and setting the second switch element Tr2 to a conductive state, a period of executing a first initialization step of applying an initialization voltage to the second electrode of the capacitive element Cs is there.

  FIG. 18 is a circuit diagram illustrating a state of the display pixel PX in the first initialization period.

  Specifically, in the period T22, as shown in FIGS. 17 and 18, the second switch element Tr2 and the third switch element Tr3 are in a conductive state, and the first switch element Tr1 and the fourth switch element Tr4 are in a nonconductive state. Set to

  The scanning line drive circuit 13 maintains the first switch element Tr1 and the fourth switch element Tr4 in the OFF state by maintaining the voltage of the signal line Scan and the signal line Enable at the L level. Further, the scanning line driving circuit 13 maintains the voltage of the signal line Init at the H level, thereby maintaining the second switch element Tr2 in the on state.

  In addition, the scanning line driving circuit 13 causes the third switch element Tr3 to transition from the off state to the on state by transitioning the voltage of the signal line Ref from the L level to the H level at the start of the period T22. Here, the current flowing through the second switch element Tr2 is only a current necessary for charging / discharging the capacitive element Cs, and when the charging / discharging of the capacitive element Cs converges, the current is almost zero. The on-resistance of the two switch element Tr2 can be almost ignored. That is, the second switch element Tr2 operates as a switch.

  Thereby, the voltage of the node N1 is set to the voltage of the power supply line VREF. Here, since the second switch element Tr2 is in the conductive state, the voltage of the node N2 is set to the voltage of the power supply line VINI. That is, in the drive transistor Trd, the voltage of the power supply line VREF is applied to the gate electrode, and the voltage of the power supply line VINI is applied to the source electrode.

  For example, the first initialization period is set to a length that can sufficiently charge and discharge the node N1.

  Here, as an example of setting the first initialization period, a case of a 40-inch 4K2K display will be described. Note that the setting of the first initialization period is an example, and even if the display is the same type, the result of the first initialization period is different if any of the following conditions is different.

  Design values of the 40-inch 4K2K display include VthMargin = 2.5V, VthMax = 2V, VthMin = 0V, Vgs_Peak = 6.5V, Id_Peak = 4.5 μA, Cs = 0.5 pF, Coled = 2.5 pF, Ron_Sw = 0 Consider the case of 6 MΩ and Ron_Drv = 1 MΩ. Cs is a capacitance value of the capacitive element of the display pixel PX. Coled is the capacity of the organic EL element OEL. Ron_Sw is the ON resistance of the second switch element Tr2. Ron_Drv is the ON resistance of the drive transistor Trd. For simplification, detailed parasitic capacitances associated with the intersections of the transistors and the wirings in the display pixels are omitted below.

  The first initialization period is a value obtained by adding the charge / discharge time in the display pixel PX at the node N1 and the time required for charging / discharging the CR load of the power supply line VREF.

  (1) The charge / discharge time in the display pixel PX at the node N1 will be described. Here, the CR time constant of the display pixel PX when the node N1 is initialized, and the time constant necessary for charging / discharging the node N1 to 99.9% is 6.9 tau. Then, the charge / discharge time in the display pixel PX is CR coefficient × Cs × Ron_Sw = 6.9 × 0.5 pF × 0.6 MΩ = 2.1 μsec.

  (2) The time required for charging / discharging the CR load of the power supply line VREF will be described. Here, when the resistance of the power supply line VREF = 3 KΩ (sheet resistance 0.1Ω / □) and the wiring load = 500 pF, the CR time constant of the power supply line VREF is 3 KΩ × 500 pF = 1.5 μsec. Therefore, the time required for charging and discharging the CR load of the power supply line VREF is 1.5 μsec × 6.9 tau = 10.4 μsec.

  From the above, the first initialization period is 2.1 μsec + 10.4 μsec = 12.5 μsec.

  Further, as described above, in the first initialization period, the gate-source voltage Vgs of the drive transistor Trd is set to a voltage that can secure the initial drain current necessary for performing the threshold value correcting operation. Therefore, the voltage difference between the voltage of the power supply line VREF and the voltage of the power supply line VINI is set to a voltage higher than the maximum threshold voltage VthMax of the drive transistor Trd. Specifically, the maximum threshold voltage VthMax of the driving transistor Trd is set to a value obtained by adding a voltage margin VthMargin necessary for threshold voltage compensation. Further, the voltage of the power supply line VREF and the voltage of the power supply line VINI are such that the current of the power supply line VINI <the voltage of the power supply line VEL + the forward current threshold voltage of the organic EL element OEL, so that no current flows through the organic EL element OEL. Also, the voltage of the power supply line VREF <the voltage of the power supply line VEL + the forward current threshold voltage of the organic EL element OEL + the minimum threshold voltage VthMin of the drive transistor Trd is set.

(Period T23: Second initialization period)
A period T23 from time t2 to time t3 shown in FIG. 17 is a second initialization period in which the voltage of the source electrode of the drive transistor Trd is corrected in order to reduce the variation in time necessary for performing threshold voltage compensation. .

  In other words, in the second initialization period, the control unit 20 maintains the application of the reference voltage to the first electrode of the capacitive element Cs after the first initialization step, and the second switch element Tr2 is in a conductive state. This is a period in which the second initialization step for starting the application of the drive voltage to the drain electrode of the drive transistor Trd is executed in the state maintained in the state.

  FIG. 19 is a circuit diagram illustrating a state of the display pixel PX in the second initialization period.

  Specifically, in the period T23, as shown in FIGS. 17 and 19, the second switch element Tr2, the third switch element Tr3, and the fourth switch element Tr4 are in the conductive state, and the first switch element Tr1 is in the non-conductive state. Set to

  The scanning line drive circuit 13 maintains the first switch element Tr1 in the OFF state by maintaining the voltage of the signal line Scan at the L level. Further, the scanning line driving circuit 13 maintains the voltage of the signal line Init and the signal line Ref at the H level, thereby maintaining the second switch element Tr2 and the third switch element Tr3 in the on state.

  Further, the scanning line driving circuit 13 causes the fourth switch element Tr4 to transition from the off state to the on state by transitioning the voltage of the signal line Enable from the L level to the H level at the start of the period T23.

  At this time, as shown in FIG. 19, a through current flows from the power supply line VTFT to the power supply line VINI via the fourth switch element Tr4, the drive transistor Trd, and the second switch element Tr2. Since the through current continues to flow during the conduction period of the second switch element Tr2, the voltage applied to the source electrode of the drive transistor Trd is affected by the on-resistance of the second switch element. That is, the second switch element Tr2 operates as a resistance unit. Therefore, the voltage of the source electrode of the drive transistor Trd at the end of the second initialization period is VTFT− (VINI + Ron × Id).

  Here, as described above, the through current Id decreases as the threshold voltage Vth of the drive transistor Trd increases, and increases as the threshold voltage Vth decreases. That is, the larger the threshold voltage Vth, the smaller Ron × Id (voltage drop amount in the resistance portion), and the voltage of the source electrode of the drive transistor Trd at the end of the initialization period and at the start of the subsequent threshold voltage compensation period is Get smaller.

  FIG. 20 is a graph showing a change in the voltage of the source electrode of the driving transistor in the second initialization period and the threshold voltage compensation period. FIG. 20 is a graph corresponding to a portion surrounded by a broken line in FIG.

  As shown in FIG. 20, the increase amount (ΔV1 to ΔV3) of the source electrode voltage of the drive transistor Trd in the second initialization period T23 increases as the threshold voltage Vth decreases (Vth1 <Vth2 <Vth3). (ΔV1> ΔV2> ΔV3).

  The second initialization period only needs to be long enough to charge and discharge the node N2 by a certain voltage. Note that the length of the node N2 that can be charged and discharged by a certain voltage varies depending on the value of the threshold voltage Vth.

  For example, the voltage Vgs = VREF−VINI = 6.5V between the gate electrode and the source electrode of the drive transistor Trd before the start of the threshold voltage compensation period. When the threshold voltage Vth is VthMin = 0V, Vgs of the drive transistor Trd at the start of the threshold voltage compensation period may be VthMin + VthMargin = 2.5V. In this case, it is only necessary to discharge 6.5-2.5 = 4.0 V of charge from the node N2. Note that, during the threshold voltage compensation period, electric charges are gradually discharged so that the Vgs of the drive transistor Trd changes from 2.5V to 0V.

  On the other hand, when the threshold voltage Vth is VthMax = 2V, Vgs of the drive transistor Trd at the start of the threshold voltage compensation period may be VthMax + VthMargin = 4.5V. In this case, it is only necessary that the charge of 6.5-4.5 = 2.0 V can be discharged from the node N2. During the threshold voltage compensation period, electric charges are gradually discharged so that the Vgs of the drive transistor Trd is changed from 4.5V to 2V. In this case, the discharge amount of the drive transistor Trd during the threshold voltage compensation period is almost the same as that when the threshold voltage Vth is VthMin. In other words, when the threshold voltage Vth becomes VthMin and when it becomes VthMax, the time for the operation relating to the threshold voltage compensation to converge is almost the same.

  In the worst case (when the threshold voltage Vth is VthMin), the second initialization period may be a length that can charge and discharge the node N2 by a certain voltage. The second initialization period is a value obtained by adding the charge / discharge time in the display pixel PX at the node N2 and the time required for charging / discharging the CR load of the power supply line VINI. Therefore, the CR time constant of the node N2 is 4V / 6.5V = 62% charge / discharge time (= 0.96 tau).

  (1) The charge / discharge time in the display pixel PX at the node N2 will be described. The charge / discharge time in the display pixel PX is CR time constant × (Cs + Coled) × Ron_Drv = 0.96 × (0.5 pF + 2.5 pF) × 1 MΩ = 2.88 μsec. Note that the values of Cs, Coled, and Ron_Drv are the same as in the first initialization period.

  (2) The time required for charging / discharging the CR load of the power supply line VINI will be described. Here, it is assumed that the CR time constant of the power supply line VINI is substantially the same as the CR time constant of the power supply line VREF. Then, the time required for charging / discharging the CR load of the power supply line VINI is 1.5 μsec × 0.96 tau = 1.44 μsec.

  From the above, the second initialization period is 2.88 μsec + 1.44 μsec = 4.32 μsec. Since the first initialization period is 12.5 μsec as described above, the second initialization period is shorter than the first initialization period.

(Period T24: threshold voltage compensation period)
A period T24 from time t3 to time t4 shown in FIG. 17 is a threshold voltage compensation period for compensating the threshold voltage of the drive transistor Trd.

  In the threshold voltage compensation period, the controller 20 maintains the application of the reference voltage to the first electrode of the capacitive element Cs and the application of the drive voltage to the drain electrode of the drive transistor Trd after the execution of the second initialization step. While maintaining the above, the step of bringing the second switch element into a non-conductive state is executed.

  Specifically, in the period T24, as shown in FIG. 17, the third switch element Tr3 and the fourth switch element Tr4 are set in a conductive state, and the first switch element Tr1 and the second switch element Tr2 are set in a non-conductive state. The

  The scanning line drive circuit 13 maintains the first switch element Tr1 in the OFF state by maintaining the voltage of the signal line Scan at the L level. Further, the scanning line driving circuit 13 maintains the third switch element Tr3 and the fourth switch element Tr4 in the on state by maintaining the voltages of the signal line Ref and the signal line Enable at the H level.

  In addition, the scanning line driving circuit 13 causes the second switch element Tr2 to transition from the on state to the off state by transitioning the voltage of the signal line Init from the H level to the L level at the start of the period T24.

  In this way, the voltage of the power supply line VREF is input to the gate electrode of the drive transistor Trd, and the second switch element Tr2 is in a non-conductive state (off state) while the fourth switch element Tr4 is in a conductive state (on state). Then, the threshold compensation operation of the drive transistor Trd can be started.

  For example, as shown in FIG. 20, in the case of the threshold voltage Vth1, the voltage of the source electrode of the drive transistor Trd at the end of the second initialization period T23 is the power supply line VREF-Vth1. Therefore, in threshold voltage compensation, the voltage of the source electrode of the drive transistor Trd needs to be increased by ΔV1b when only the first initialization period is provided, but when the second initialization period is provided, What is necessary is just to raise only (DELTA) V1a smaller than (DELTA) V1b. In other words, the voltage increase during the second initialization period T23 and the voltage increase during the threshold voltage compensation period can be kept small. In the case of the threshold voltage Vth1, the amount of voltage increase in the threshold voltage compensation is smaller by ΔV1 in the present embodiment than in the comparative example.

  Also in the case of the threshold voltage Vth2 or Vth3, the amount of voltage increase in the threshold voltage compensation is reduced by ΔV2 or ΔV3.

  Further, in the second initialization period T23 of the present embodiment, as described above, the amount of increase in the voltage of the source electrode (ΔV1 to ΔV3) of the drive transistor Trd decreases as the value of the threshold voltage Vth decreases (Vth1 <Vth2). <Vth3), which is larger (ΔV1> ΔV2> ΔV3). Comparing the graphs of FIG. 3 and FIG. 20, the variation in the period of threshold voltage compensation (Tth1−Tth3 = ΔTth) is shorter in ΔTth of the present embodiment than in ΔTth of the comparative example. Recognize. That is, in the case of the present embodiment, it can be seen from FIGS. 3 and 20 that the variation in the period for threshold voltage compensation is reduced.

(Period T25: Third period)
A period T25 from time t4 to time t5 illustrated in FIG. 17 is a period for ending the threshold compensation operation.

  Specifically, in the period T25, as shown in FIG. 17, the first switch element Tr1, the second switch element Tr2, and the third switch element Tr3 are set in a conductive state, and the fourth switch element Tr4 is set in a non-conductive state. The The control unit 20 executes the third step of changing the fourth switch element Tr4 from the conductive state to the nonconductive state at the start of the period T25.

  The scanning line drive circuit 13 maintains the first switch element Tr1 and the second switch element Tr2 in the OFF state by maintaining the voltage of the signal line Scan and the signal line Init at the L level. Furthermore, the scanning line driving circuit 13 maintains the third switch element Tr3 in the ON state by maintaining the voltage of the signal line Ref at the H level.

  In addition, the scanning line drive circuit 13 causes the fourth switch element Tr4 to transition from the on state to the off state by transitioning the voltage of the signal line Enable from the H level to the L level at the start of the period T25.

  In this way, by providing the period T25 in which the fourth switch element Tr4 is turned off by the operation of the signal line Enable after the threshold voltage compensation period, the drive transistor Trd is connected to the anode electrode of the organic EL element OEL. The supply of current from the power supply line VEL to the node N2 can be eliminated, and the next operation can be performed after the threshold value compensation operation has been completed with certainty.

(Period T26: Fourth period)
In the period T26 between time t5 and time t6 shown in FIG. 17, the voltage of the data signal supplied via the signal line Data and the power line VREF are set by turning off the third switch element Tr3. This is a period for preventing the voltage from being simultaneously applied to the node N1.

  Specifically, in the period T26, as shown in FIG. 17, the first switch element Tr1 to the fourth switch element Tr4 are set in a non-conductive state. The control unit 20 executes the fourth step of setting the third switch element Tr3 from the conductive state to the non-conductive state at the start of the period T26.

  The scanning line driving circuit 13 maintains the voltages of the signal line Scan, the signal line Init, and the signal line Enable at the L level, thereby turning off the first switch element Tr1, the second switch element Tr2, and the fourth switch element Tr4. To maintain. Furthermore, the scanning line driving circuit 13 causes the third switch element Tr3 to transition from the on state to the off state by causing the voltage of the signal line Ref to transition from the H level to the L level at the start of the period T26.

  In this way, by providing the period T26 in which the third switch element Tr3 is further turned off by the operation of the signal line Ref, and the first switch element Tr1 and the third switch element Tr3 are turned off at the same time (off state). The voltage of the data signal supplied from the first switch element Tr1 via the signal line Data and the voltage of the power supply line VREF can be prevented from being simultaneously applied to the node N1.

  Note that the third switch element Tr3 and the fourth switch element Tr4 may be simultaneously turned off (off state), and the periods T25 and T26 may be combined into one.

  In the case where the period T25 and the period T26 are divided into two stages, there are advantages described below. That is, by providing the period T25 and the period T26, the period in which the voltage of the node N1, which is the gate voltage of the drive transistor Trd, becomes unstable is shortened as much as possible, voltage fluctuation that may occur in the indefinite period is suppressed, and the video signal Can be displayed more accurately.

  Further, gradation display is performed when writing of the voltage of the node N1 at the end (time t6) of the period T26 and the voltage of the data signal input via the signal line Data (voltage corresponding to the video signal) is completed (time t27). This is performed by the voltage difference with the voltage of the node N1. For this reason, it is preferable that the voltage fluctuation of the node N1 in the period T26 is small. Ideally, the voltage of the power supply line VREF is applied to the node N1 in the period T24, and the voltage of the node N1 is held in the period T25, so that the voltage difference (the voltage of the data signal−the voltage of the power supply line VREF) is increased. Based on this, the display luminance of the organic EL element OEL is determined.

  Note that the period T26 is preferably as short as possible in order to accurately reflect the voltage of the data signal and the voltage of the power supply line VREF.

  Further, the fourth switch element Tr4 connected to the signal line Enable is connected to the drain side of the drive transistor Trd as shown in FIG. When the fourth switch element Tr4 is formed of an n-type transistor, the on-resistance of the fourth switch element Tr4 tends to be high, and the voltage drop due to the on-resistance affects the power consumption of the organic EL panel 11. Therefore, the fourth switch element Tr4 is formed with as low an on-resistance as possible. As a method of reducing the on-resistance of the fourth switch element Tr4, generally, a method of increasing the channel size of the fourth switch element Tr4 or setting a high voltage (on-state control voltage) of the signal line Enable is set. Although any method is known, the falling time of the signal line Enable becomes longer in any method.

  Therefore, in this embodiment, by providing the period T25 during which the signal line Enable is first lowered with respect to the signal line Ref, the period during which the voltage at the node N1 becomes unstable can be shortened. Time can be shortened.

(Period T27: Write period)
In a period T27 from time t6 to time t7 shown in FIG. 17, the voltage of the data signal having a voltage value corresponding to the gradation value included in the video signal is applied from the signal line Data to the capacitive element Cs via the first switch element Tr1. This is a writing period for writing to.

  Specifically, in the period T27, as shown in FIG. 17, the first switch element Tr1 is set in a conductive state, and the second switch element Tr2, the third switch element Tr3, and the fourth switch element Tr4 are set in a nonconductive state. The

  The scanning line driving circuit 13 maintains the voltage of the signal line Init, the signal line Ref, and the signal line Enable at the L level, thereby turning off the second switch element Tr2, the third switch element Tr3, and the fourth switch element Tr4. To maintain. Further, the scanning line driving circuit 13 causes the first switch element Tr1 to transition from the off state to the on state by transitioning the voltage of the signal line Scan from the L level to the H level at the start of the period T27.

  Thereby, in addition to the threshold voltage Vth of the driving transistor Trd stored in the threshold voltage compensation period, the voltage difference between the voltage of the data signal and the voltage of the power supply line VREF is (capacitor of the organic EL element OEL). (Capacitance) / (Capacity of organic EL element OEL + Capacitance element Cs) is multiplied and stored (held). Here, since the fourth switch element Tr4 is in a non-conductive state, the drive transistor Trd does not pass a drain current. Therefore, the voltage of the node N2 does not change significantly during the period T27.

  As the screen becomes larger (the size of the organic EL panel 11 becomes larger) and the number of display pixels PX increases, the period for writing data signals to the display pixels PX (horizontal scanning period) becomes shorter. As the screen becomes larger, the wiring time constant of the signal line Scan also increases. By shortening the horizontal scanning period and increasing the wiring time constant of the signal line Scan, it becomes difficult to write a voltage corresponding to a desired gradation value to the display pixel PX, as compared with the conventional organic EL panel 11.

  Therefore, in the present embodiment, as shown in FIG. 3, in order to capture the video signal (data signal voltage) in a limited time, the time (period T27) for turning on the first switch element Tr1 is increased. ing. In the present embodiment, even when the waveform of the signal line Scan is rounded, the signal line Scan is completed to rise before the voltage of the data signal is input to the signal line Data, and the first switch element Tr1 becomes conductive. The state (ON state) is set. This is because the node N2 voltage fluctuation does not occur greatly in the period T27.

  As a result, even an organic EL panel 11 having a large screen and a large number of pixels that has a large load (wiring time constant) on the signal line Scan and takes a long time to start can be reliably written.

  Note that since the driving is performed in this manner, the wiring width of the signal line Scan can be further reduced. In that case, the display performance may be improved by using the thinned wiring width for enlarging the size (capacitance) of the capacitive element Cs.

  The display performance is such that when the capacitive element Cs is small, the parasitic capacitance between the drain and gate of the drive transistor Trd, the capacitive element Cs, and the capacitance of the organic EL element OEL are in series. The problem that the amount of electric charge written in is changed becomes significant. Therefore, for display performance, the ratio of parasitic capacitance to storage capacitance is important, and it is preferable that storage capacitance / parasitic capacitance >> 1.

  Thus, in the period T27 (writing period), the voltage according to the voltage of the data signal and the threshold voltage of the drive transistor Trd is stored (held) in the capacitor Cs.

(Period T28)
A period T28 from time t7 to time t8 shown in FIG. 17 is a period for surely turning off the first switch element Tr1.

  Specifically, in the period T28, as shown in FIG. 17, the first switch element Tr1 to the fourth switch element Tr4 are set in a non-conductive state.

  The scanning line driving circuit 13 maintains the voltage of the signal line Init, the signal line Ref, and the signal line Enable at the L level, thereby turning off the second switch element Tr2, the third switch element Tr3, and the fourth switch element Tr4. To maintain. Further, the scanning line driving circuit 13 changes the voltage of the signal line Scan from the H level to the L level at the start of the period T28, thereby changing the first switch element Tr1 from the on state to the off state.

  Accordingly, the first switch element Tr1 can be surely turned off (off state) before the fourth switch element Tr4 is turned on (on state) in the subsequent period T29 (light emission period).

  When the period T28 is not provided and the first switch element Tr1 and the fourth switch element Tr4 are simultaneously turned on (on state), the voltage at the node N2 increases due to the drain current of the drive transistor Trd. The voltage at the node N1 becomes the voltage of the data signal. As a result, the source-gate voltage of the drive transistor Trd becomes small. In this case, there is a problem that light is emitted with less luminance than desired luminance. In order to prevent this, in the present embodiment, the period T28 is provided to ensure that the first switch element Tr1 is non-conductive, and then the fourth switch element Tr4 is set to conductive in the subsequent period T29.

(Period T29: Light emission period)
A period T29 from time t8 to time t9 shown in FIG. 17 is a light emission period.

  Specifically, in the period T29, as shown in FIG. 17, the fourth switch element Tr4 is set in a conductive state, and the first switch element Tr1, the second switch element Tr2, and the third switch element Tr3 are set in a non-conductive state. The

  The scanning line driving circuit 13 maintains the voltage of the signal line Scan, the signal line Init, and the signal line Ref at the L level, thereby turning off the first switch element Tr1, the second switch element Tr2, and the third switch element Tr3. To maintain. Further, the scanning line driving circuit 13 causes the fourth switch element Tr4 to transition from the off state to the on state by transitioning the voltage of the signal line Enable from the L level to the H level at the start of the period T29.

  As described above, when the fourth switch element Tr4 becomes conductive (on state), the drive transistor Trd supplies a drive current corresponding to the voltage stored in the capacitive element Cs to the organic EL element OEL. Thereby, the organic EL element OEL can emit light.

(Period T30: Second period)
A period T30 from time t9 to time t0 illustrated in FIG. 17 is a period for setting all the switches to a non-conductive state and changing the voltages at the nodes N1 and N2 to a voltage close to a voltage necessary for the period T21.

  Specifically, in the period T30, as shown in FIG. 17, the first switch element Tr1 to the fourth switch element Tr4 are set in a non-conductive state. At the start of the period T30, the control unit 20 executes a second step of bringing the first switch element Tr1, the second switch element Tr2, the third switch element Tr3, and the fourth switch element Tr4 into a non-conductive state.

  The scanning line driving circuit 13 maintains the voltage of the signal line Scan, the signal line Init, and the signal line Ref at the L level, thereby turning off the first switch element Tr1, the second switch element Tr2, and the third switch element Tr3. To maintain. Furthermore, the scanning line driving circuit 13 causes the fourth switch element Tr4 to transition from the on state to the off state by transitioning the voltage of the signal line Enable from the H level to the L level at the start of the period T30.

  By providing the period T30 between the period T29 and the period T21, the voltage at the node N1 and the node N2 can be changed to a voltage close to a voltage necessary for the period T21 without charging and discharging of current with the power supply line.

  More specifically, the node N2 converges to the voltage of the power supply line VEL + the threshold voltage of the organic EL element OEL in the period T30. Further, in the period T30, the node N1 has a voltage stored in the voltage of the node N2 and the capacitor Cs.

  That is, by providing the period T30, the voltage at the node N1 and the node N2 at the start time (time t0) of the period T21 is set to the voltage at the time of light emission of the organic EL element OEL−threshold compared to the end time (time t9) of the period T29 It can be lowered by the voltage.

  Due to this voltage drop, the load of the charging / discharging operation due to the voltage of the power supply line VINI and the voltage of the power supply line VREF in the period T21 is reduced.

  With the above sequence, the display pixel PX performs gradation display.

  Note that the control unit 20 performs the same control method line-sequentially for the other display pixels PX constituting the organic EL panel 11.

[3. Effect]
In the organic EL display 10 of the present embodiment, the node N2 that is the connection point between the second electrode of the capacitive element Cs and the anode electrode of the organic EL element OEL is between the first initialization period and the threshold voltage compensation period. On the other hand, a second initialization period is provided in which charging and discharging are performed in a direction in which the period for threshold voltage compensation is shortened.

  Here, in the organic EL display 10 of the present embodiment, an extra second initialization period is provided, which means that the threshold voltage compensation period is increased by the length of the second initialization period. However, the shortening time of the threshold voltage compensation period by providing the second initialization period is longer than the length of the second initialization period. That is, since the conventional threshold voltage compensation period> the threshold voltage compensation period of the present embodiment + the second initialization period, the total time can be reduced.

  Furthermore, in the organic EL display 10 of the present embodiment, by providing the second initialization period, the period required for threshold voltage compensation (for example, Tth1 to Tth3 in FIG. 20) is shortened, so that the threshold voltage compensation period (for example, Even if 1H) becomes shorter, the threshold voltage compensation can be performed satisfactorily. In the organic EL display 10 according to the present embodiment, the threshold voltage compensation period is extended, for example, by setting the threshold voltage compensation period to 2H even when one horizontal scanning period is shortened due to enlargement or high definition. Thus, the threshold voltage compensation can be performed satisfactorily.

  In the organic EL display 10 of the present embodiment, in order to shorten the time required for threshold voltage compensation, the voltage of the source electrode of the drive transistor Trd is used for threshold voltage compensation by using a voltage drop due to the second switch element. Adjustment is made in such a direction that the period becomes shorter. That is, in the present embodiment, the threshold voltage compensation period is shortened and the threshold voltage compensation is performed without increasing the manufacturing cost with a simple configuration in which the second switch element is operated as a resistance unit and the second initialization period is provided. Accuracy can be improved.

(Other embodiments)
While the display device has been described based on the embodiment, the present invention is not limited to this embodiment. Unless it deviates from the gist of the present invention, various modifications conceived by those skilled in the art have been made in this embodiment, and forms constructed by combining components in different embodiments are also within the scope of one or more aspects. May be included.

  (1) For example, in the above embodiment, the function as the resistance unit is realized by reducing the W / L ratio of the second switch element Tr2 as compared with other switch elements. However, the present invention is not limited to this. . For example, the on-resistance of the second switch element Tr2 is increased by lowering the on-voltage of the signal line Init (the voltage of the gate electrode when the switch element is turned on) compared to the on-voltage of other signal lines. You may do it. At this time, the on-voltage of the signal line Init may be changed between the first initialization period and the second initialization period. Alternatively, the W / L ratio of the second switch element Tr2 may be adjusted in the manufacturing process so that the mobility of the second switch element Tr2 is lower than that of the other switch elements while keeping the same W / L ratio as the other switch elements. .

  (2) Moreover, in the said embodiment, although 2nd switch element Tr2 was used as a resistance part, it does not restrict to this. The resistance unit may include a resistance element separate from the second switch element Tr2. For example, the resistance unit may have a configuration in which a series circuit of a switch element and a resistance element and a second switch element Tr2 are connected in parallel.

  FIG. 21 is a circuit diagram illustrating another example of the resistance unit. In this case, since the second switch element Tr2 only needs to operate as a switch, the on-resistance may be the same as other switch elements. That is, the W / L ratio may be the same as other switch elements. Further, when the switch element of the series circuit is a transistor, the control unit 20 controls the voltage of the gate electrode of the switch element so that the switch element operates with a sufficiently low resistance to be regarded as a switch. It doesn't matter.

  22 and 23 are circuit diagrams showing other examples of the resistance section. In FIG. 22 and FIG. 23, a resistance element is directly connected to the second switch element Tr2.

  (3) Further, the resistance portion may be shared by the plurality of display pixels PX.

  FIG. 24 is a circuit diagram illustrating an example in which one display unit PX shares one resistance unit. In FIG. 24, one end of the second switch element Tr2 of the display pixel PX1 and one end of the second switch element Tr2 of the display pixel PX2 are connected to one end of the switch element TrB. In the modification shown in FIG. 24, the second switch element Tr2 of the display pixels PX1 and PX2 is operated as a switch, and the switch element TrB is operated as a resistance unit. Specifically, the second switch element Tr2 has the same on-resistance and the same W / L ratio as the first switch element Tr1, and the switch element TrB is more than the first switch element Tr1 and the second switch element Tr2. The W / L ratio is reduced so as to achieve a high on-resistance.

  Here, the through current flowing through the switch element TrB during the second initialization period is approximately twice that in the case where the resistance portion is provided for each sub-pixel. Therefore, the voltage of the source electrode of the drive transistor in the initialization period is VTFT− (VINI + Ron × 2 × Id), and the voltage fluctuation amount of the source electrode of the drive transistor at the start of the threshold voltage compensation period in the above embodiment. Is approximately doubled. In other words, the Ron resistance for realizing the target ∂Vgs / ∂Vth is about ½ that of the above embodiment, and the area of the resistance portion itself can be reduced to half. That is, when one display unit PX shares one resistance unit, the increase in layout size per subpixel can be suppressed to about ¼ compared to the above embodiment.

  As a result, by sharing the resistance portion among a plurality of pixels, not only the increase in the layout size per sub-pixel can be suppressed, but also the area of the resistance portion itself can be reduced, so a higher definition panel can be designed. Is possible.

  (4) In the above embodiment, the case where the voltage applied to the gate electrode of the second switch element Tr2 has the same value in the first initialization period and the second initialization period has been described as an example. This is not a limitation.

  FIG. 25 is a circuit diagram showing another example of the signal waveform of the organic EL display 10. In the example shown in FIG. 25, in the first initialization period, a voltage having the same level as that of the gate electrode of another switch element is applied to the gate electrode in order to operate the switch as low resistance as possible. Thereby, the on-resistance of the second switch element Tr2 is larger than that in the second initialization period. On the other hand, in order to operate as a resistor in the second initialization period, a voltage smaller than that in the first initialization period is applied to the gate electrode of the second switch element Tr2. The on-resistance of the second switch element Tr2 is smaller than that in the first initialization period.

  The present invention can be used for a display device such as an organic EL display.

10, 100 Organic EL display 11, 110 Organic EL panel 12, 120 Data line drive circuit 13, 130 Scan line drive circuit 20, 200 Control unit Cs Capacitance element N1, N2 Node OEL Organic EL elements P0, PX, PX1, PX2 display Pixel Tr1 First switch element Tr2, Tr20 Second switch element Tr3 Third switch element Tr4 Fourth switch element Trd Drive transistor VTFT, VINI, VREF, VEL Power line

Claims (13)

A display device comprising display pixels,
The display pixel is
A light emitting element;
A capacitive element for holding the voltage;
A drive transistor in which a gate electrode is connected to the first electrode of the capacitive element, and a source electrode is connected to the second electrode of the capacitive element and the anode of the light emitting element;
A display pixel having a signal line for supplying a voltage corresponding to a data signal and a first switch element that switches between conduction and non-conduction with the first electrode of the capacitive element;
A control unit for controlling the driving of the display pixels,
The controller is
A reference voltage is applied to the first electrode of the capacitive element at a start of the first initialization period in a state where application of a drive voltage for driving the light emitting element to the drain electrode of the drive transistor is stopped. A first initialization step of applying an initialization voltage to the second electrode;
At the start of the second initialization period set after execution of the first initialization step, the application of the reference voltage to the first electrode is maintained, and the application of the initialization voltage to the second electrode is maintained. A second initialization step of starting application of the drive voltage to the drain electrode of the drive transistor in the state of
Maintaining the application of the reference voltage to the first electrode at the start of a threshold voltage compensation period for compensating the threshold voltage of the drive transistor set after the execution of the second initialization step, and draining the drive transistor A threshold voltage compensation step of stopping application of the initialization voltage to the second electrode of the capacitive element while maintaining application of the drive voltage to the electrode;
Display device. The display pixel further includes:
A second switch element that switches between conduction and non-conduction between the power supply line that supplies the initialization voltage and the second electrode of the capacitive element;
A third switch element that switches between conduction and non-conduction between a connection point of the first electrode of the capacitive element and the gate electrode of the drive transistor and a power supply line that supplies the reference voltage;
A fourth switch element that switches between conduction and non-conduction between a drain electrode of the drive transistor and a power supply line that supplies the drive voltage;
The controller is
In the first initialization step, the first switch element and the fourth switch element are turned off, the second switch element and the third switch element are turned on,
In the second initialization step, the first switch element is turned off, the second switch element, the third switch element and the fourth switch element are turned on,
In the threshold voltage compensation step, the first switch element and the second switch element are made non-conductive, and the third switch element and the fourth switch element are made conductive.
The display device according to claim 1. The second switch element is a transistor;
The control unit operates the second switch element as a resistance unit in the second initialization step.
The display device according to claim 2. The second switch element is a transistor;
The on-resistance of the second switch element is higher than the on-resistance of the other switch elements;
The display device according to claim 3. The second switch element is a transistor;
The W / L ratio of the second switch element is smaller than the W / L ratio of the other switch elements;
The display device according to claim 3. The second switch element is a transistor;
The control unit controls the voltage applied to the gate electrode of the second switch element to be lower than the voltage applied to the gate electrode of another switch element in the second initialization step.
The display device according to claim 3. The second switch element is a transistor;
In the second initialization step, the control unit is configured such that a voltage applied to the gate electrode of the second switch element is greater than a voltage applied to the gate electrode of the second switch element in the first initialization step. Control to be lower,
The display device according to claim 3. The first initialization period is longer than the second initialization period,
The display device according to claim 1. The control unit further includes:
Performing a first step of bringing the second switch into a conducting state at the start of a first period set before the first initialization period;
The display device according to claim 2. The power supply line that supplies the initialization voltage is disposed so as to intersect the power supply line that supplies the drive voltage and the power supply line that supplies the reference voltage.
The display device according to claim 9. The control unit further includes:
The first switch element, the second switch element, the third switch element, and the fourth switch element at the start of the second period set before the first period and after the light emission period for causing the light emitting element to emit light Performing a second step of bringing the device into a non-conductive state,
The display device according to claim 9 or 10. The control unit further includes:
Performing a third step of bringing the fourth switch element into a non-conductive state at the start of a third period set after the threshold voltage compensation period;
At the start of the writing period set after the end of the third period, the first switch element is turned on, and the second switch element, the third switch element, and the fourth switch element are turned off. To perform a writing step of writing a voltage to the storage capacitor,
The display device according to claim 2. The control unit further includes:
After the third period, at the start of the fourth period set before the writing period, execute a fourth step of bringing the third switch element into a non-conductive state.
The display device according to claim 12.

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