JPH11202299A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal display device in which no afterimage is left. SOLUTION: Based on a reference voltage outputted by a reference voltage generating circuit, a source driver IC 2 supplies a gradation signal corresponding to the gradation to be displayed by a display part to a switching element, but the reference voltage generating circuit is connected between first and second potential supply sources and has plural ladder resistors 7, 8 and 9 constituted so as to output the reference voltages from the respective nodes of plural serially connected resistors 4. In this case, one of plural ladder resistors 7, 8 and 9 is selected by switches 5 and 6. Thus, the reference voltage is varied and the contrast of the display part is changed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a liquid crystal display device, and more particularly to a gradation voltage setting circuit.

[0002]

2. Description of the Related Art Liquid crystal display devices have been used both as monitors for personal computers and as television receivers.
mode-Ray Tube). In a liquid crystal display device for digitally inputting RGB signals, a source driver IC includes:
In accordance with the RGB signals, a voltage corresponding to the gradation is output to the pixel. The voltage for each gradation is generated by a ladder resistance circuit. When the image signal input is an 18-bit RGB signal, the image signal has 64 gradations, and 128 voltage levels are generated inside the source driver IC including the case of positive polarity and the case of negative polarity. . A plurality of voltage levels are generated by a ladder resistance circuit outside the source driver IC,
The voltage is input to the source driver IC, and based on the input, 128 voltage levels are generated by the source driver IC. This voltage level is supplied to the switching element through a signal line as a gradation signal. The switching elements are arranged at intersections of scanning lines that intersect signal lines, and scanning signals are supplied to the scanning lines from a gate driver IC (not shown).

FIG. 7 is a diagram showing a ladder resistance circuit of a conventional liquid crystal display device. In FIG. 7, 1 is a DC power supply, 2 is a source driver IC, 3 is a bus line, 4 is a resistor constituting a ladder resistor circuit, and outputs V0 to V9.
Are supplied to the source driver IC 2 via the bus line 3.

FIG. 8 is a diagram showing an example of a luminance-voltage characteristic of a liquid crystal cell. In FIG. 8, the luminance is standardized, and the luminance of 1 corresponds to the luminance when the voltage of the gradation signal is the maximum. The voltage is also standardized. FIG.
Is a ladder resistance circuit in the case of dot inversion drive in which the polarity is opposite to that of the upper, lower, left and right pixels adjacent to each other on the liquid crystal display device. In this ladder resistance circuit, a total of ten voltages of, for example, 1, 8, 16, 32, 48, and 64 gradations of positive polarity and negative polarity are generated and input to the source driver IC2, and the voltages between the voltages are generated. The voltage adjustment level is generated by a ladder resistor or the like inside the source driver IC. The gray scale voltage generated outside the source driver IC is called a reference voltage. These reference voltages are generated by a ladder resistance circuit in the liquid crystal display device.

FIG. 9 is a diagram showing a typical example of gradation-luminance characteristics of a liquid crystal display device having 64 gradations. In FIG. 9, for example, the transmittance of the 0th gradation, that is, the luminance
Therefore, a voltage having an amplitude of 1.0 at the normalized voltage in FIG. 9 is output to V0 and V9 in FIG. 7, and the transmittance is about 0. 8, the voltage value corresponding to the transmittance is read from FIG. 8, and the voltages from V2 to V7 are determined in the same manner so that the voltage having this voltage amplitude is output to V1 and V9. The value of the resistor 4 is determined so that the obtained voltage is output. In this way, a gradation-luminance characteristic as shown in FIG. 9 can be obtained from a liquid crystal cell having the luminance-voltage characteristic as shown in FIG.

[0006]

In a conventional liquid crystal display device, when a still image is displayed for a long time and then halftones are displayed on the entire display, there is a phenomenon that a still image remains as an afterimage. A similar phenomenon occurs in a CRT, but this phenomenon occurs in a liquid crystal display device in a shorter time than in a CRT. In the case of a CRT, an electron beam burns the aperture grill, so that an afterimage remains irreversibly. However, in the case of a liquid crystal display device, the afterimage disappears when left for a while.

It is considered that the afterimage in the liquid crystal display device is caused by charged particles accumulating at the film interface on the opposing substrate surface or the film interface on the TFT array substrate side due to a DC voltage component generated in the liquid crystal cell. This phenomenon is remarkable in a liquid crystal display device having a high contrast. In addition, staring at a screen having high contrast for a long time may cause eyestrain. The present invention has been made to solve such a problem, and an object of the present invention is to obtain a liquid crystal display device having no afterimage.

[0008]

A liquid crystal display device according to the present invention has switching elements arranged in a matrix at intersections of a plurality of scanning lines and a plurality of signal lines, and has a predetermined luminance-gradation level. A display unit that performs display according to characteristics, a first drive circuit that is connected to a scan line and supplies a first signal to a switching element, and is connected between a first potential supply source and a second potential supply source. A reference voltage generating circuit that outputs a reference voltage, and a second signal that is connected to the signal line and that corresponds to a gray scale displayed by the display unit on the switching element based on the reference voltage output from the reference voltage generating circuit. A second drive circuit for supplying the reference voltage is provided, and the reference voltage generation circuit varies the reference voltage so that the luminance-gradation characteristics of the display unit change.

The reference voltage generation circuit has a plurality of ladder resistors configured to output a reference voltage from each connection point of the plurality of resistors connected in series, and one of the ladder resistors is selected from the plurality of ladder resistors. It is configured to be selected.
Further, the reference voltage generation circuit has a ladder resistor configured to output a reference voltage from each connection point of the plurality of resistors connected in series, and at least one end of the ladder resistor is connected in parallel. And a plurality of resistors having different resistance values, and one of the plurality of resistors is selected. Further, the reference voltage generation circuit has a ladder resistor configured to output a reference voltage from each connection point of a plurality of resistors connected in series, and a resistor at a central portion of the ladder resistor is connected in parallel. And a plurality of resistors having different resistance values, and one of the plurality of resistors is selected.

The reference voltage generating circuit is inserted between the ladder resistor and a first potential supply source or a second potential supply source to which the ladder resistor is connected, and receives a digital signal to input the ladder resistance. It has a digital-to-analog converter configured so that the potential applied to the resistor is variable. The reference voltage generation circuit has a ladder resistor configured to output a reference voltage from each connection point of a plurality of resistors connected in series, and a part of the ladder resistor receives a digital signal. Is replaced by a digital-to-analog converter configured so that the potential output by the ladder resistor becomes variable. The change in the luminance-gradation characteristic is performed by changing the maximum luminance or the minimum luminance.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 A liquid crystal display device generally includes a matrix of pixels each having a thin film transistor having a gate electrode connected to a scanning line, a source electrode connected to a signal line, and a drain electrode connected to a pixel electrode at intersections of a plurality of scanning lines and a plurality of signal lines. A gate driver I, which is a first drive circuit, is provided in a scanning line.
By sending a scanning signal from C and a pixel signal from a source driver IC as a second driving circuit to a signal line, and applying an electric field to the liquid crystal sandwiched between the two substrates, the The display is performed by changing the light transmittance. At this time, the source driver IC forms an image signal based on a reference voltage having a plurality of voltage levels generated by the reference voltage generation circuit. The present invention relates to this reference voltage generating circuit, wherein the voltage level of a reference voltage output from the circuit is made variable. FIG.
FIG. 2 is a diagram showing a reference voltage generation circuit of the liquid crystal display device according to the first embodiment of the present invention, which is a reference voltage generation circuit when performing dot inversion driving. In the figure, 1 is a DC power supply, and 2 is a source driver I which is a second drive circuit.
C, 3 are bus lines for transmitting the reference voltage output from the reference voltage generation circuit to the source driver IC 2; A reference voltage is output from the point.
5 is a ladder resistance switch, 6 is a reference voltage switch, 7 is the first ladder resistor, and 8 is the second ladder resistor.
The third ladder resistor 9 is a third ladder resistor, the resistance values of which are set so as to have different luminance-gradation characteristics.

FIG. 2 is a diagram showing luminance-gradation characteristics of the liquid crystal display device according to the first embodiment of the present invention.
In FIG. 2, the axis of the normalized luminance is the same scale as the normalized luminance shown in FIG. A curve 10 is a typical luminance-gradation characteristic. In FIG. 8, the voltage region where the luminance changes in FIG. A curve 11 is a luminance-gradation characteristic when the contrast is lowered by using the voltage region on the low luminance side, and the contrast is half that of the typical luminance-gradation characteristic. A curve 12 is a gradation-luminance characteristic when the contrast is lowered by using the voltage region on the high luminance side, and the contrast value is 10.

Next, the operation will be described. In FIG. 1, three types of ladder resistance circuits can be switched by simultaneously switching the ladder resistance switching switch 5 and the reference voltage switching switch 6, and the three ladder resistance circuits shown in FIG.
Two luminance-gradation characteristics can be selected. The curve 10 in FIG. 2 is obtained by the ladder resistance 7 in FIG. 1, and the curve 11 in FIG. 2 and the ladder resistance 9 in FIG.
Gives the curve 12 in FIG. Ladder resistance 7 in FIG.
R7-1, R7-2 ... R7 from the high potential side
-9, and the resistance value of the ladder resistor 9 in FIG. 1 is changed from the high potential side to R8-1, R8-2... R8-9, and similarly, the resistance value of the ladder resistor 9 in FIG. R9-1, R9-
1 ... R9-9, and assuming that the sum of the resistance values of the ladder resistors 7, 8, and 9 is constant, R8-6> R7-6, R9-1> R7-1, and R9 -9> R7-9
Is the minimum requirement. As described above, by preparing a plurality of ladder resistors capable of reducing the contrast, it is possible to select a gradation-luminance characteristic that is easy for the user to see even when the contrast is reduced, as compared with a typical gradation-luminance characteristic. It can be so.

Embodiment 2 FIG. FIG. 3 is a diagram showing a reference voltage generating circuit of a liquid crystal display device according to a second embodiment of the present invention, which is a reference voltage generating circuit when performing dot inversion driving. In the figure, 1 to 4 are the same as those in FIG. Reference numerals 5a and 5b denote resistance switching switches, and reference numerals 4a to 4f denote resistors. Embodiment 2
In the above, the resistors at both ends of the ladder resistor are respectively constituted by a plurality of resistors 4a to 4c and 4d to 4f so that they can be switched.

Next, the operation will be described with reference to FIGS. The contrast can be changed even by using the reference voltage generating circuit as shown in FIG. When the resistance 4a is selected by the resistance changeover switch 5a and the resistance 4d is selected by the resistance changeover switch 5b, the luminance-gradation characteristic of the curve 10 in FIG. 2 is obtained. Here, the resistance values are expressed as resistance 4a <resistance 4b and resistance 4d <resistance 4e.
If the resistances 4b and 4e are selected by the resistance switching switches 5a and 5b, respectively, the maximum amplitude of the voltage applied to the liquid crystal decreases, and the luminance of the 0 gradation level increases. 8, a liquid crystal having a luminance-voltage characteristic as shown in FIG.
Gradation characteristics can be obtained. In addition, the resistance value is expressed as resistance 4b <resistance 4
c, the resistance 4d <the resistance 4f, and the contrast can be further reduced by selecting the resistances 4c and 4f with the resistance switching switches 5a and 5b, respectively. As described above, the contrast can be reduced even in a circuit as shown in FIG. 4 which is simpler than the circuit in FIG.

Embodiment 3 FIG. 4 is a diagram showing a reference voltage generation circuit of a liquid crystal display device according to Embodiment 3 of the present invention. In the figure, 1 to 4 are the same as those in FIG. 14 and 15 are DA converters 1 and 2 provided at both ends of the ladder resistance circuit, respectively, and 16 is a switch for generating a digital signal to be input to the DA converters 1 and 2.

Next, the operation of the reference voltage generating circuit shown in FIG. 4 will be described with reference to FIGS. The DA converters 1 and 2 output a voltage proportional to the value of the digital signal to the ladder resistor side. The same digital value is input to the DA converter 1 and the DA converter 2. As shown in FIG. 4, the output of the DA converter 1 has a high potential side connected to the positive side of a constant voltage power supply and a low potential side connected to a ladder resistance circuit. As for the output of the DA converter 2, the high potential side is connected to the ladder resistance circuit, and the low potential side is connected to the negative side of the constant voltage power supply.

0 for DA converter 1, DA converter 2
When a digital value of 0 is input to the LCD, a curve 10 in FIG. When a value larger than 0 is input to the DA converter 1 and the DA converter 2, the voltage applied to the ladder resistance circuit decreases as a whole because the potential on the high potential side becomes low and the low potential side is raised. Moreover, the amplitude center voltage of the source signal remains constant. Therefore, the maximum amplitude of the voltage applied to the liquid crystal decreases, and FIG.
In the case of a liquid crystal having a luminance-voltage characteristic as shown in FIG. 2, as a result, a luminance-gradation characteristic with reduced contrast as shown by a curve 12 in FIG. 2 is obtained.

Embodiment 4 FIG. 5 is a diagram showing a reference voltage generation circuit of a liquid crystal display device according to a fourth embodiment of the present invention, which is a reference voltage generation circuit when performing dot inversion driving. In the figure, 1 to 4 are the same as those in FIG. 5c is a resistance switching switch, and 4g to 4j are resistors. In the fifth embodiment,
The resistance between the positive polarity part and the negative polarity part of the reference voltage, which is the center of the ladder resistance, is constituted by a plurality of resistances 4g to 4j, respectively, so that they can be switched.

Next, the operation will be described with reference to FIGS. The contrast can be changed even by using the reference voltage generating circuit as shown in FIG. When the resistor 4g is selected by the switch 5c, the luminance-gradation characteristic of the curve 10 in FIG. 2 is obtained. Here, the resistance value is 4
If g <4h, if the resistor 4h is selected by the switch 5c, the maximum amplitude of the voltage applied to the liquid crystal decreases, and in the case of the liquid crystal having the luminance-voltage characteristic as shown in FIG.
Since the luminance at the high gradation level decreases, a luminance-gradation characteristic as shown by a curve 11 in FIG. 2 is obtained. Also,
The resistance value is set to be 4h <4j, and the switch 5c
When the resistor 4j is selected, the contrast can be further reduced. As described above, the contrast can be reduced even in a circuit as shown in FIG. 5 which is simpler than the circuit in FIG.

Embodiment 5 FIG. 6 is a diagram showing a reference voltage generation circuit of a liquid crystal display device according to a fifth embodiment of the present invention, which is a reference voltage generation circuit when performing dot inversion driving. In the figure, 1 to 4 and 16 are the same as those in FIG. Reference numeral 17 denotes a DA converter provided at the center of the ladder resistance circuit.

Next, the operation of the reference voltage generating circuit shown in FIG. 6 will be described with reference to FIGS. DA converter 1
7 outputs a voltage proportional to the value of the digital signal. D
As shown in FIG. 6, the A converter 17 is inserted between the positive polarity part and the negative polarity part of the reference voltage, and the high potential side of the output is connected to the positive polarity side and the low potential side is connected to the negative polarity ladder resistance circuit. Connecting. When the digital value of 1 is input from the switch 16 to the DA converter 17, the curve 10 in FIG. DA converter 1
When a value larger than 1 is input to 7, the maximum amplitude of the voltage applied to the liquid crystal decreases, and in the case of the liquid crystal having the luminance-voltage characteristic as shown in FIG. 8, the luminance of the high gradation level decreases. Therefore, a luminance-gradation characteristic as shown by a curve 11 in FIG. 2 is obtained. When a larger digital value is input to the DA converter 17, the contrast can be further reduced. As described above, the contrast can be reduced even in the circuit as shown in FIG. 6 which is simpler than the circuit in FIG.

[0023]

Since the present invention is configured as described above, it has the following effects. A display portion that has switching elements arranged in a matrix at intersections of the plurality of scanning lines and the plurality of signal lines, and performs display according to predetermined luminance-gradation characteristics; A first drive circuit for supplying one signal, connected between a first potential supply source and a second potential supply source,
A reference voltage generation circuit that outputs a reference voltage; and a second signal that is connected to the signal line and that supplies a second signal corresponding to a gray scale displayed by the display unit to the switching element based on the reference voltage output from the reference voltage generation circuit. A second driving circuit, wherein the reference voltage generation circuit changes the reference voltage so that the luminance-gradation characteristic of the display unit changes, so that the contrast of the display can be switched, and the contrast is reduced to a low level. No indication can be made.

Further, the reference voltage generation circuit has a plurality of ladder resistors configured to output a reference voltage from each connection point of the plurality of resistors connected in series, and one of the ladder resistors is selected from the plurality of ladder resistors. Since the display contrast is selected, a plurality of display contrasts can be switched. Further, the reference voltage generation circuit has a ladder resistor configured to output a reference voltage from each connection point of the plurality of resistors connected in series, and at least one end of the ladder resistor is connected in parallel. Since a plurality of resistors having different resistance values are used and one of the plurality of resistors is selected, a plurality of display contrasts can be switched with a simple circuit.

The reference voltage generating circuit is inserted between the ladder resistor and the first potential supply source or the second potential supply source to which the ladder resistor is connected, and receives a digital signal to input the ladder resistance. Since the digital-to-analog converter is configured so that the potential applied to the resistor is variable, the display contrast can be changed by changing the digital signal. Further, since the change of the luminance-gradation characteristic is performed by changing the maximum luminance or the minimum luminance, the display contrast can be switched. Further, since the change of the luminance-gradation characteristic is performed by changing the maximum luminance or the minimum luminance, the display contrast can be switched around the intermediate luminance.

[Brief description of the drawings]

FIG. 1 is a diagram showing a reference voltage generation circuit of a liquid crystal display device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a luminance-gradation characteristic of the liquid crystal display device according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a reference voltage generation circuit of a liquid crystal display device according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a reference voltage generation circuit of a liquid crystal display device according to a third embodiment of the present invention.

FIG. 5 is a diagram showing a reference voltage generation circuit of a liquid crystal display device according to a fourth embodiment of the present invention.

FIG. 6 is a diagram showing a reference voltage generation circuit of a liquid crystal display device according to a fifth embodiment of the present invention.

FIG. 7 is a diagram showing a conventional reference voltage generation circuit.

FIG. 8 is a diagram illustrating an example of a luminance-voltage characteristic of a liquid crystal cell.

FIG. 9 is a diagram showing typical luminance-gradation characteristics of a conventional liquid crystal display.

[Explanation of symbols]

1 DC power supply, 2 source driver IC, 3 bus line, 4 resistor, 4a, 4b, 4d, 4e, 4f
Resistance, 7 first ladder resistance, 8 second ladder resistance, 9 third ladder resistance, 10 typical luminance-gradation curve of liquid crystal display, 11 contrast using low-luminance voltage region Dropped luminance-gradation curve, 12 contrast-reduced luminance-gradation curve using high-luminance voltage region, 13 luminance-gradation curve using half-tone voltage region, 14, 15 DA converter.

Claims (7)

[Claims] 1. A display unit having switching elements arranged in a matrix at intersections of a plurality of scanning lines and a plurality of signal lines, and performing a display according to predetermined luminance-gradation characteristics, connected to the scanning lines. A first drive circuit that supplies a first signal to the switching element, a reference voltage generation circuit that is connected between a first potential supply source and a second potential supply source, and outputs a reference voltage, A second drive circuit connected to a line, and supplying a second signal corresponding to a gray scale displayed by the display unit to the switching element based on a reference voltage output from the reference voltage generation circuit, The reference voltage generation circuit
A liquid crystal display device, wherein a reference voltage is varied so that a luminance-gradation characteristic of the display unit changes. 2. The reference voltage generating circuit includes a plurality of ladder resistors configured to output a reference voltage from each connection point of a plurality of resistors connected in series, and one of the ladder resistors is selected from the plurality of ladder resistors. 2. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is configured to select. 3. A reference voltage generating circuit having a ladder resistor configured to output a reference voltage from each connection point of a plurality of resistors connected in series, wherein at least one end of the ladder resistor is connected in parallel. 2. A liquid crystal display device according to claim 1, comprising a plurality of resistors having different resistance values connected to each other, wherein one of the plurality of resistors is selected. 4. A reference voltage generating circuit having a ladder resistor configured to output a reference voltage from each connection point of a plurality of resistors connected in series, wherein a resistance at a central portion of the ladder resistor is connected in parallel. 2. A liquid crystal display device according to claim 1, comprising a plurality of resistors having different resistance values connected to each other, wherein one of the plurality of resistors is selected. 5. The reference voltage generating circuit is inserted between a ladder resistor and a first potential supply source or a second potential supply source to which the ladder resistor is connected, and receives the digital signal so as to receive the reference signal. 2. The liquid crystal display device according to claim 1, further comprising a digital-to-analog converter configured to change a potential applied to the ladder resistor. 6. A reference voltage generating circuit includes a ladder resistor configured to output a reference voltage from each connection point of a plurality of resistors connected in series, and a part of the ladder resistor converts a digital signal. 2. The liquid crystal display device according to claim 1, wherein said digital ladder resistor is replaced by a digital / analog converter configured so that the potential output from said ladder resistor can be varied by inputting. 7. The liquid crystal display device according to claim 1, wherein the change of the luminance-gradation characteristic is performed by changing the maximum luminance or the minimum luminance.