RU2494472C1 - Pixel cell driver for oled display - Google Patents

Abstract

FIELD: information technology.

SUBSTANCE: device has a current source on a first MOS transistor connected to a first supply voltage line and an organic light-emitting diode (OLED), a capacitor which stores the current signal value, a signal voltage to capacitor voltage converter when programming the pixel cell, consisting of a second and a third MOS transistor, fourth and fifth switch MOS transistors. The second MOS transistor is connected to the signal voltage line and the second supply voltage line. The fourth and fifth switch MOS transistors are connected to a line for enabling the recording of the signal voltage to the pixel cell during programming thereof.

EFFECT: high operating speed, contrast, uniformity and stability of brightness of a microdisplay.

2 dwg

Description

The invention relates to microelectronic devices for image visualization, namely to microdisplays, and more particularly to ultra-large integrated circuits for controlling displays with active OLED arrays (light emitting diode arrays based on organic semiconductors) formed on a common silicon substrate with pixel cell driver circuits.

The known pixel cell driver for OLED active matrix displays (G. Levy et. Al. An 852 × 600 Pixel OLED-on-Silicon Color Microdisplay Using CMOS Subthreshol-Voltage-Scaling Current Drivers. IEEE J. Of Solid-State Circuits, vol. 37, # 12, Dec. 2002, pp. 1879-1889), containing a current source on a MOS transistor that sets the operating current to a light emitting diode based on organic semiconductors, a MOS transistor switch for sampling from a matrix of a given pixel cell and for programming a pixel cell, MOS a transistor key for entering information about the current brightness level in the form of a voltage on a storage capacitor, which connected to the gate of the MOS transistor of the current source. (In addition, the key MOS transistor for switching off a light-emitting diode based on organic semiconductors in programming mode and the key MOS transistor in diode switching on to limit the voltage on the electric circuit when disconnecting a light-emitting diode based on organic semiconductors are included in this circuit). high speed when programming a pixel cell, the current from the information bus to the storage capacitor and to the driver MOS transistor to the diode m inclusion significantly increased (e.g., 100 times). In working condition, the current through the driver MOS transistor, respectively, is reduced by the same amount by supplying a bias locking voltage through a storage capacitor to the gate of the MOS transistor of the current source. For this purpose, an additional current scaling bus is introduced into the cell.

The main reasons for the need to introduce scaling are to increase the speed of the driver and reduce the influence of technological variation of the parameters of MOS transistors on the variation in brightness of the pixel cells of the display. When scaling the current, the exponential dependence of the drain current of the MOS transistor in the pre-threshold mode on the voltage at its gate is used, which allows the mathematical operation of multiplying by the scaling factor to be replaced by the addition operation of the programming voltage source and the bias voltage on the additional current scaling bus on the Mop gate of the transistor. The scaling factor is constant for the entire matrix and does not change in the operating mode. (G. Levy et. Al. An 852 × 600 Pixel OLED-on-Silicon Color Microdisplay Using CMOS Subthreshol-Voltage-Scaling Current Drivers. IEEE J. Of Solid-State Circuits, vol. 37, # 12, Dec. 2002, pp. 1879-1889).

Such a driver has the following disadvantages:

- insufficient dynamic range of working signals (in the microdisplay this limits its contrast, for example, of the order of 100: 1) due to the relatively small range of working currents of the current source on the MOS transistor in the pre-threshold mode;

- high noise level (relative to the nominal signal level), large temperature and temporal instability of the current characteristics of the current source on the MOS transistor and the brightness spread across the display screen (in the microdisplay this is manifested in uneven brightness across the screen and flickering of the image, as well as in requiring adjustment changing brightness during operation and temperature changes);

- the presence of additional elements (in the cell, an additional bus, along which the scaling bias voltage is transmitted, outside the matrix, the scaling control circuit).

A known pixel cell driver for an OLED display (US patent No. 2007/0273622 A1, publ. 11.29.07) is closest to the proposed one and contains a current source for a light-emitting diode based on organic semiconductors on a first MOS transistor (T4 in Figure 2) connected to the first bus of the power supply voltage (VDD in Figure 2), and to a light emitting diode based on organic semiconductors and connected by a gate to the first capacitor plate (C in Figure 2), which stores the current value of the signal, the second plate of which is connected to ervoy bus supply voltage. The second MOS transistor (T3 in Figure 2), which converts the signal input current to the voltage on the capacitor C when programming the pixel cell, is connected by the source to the power supply source, and the gate is connected to the first capacitor plate C, the second plate of which is connected to the power supply source. In addition, the driver includes a first MOSFET transistor (T1 in Figure 2) connecting the programmable signal current bus (Data) to the T3 drain, and containing a second MOSFET transistor (T2 in Figure 2) connecting the drain of the second MOS transistor (T3 in the drawing 2) with the first lining of the capacitor C, the gates of the first and second key MOS transistors are connected to the sampling bus of the pixel cell (Scan) during its programming. Unlike the electrical circuit of the pixel cell driver of the analogue above, to achieve the required high programming speed when programming, current scaling is possible only by increasing the size of the second MOS transistor, then the scaling factor will be equal to the ratio W / L of the second to W / L of the first MOS transistors, W is the width, L is the length of the channel. To obtain a sufficiently large scaling factor (for example, 100: 1), the dimensions of the second MOS transistor become so large that it may not fit in the dimensions of a pixel cell, and with a small scaling factor for full-format microdisplays it is impossible to achieve sufficient speed.

Such a driver has the following disadvantages:

- for full-format microdisplays, unattainable high speed when programming a pixel cell (due to the need to achieve a sufficient scaling factor, for example equal to 100);

- the introduction of current scaling leads to a corresponding increase in energy consumption of circuits programming pixel microdisplay cells;

- due to the significantly different sizes of the first and second MOS transistors, the error in converting the input (programming) current to the operating current increases significantly.

The present invention solves the problem of achieving sufficient performance when programming pixel cells without the need to scale the current when programming a pixel cell and reduce the error of conversion and power consumption of schemes programming pixel cells.

To achieve this technical result, the pixel cell driver of the OLED display contains, for a light emitting diode based on organic semiconductors, a current source on the first MOS transistor connected to the first bus of the power supply voltage and to a light emitting diode based on organic semiconductors and connected to the first capacitor plate by a gate, which stores the current value of the signal, the second lining of which is connected to the first bus of the power supply, an additional converter the voltage of the signal to the voltage across the capacitor when programming the pixel cell (recording information in it), which consists of the second and third MOS transistors and which is connected through the key fourth and fifth MOS transistors to the first capacitor plate, the second MOS transistor is connected by a gate to the signal voltage bus and is connected to the second bus of the power supply voltage source, and through the key fourth MOS transistor to the third MOS transistor, which is connected to the first bus of the power voltage source, and its gate p is connected to the first plate of the capacitor, the key fifth MOS transistor connects the first plate of the capacitor to the third MOS transistor, the gates of the key fourth and fifth MOS transistors are connected to the bus write enable signal voltage to the pixel cell when it is programmed.

The features distinguishing the proposed pixel cell driver of the OLED display from the closest known US patent No. 2007/0273622 A1 (prototype) characterize the presence of a signal to voltage converter on a capacitor when programming a pixel cell (recording information into it), which consists of the second and third MOS transistors and which is connected through the key fourth and fifth MOS transistors to the first capacitor plate, the second MOS transistor is connected by a gate to the signal voltage bus and connected to to the other side of the power supply bus, and through the fourth key MOSFET to the third MOS transistor, which is connected to the first bus of the power supply, and its gate is connected to the first capacitor plate, the fifth fifth MOSFET transistor connects the first capacitor plate to the third MOS transistor, the gates of the key the fourth and fifth MOS transistors are connected to the bus for recording the voltage of the signal in the pixel cell when it is programmed.

Figure 1 shows a circuit diagram of an OLED display pixel cell driver driver, comprising: D is a light emitting diode based on organic semiconductors, MOSF1 is a MOS current source transistor for a light emitting diode based on organic semiconductors, K is a capacitor storing the current signal value, MOSFET 2 and MOPT3 - MOS transistors of the voltage-to-voltage converter of a capacitor when programming a pixel cell; MOPT4 and MOSPT5 are key MOS transistors providing information recording in capacitor K when programming, E11 and E12 - the first bus of the power supply voltage of the MOS transistor MOSFET1 and the signal voltage converter, E21 - the second bus of the power supply voltage of the signal voltage converter, E22 - the second bus of the power supply voltage D, Ш1 - the signal voltage bus, Ш2 - a bus for permitting the recording of signal voltage into a pixel cell during its programming. When using MOSFETs of the same type of channel conductivity (p- or n-type), the sources of MOSFET 1 and MOSFET3 are connected to the first bus of the power supply source, the drain of MOSFET2 to the second bus of the power supply source. In this case, the signal voltage converter on the MOSFET MOSFET 2 and MOSFET 3 operates as a source voltage follower. If the types of channel conductivity of the MOSFET MOSFET 2 and MOSFET 3 are different, then in this case the signal voltage converter on the MOSFET MOSFET 2 and MOSFET 3 transistors works as a signal voltage amplifier.

Figure 2 (prototype) shows a circuit diagram of a driver of a pixel cell OLED display prototype containing: T1 - the first key MOS transistor, T2 - the second key MOS transistor, T3 - the second MOS transistor, T4 - the first MOS transistor, C - the capacitor, VDD - power supply source. Data - programmable current bus, Scan - pixel cell sampling bus.

The pixel cell driver of an OLED display operates as follows. In the programming mode of the cell (recording information), a control pulse is supplied to the Ш2 bus, which opens the MOSFET 4 and MOPT5 key MOS transistors, the signal voltage supplied to the Ш1 bus through the signal voltage converter on the MOSFET 2 and MOSFET 3 transistors is supplied to the capacitor K. After the programming mode MOSFET transistors MOPT4 and MOPT5 disconnect the capacitor K from the signal voltage converter on the MOSFET transistors MOPT2 and MOPT3 and a charge corresponding to the signal voltage is stored on it throughout Static preparation mode. Since the signal voltage (from a low-resistance signal voltage source) is not supplied to the pixel cell, as in the prototype, it is not necessary to scale the current (to introduce a multiply increased current into the pixel cell) to achieve the required speed. In this case, the entire range of operating currents of the MOS transistor MOSFET1 can be used, and not only pre-threshold drain currents, which can significantly expand the dynamic range and contrast ratio. Accordingly, the influence of external and intrinsic noise is reduced, as well as the influence of the spread of threshold voltages of the MOS transistors MOSFET 1 and MOSFET 3 on the brightness spread across the display field.

The proposed electrical circuit of the light emitting cell provides:

- high speed without the need for scaling current when programming a pixel cell (respectively, without the need to increase the power consumption of circuits programming pixel cells, and ensure scaling, for example, by increasing the size of the corresponding MOS transistors);

- high contrast;

- uniformity and stability of the brightness of the microdisplay.

Claims (1)

An OLED display pixel cell driver comprising a current source for a light emitting diode based on organic semiconductors on a first MOS transistor connected to a first bus of a power supply voltage source and a light emitting diode based on organic semiconductors and connected to a first capacitor plate storing the current signal value, the second lining of which is connected to the first bus of the power supply source, characterized in that a signal voltage converter is additionally introduced into it When the pixel cell is programmed (writing information to it), which consists of the second and third MOS transistors and which is connected through the key fourth and fifth MOS transistors to the first capacitor plate, the second MOS transistor is connected by a gate to the voltage bus signal and is connected to the second bus of the voltage source, and through the fourth key MOSFET to the third MOS transistor, which is connected to the first bus of the voltage source, and its gate is connected to the first the capacitor plate, the fifth key MOSFET connects the first capacitor plate to the third MOSFET, the gates of the key fourth and fifth MOSFETs are connected to the bus for recording the voltage of the signal in the pixel cell when programming it.

Images