JPH08227067A - Liquid crystal display device and its driving method - Google Patents

Abstract

PURPOSE: To provide an active matrix type liquid crystal display device of which the luminance gradient is lessened by decreasing the leak currents of thin-film transistors and lessening the influence of the leak currents in the vertical direction of a screen and its driving method. CONSTITUTION: This driving method for the active matrix type liquid crystal display device is a driving system of applying signal line potential 15 on the potential impressed on transparent electrodes (common electrodes). Scanning line potential 16 is set at low level potential 19 higher than low level potential 18 in another holding period in a holding period when the polarity of the signal line potential 15 is positive and the polarity of pixel electrode potential 17 is positive.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix type liquid crystal display device using thin film transistors and a driving method thereof.

[0002]

2. Description of the Related Art A thin film transistor (hereinafter referred to as a TFT) of an active matrix type liquid crystal display device is usually
Amorphous silicon TFT or polysilicon TFT
Is used.

FIG. 7 is a plan view showing a schematic structure of a conventional active matrix type liquid crystal display device. In the figure, 1 is an array substrate made of glass or the like, 2 is a plurality of scanning lines arranged in parallel on the array substrate 1, 3 is a signal line arranged perpendicular to the scanning line 2, and 4 is a scanning line 2. Scan line driving circuit for sequentially and selectively applying signals, 5
Is a signal line drive circuit for applying a signal for displaying an image to the signal line 3.

FIG. 8 shows the scanning line 2 and the signal line 3 shown in FIG.
6 is an enlarged plan view of one pixel formed between the two
T and 7 are source electrodes of the TFT 6, 8 is a drain electrode of the TFT 6, and 9 is a pixel electrode. A gate electrode (not shown) of the TFT 6 is a scanning line 2, a source electrode 7 is a signal line 3, and a drain electrode. 8 are connected to the pixel electrodes 9 respectively,
The scanning line potential (gate voltage) from the scanning line 2 to the gate electrode,
Signal line potential (source voltage) from signal line 3 to source electrode 7
Are respectively applied, and a drain current flows.

FIG. 9 shows the drain current (DRA
IN CURRENT vs. gate voltage (GATE VO)
As shown in the figure, the drain current tends to increase on the negative side of the gate voltage.

The effective gate voltage of the TFT 6 is the source potential of the source electrode 7 (pixel electrode potential) or the drain electrode 8.
In the liquid crystal display device which is determined by the relative potential with the lower one of the drain potentials of the above, and the AC drive is performed, the effective gate voltage is greatly different depending on whether the signal line potential is the positive polarity or the negative polarity. As described above, the difference in the effective gate voltage appears as the difference in the drain current when the TFT 6 is OFF, that is, the leak current, which directly impairs the display characteristics.

In order to avoid the above problems, Japanese Patent Laid-Open No. 304420/1992 proposes a driving method shown in the timing chart of FIG. In the figure, the horizontal axis represents time (TIME) and the vertical axis represents voltage (VOLTAGE).
Is a scanning line potential, 12 is a signal line potential, 13 is a pixel electrode potential, 14 is a common electrode potential, 10 is a high level potential applied to the gate electrode during the selection period, 11 bp and 1
1bn is a low bell potential given to the scanning line potential 11 during a period in which the polarity of the pixel electrode potential 13 with respect to the common electrode potential 14 is positive and negative during holding, and there is a gap between adjacent high-level potentials 10. It is a vertical scanning period, that is, one frame period for displaying one screen.

The driving method shown in FIG. 10 corresponds to the positive / negative of the pixel electrode potential 13, and the binary low-level potential 11
By setting bp and 11bn, the fluctuation range of the effective gate voltage during holding is narrowed as much as possible, and the drain current during holding, that is, the leak current is reduced.

[0009]

However, the influence of the leak current differs depending on the screen position. FIG.
In a conventional liquid crystal display device, a timing chart showing that the influence of the leak current varies depending on the screen position, the horizontal axis represents time and the vertical axis represents potential.

In the figure, the first, second and third parts of the screen are shown according to the upper part (a), the central part (b) and the lower part (c).
11 shows a timing chart when a scanning line is selected and a holding time is started without a frame period.
Is a scanning line potential, 12 is a signal line potential, 13 is a pixel electrode potential, and the pixel electrode potential 13 is shown only in the central portion (a) of the screen.

Further, 15a and 15b are fluctuation amounts of the pixel electrode potential 13, which are generated by fluctuations of the gate voltage through the capacitance, and correspond to the positive polarity side and the negative polarity side, respectively.

Also, 16a, 16b, 16c and 16
d is the effective gate voltage corresponding to the holding time regions A, B, C, and D, respectively, and is the gate voltage seen from the signal line potential 12 or the pixel electrode potential 13 whichever is lower.

As is apparent from FIG. 11, the value of the effective gate voltage 16a is equal to the effective gate voltages 16b, 16c and 1.
The value of the leak current is larger than 6d and becomes a large value relative to the effective gate voltage on the more negative side in FIG.

The fluctuation of the pixel electrode potential 13 is larger in the holding time region A than in the holding time regions B, C and D. Here, comparing the lengths of the holding time regions A, B, C and D in the upper part (a), the central part (b) and the lower part (c) of the screen, the length of the holding time region A is ) Is longer than others, and becomes shorter as it goes to the lower part (c) of the screen.

That is, the time of large leak current is longest in the upper part (a) of the screen, and the time of large leak current becomes shorter as it moves to the lower part (c) of the screen. The influence of the leak current on the image appears as a defect that the brightness is different between the upper part (a) and the lower part (c) of the screen.

However, the driving method shown in FIG. 10 sets the low level of the scanning line potential to a different value for every fixed frame period, and the low level of the scanning line potential is set within one frame period. Since it is kept constant without changing, it becomes the same as that shown in FIG. 11, and inevitably allows the effective gate voltage to change within the frame period. However, there is a problem in that the brightness is inclined.

The present invention has been made in order to solve the above-mentioned problems, and it is possible to reduce the leakage current and to reduce the influence of the leakage current in the vertical direction of the screen to reduce the luminance gradient. An object is to provide an active matrix type liquid crystal display device and a driving method thereof.

[0018]

The invention according to claim 1 is
A plurality of thin film transistors arranged in a matrix, a pixel electrode electrically connected to the thin film transistor, a scanning line for applying a scanning line potential to scan the thin film transistor, and a signal line potential input to the thin film transistor. An active matrix liquid crystal display device in which a liquid crystal is held in a gap between an array substrate having a signal line for performing the above and a counter substrate having a transparent electrode formed thereon, wherein the signal line potential is applied to the transparent electrode. A driving method in which the scanning line potential is applied to an electric potential, the polarity of the signal line potential is positive, and the polarity of the pixel electrode potential is positive; It is an active matrix type liquid crystal display device characterized by being set high.

According to a second aspect of the invention, in the active matrix type liquid crystal display device according to the first aspect, during the holding period in which the polarity of the signal line potential is positive and the polarity of the pixel electrode potential is positive. The potential difference between the set scanning line potential and the scanning line potential set in another holding period is set to the amplitude of the signal line voltage ± 1V.

The invention according to claim 3 is the invention according to claim 1 or 2.
In the active matrix type liquid crystal display device described,
The drive system is a frame inversion drive system in which the polarities of pixel electrodes adjacent to each other in the signal line direction are the same, and the polarities are inverted every frame period.

The invention according to claim 4 is the invention according to claim 1 or 2.
In the active matrix type liquid crystal display device described,
The driving method is a gate line inversion driving method in which the polarities of the pixel electrodes adjacent in the scanning line direction are the same and the polarities of the pixel electrodes adjacent in the signal line direction are alternately opposite.

According to a fifth aspect of the present invention, a plurality of thin film transistors arranged in a matrix, pixel electrodes electrically connected to the thin film transistors, and scanning lines for applying a scanning line potential to scan the thin film transistors. A method of driving an active matrix liquid crystal display device in which a liquid crystal is held in a gap between an array substrate having a signal line for inputting a signal line potential to the thin film transistor and a counter substrate having a transparent electrode formed thereon. And a holding period in which the signal line potential is applied to the potential applied to the transparent electrode and the polarity of the signal line potential is positive and the potential of the pixel electrode is positive. In the driving method of the active matrix type liquid crystal display device, the scanning line potential is set to be higher than that in other holding periods.

According to a sixth aspect of the invention, in the method of driving the active matrix type liquid crystal display device according to the fifth aspect, the polarity of the signal line potential is positive and the polarity of the potential of the pixel electrode is positive. The potential difference between the scanning line potential set in the holding period and the scanning line potential set in another holding period is set to ± 1V of the amplitude of the signal line voltage.

The invention according to claim 7 is the invention according to claim 5 or 6.
In the driving method of the active matrix type liquid crystal display device described above, the driving method is a frame inversion driving method in which the polarities of the pixel electrodes adjacent in the signal line direction are the same and the polarities are inverted every frame period.

The invention according to claim 8 is claim 5 or 6
In the driving method of the active matrix liquid crystal display device described above, the driving method is a gate line in which the polarities of the pixel electrodes adjacent in the scanning line direction are the same and the polarities of the pixel electrodes adjacent in the signal line direction are alternately opposite. It is an inversion driving method.

[0026]

According to the first and fifth aspects of the present invention, the scanning line potential is less than that in the other holding period during the holding period when the polarity of the signal line potential is positive and the polarity of the potential of the pixel electrode is positive. Is set to a high value, the leak current of the thin film transistor can be reduced, and the influence of the leak current of the thin film transistor in the vertical direction of the screen can be reduced to reduce the luminance gradient.

According to the second and sixth aspects of the present invention, the scanning line potential and other holdings set in the holding period in which the polarity of the signal line potential is positive and the polarity of the pixel electrode potential is positive. By setting the potential difference from the scanning line potential set in the period to half the amplitude of the signal line potential ± 1 V, more preferable results can be obtained for reducing the leak current of the thin film transistor and reducing the brightness gradient in the vertical direction of the screen. .

According to the third and seventh aspects of the invention, in the frame inversion drive system in which the polarities of the pixel electrodes adjacent to each other in the signal line direction are the same, and the polarities are inverted every frame period, the leakage current of the thin film transistor is reduced. Further, it is possible to reduce the brightness gradient in the vertical direction of the screen.

According to the fourth and eighth aspects of the present invention, the gate line inversion drive in which the polarities of the pixel electrodes adjacent in the scanning line direction are the same and the polarities of the pixel electrodes adjacent in the signal line direction are alternately opposite polarities. In this method, it is possible to reduce the leakage current of the thin film transistor and the luminance gradient in the vertical direction of the screen.

[0030]

【Example】

Example 1. FIG. 1 is a timing chart showing an embodiment of a driving method of an active matrix type liquid crystal according to the present invention, which is a frame inversion driving method in which the polarity is inverted every frame period (driving in which adjacent signal lines have the same polarity). Method). In the figure, the horizontal axis represents the time axis, and the vertical axis represents potential, 15 denotes a signal line potential, Vs _a signal line voltage, the scanning line potential 16, the pixel electrode potential 17, 18 and 19 are low level of the scanning line potential 16 , 20 are high levels of the scanning line potential, and low level 19 sets only the holding period in which both the signal line potential 15 and the pixel electrode potential 17 are positive.

FIG. 1 shows a typical value of each potential, and the polarities of the signal line potential 15 and the pixel electrode potential 17 and the effective gate voltage in each period are shown in Comparative Example 1 (conventional case in which the low level 19 is not set). In the case of)). Period A in which the polarities of the signal line potential 15 and the pixel electrode potential 17 are both positive
In Comparative Example 1, the effective voltage is −14.5 V to −
While it was 20.5 V, this embodiment is -9.5 V.
It becomes -15.5V, and the region in which the drain current shown in FIG. 9 is small can be obtained.

In this embodiment, the set value of the low level 19 is set to -2V, which is higher than the low level 18 (-7V) by 5V. The set value of the low level 19 is preferably higher than the low level 18 by the amplitude of the signal line voltage 15 ± 1V.

FIG. 2 is a diagram showing potential changes in each mode (all black, halftone and all white) corresponding to the first embodiment of the present invention shown in FIG. In the figure, A, B, C and D correspond to each holding period shown in FIG. V _sa is a signal voltage of 6 V for all black, 3 V for halftone, and 0 V for all white.
V _s is a signal line potential, V _d is a pixel electrode potential, V _g is a scanning line potential, V _gd is a relative potential (V _g −V _d ) of a scanning line based on the pixel electrode potential V _d , and V _gs is a signal. the relative potential of the scanning line relative to the line voltage _{V s (V g -V s)} , V ds is the signal line potential V _s
Is a relative potential (V _d −V _s ) of the pixel electrode with reference to.

FIG. 3 shows a comparative example 1 (low level 1) of FIG.
9 is a diagram showing the potential change in each mode (all black, halftone and all white) corresponding to the case (9 in the conventional case not set), and each symbol in the figure shows the same as in FIG. Each potential change in each mode corresponding to this embodiment shown in FIG.
It can be seen that, compared with the respective potential changes in the conventional case shown in FIG. 3, the uniformity is remarkably improved and the brightness gradient of the screen can be reduced.

Example 2. The first embodiment shows the case of the frame inversion driving method. To prevent flicker (flickering of the screen), the gate line inversion drive method (drive in which the pixel electrodes adjacent in the scanning line direction have the same polarity and the pixel electrodes adjacent in the signal line direction have opposite polarities alternately) Method) is widely used.

FIG. 4 is a timing chart showing an embodiment of the driving method of the active matrix type liquid crystal using the gate line inversion driving method. In the figure, the horizontal axis is the time axis, the vertical axis is the potential, 15 is the signal line potential, V _sa is the signal voltage,
16 is a scanning line potential, 17 is a pixel electrode potential, 18 and 1
Reference numeral 9 is a low level of the scanning line potential 16, 20 is a high level of the scanning line potential, and the signal line potential 15 and the pixel electrode potential 1
Both 7 and 7 set the low level 19 only during the positive holding period.

FIG. 4 exemplifies typical values of the respective potentials, and the signal voltage V _sa is V _sa = 0 V in the all white (normally white mode) mode and V _sa = 6 V in the all black mode. Further, the polarities of the signal line potential 15 and the pixel electrode potential 17 and the effective gate voltage in each period are shown together with Comparative Example 2 (conventional example in which the low level 19 is not set).

In the figure, in the holding period A in which the polarities of the signal line potential 15 and the pixel electrode potential 17 are both positive, in Comparative Example 2, the effective gate voltage is from -14.5 V to -20.
While it was 5 V, in the present embodiment, -9.5 V to -1.
It becomes 5.5V, and it can be set to a region where the drain current shown in FIG. 9 is small.

FIG. 5 is a diagram showing potential changes in each mode (all black, halftone and all white) corresponding to the second embodiment of the present invention shown in FIG. In the figure, A, B, C and D correspond to the holding periods shown in FIG. V _sa is a signal voltage of 6 V for all black, 3 V for halftone, and 0 V for all white.
V _s is a signal line potential, V _d is a pixel electrode potential, V _g is a scanning line potential, V _gd is a relative potential (V _g −V _d ) of a scanning line based on the pixel electrode potential V _d , and V _gs is a signal. the relative potential of the scanning line relative to the line voltage _{V s (V g -V s)} , V ds is the signal line potential V _s
Is a relative potential (V _d −V _s ) of the pixel electrode with reference to.

FIG. 6 shows a comparative example 2 (low level 1) of FIG.
9 is a diagram showing a potential change in each mode (all black, halftone and all white) corresponding to the case (9 in which the conventional case is not set),
The reference numerals are the same as those in FIG. It can be seen that the respective potential changes in the respective modes corresponding to the present embodiment shown in FIG. 5 are remarkably improved in uniformity as compared with the respective potential changes in the conventional case shown in FIG.

In the present embodiment, the set value of the low level 19 is set to -2V, which is higher than the low level 18 (-7V) by 5V. The set value of the low level 19 is preferably higher than the low level 18 by the amplitude of the signal line voltage 15 ± 1V.

[0042]

According to the inventions of claims 1 and 5,
The polarity of the signal line potential is positive, and the scanning line potential is set higher than that in other holding periods in the holding period in which the polarity of the pixel electrode potential is positive, so that the leakage current of the thin film transistor is reduced, It is possible to reduce the influence of the leak current of the thin film transistor in the vertical direction of the screen and reduce the luminance gradient.

According to the second and sixth aspects of the present invention, the scanning line potential and other holdings set in the holding period in which the polarity of the signal line potential is positive and the polarity of the potential of the pixel electrode is positive. By setting the potential difference from the scanning line potential set in the period to half the amplitude of the signal line potential ± 1 V, more preferable results can be obtained for reducing the leak current of the thin film transistor and reducing the brightness gradient in the vertical direction of the screen. .

According to the third and seventh aspects of the present invention, the source line inversion drive in which the polarities of the pixel electrodes adjacent in the signal line direction are the same and the polarities of the pixel electrodes adjacent in the scanning line direction are alternately opposite polarities. In this method, it is possible to reduce the leakage current of the thin film transistor and the luminance gradient in the vertical direction of the screen.

According to the fourth and eighth aspects of the invention, the gate line inversion drive is such that the polarities of the pixel electrodes adjacent in the scanning line direction are the same and the polarities of the pixel electrodes adjacent in the signal line direction are alternately opposite. In this method, it is possible to reduce the leakage current of the thin film transistor and the luminance gradient in the vertical direction of the screen.

[Brief description of drawings]

FIG. 1 is a timing chart showing an embodiment of a driving method of an active matrix type liquid crystal according to the present invention.

FIG. 2 is a diagram showing a potential change in each mode (all black, halftone and all white) corresponding to one embodiment of the present invention.

FIG. 3 is a diagram showing a potential change in each mode (all black, halftone, and all white) corresponding to a comparative example (conventional case in which the low level 19 is not set).

FIG. 4 is a diagram showing potential changes in each mode (all black, halftone and all white) corresponding to another embodiment of the present invention.

FIG. 5 is a diagram showing a potential change in each mode (all black, halftone and all white) corresponding to another embodiment of the present invention.

FIG. 6 is a diagram showing a potential change in each mode (all black, halftone, and all white) corresponding to a comparative example (conventional case in which the low level 19 is not set).

FIG. 7 is a plan view showing a schematic configuration of a conventional active matrix type liquid crystal display device.

8 is an enlarged plan view of one pixel formed at the intersection of the scanning line and the signal line of FIG.

FIG. 9: Drain current (DRAIN CURRENT) vs. gate voltage (GAT) of a thin film transistor (TFT)
It is a figure which shows an E VOLTAGE) characteristic.

FIG. 10 is a timing chart showing a driving method of a conventional active matrix type liquid crystal display device.

FIG. 11 is a timing chart showing that the influence of the leak current differs depending on the screen position in the conventional active matrix type liquid crystal display device.

[Explanation of symbols]

1 array substrate, 2 scanning lines, 3 signal lines, 4 scanning line drive circuit, 5 signal line drive circuit, 6 TFT, 7 source electrode, 8 drain electrode, 9 pixel electrode, 10 and 20 high level potential of scanning potential, 11 and 1
6 scan line potentials, 11 bp, 11 bn, 18 and 19
Low level potential of scanning potential, 12 and 15 signal line potential, 13 pixel electrode potential, 14 common electrode potential, V
_sa signal line voltage, 17 pixel electrode potential, V _s signal line potential, V _d pixel electrode potential, V _g scan line potential, V _gd pixel electrode potential V _d relative to the scan line relative potential (V _g −
V _d), V _gs signal line potential V _s relative potential of the scanning line relative to the _{(V g -V s), V} ds signal line potential V relative potential of the pixel electrode relative to the _s (V _d -V _s )

Claims (8)

[Claims] 1. A plurality of thin film transistors arranged in a matrix, pixel electrodes electrically connected to the thin film transistors, scanning lines for applying a scanning line potential to scan the thin film transistors, and the thin film transistors. An active matrix liquid crystal display device in which a liquid crystal is held in a gap between an array substrate having a signal line for inputting a signal line potential and a counter substrate on which a transparent electrode is formed. The scanning line potential is applied to another electrode during a holding period in which the signal line potential has a positive polarity and the pixel electrode potential has a positive polarity. An active matrix type liquid crystal display device characterized by being set higher than in the holding period. 2. A potential difference between a scanning line potential set in a holding period and a scanning line potential set in another holding period in which the polarity of the signal line potential is positive and the polarity of the potential of the pixel electrode is positive. 2. The active matrix type liquid crystal display device according to claim 1, wherein the amplitude of the signal line voltage is ± 1V. 3. The driving method is a frame inversion driving method in which the polarities of pixel electrodes adjacent to each other in the signal line direction are the same, and the polarities are inverted every frame period. Active matrix liquid crystal display device. 4. The driving method is a gate line inversion driving method in which the polarities of the pixel electrodes adjacent in the scanning line direction are the same and the polarities of the pixel electrodes adjacent in the signal line direction are alternately opposite. The active matrix liquid crystal display device according to claim 1 or 2. 5. A plurality of thin film transistors arranged in a matrix, pixel electrodes electrically connected to the thin film transistors, scanning lines for applying a scanning line potential for scanning the thin film transistors, and the thin film transistors. A method for driving an active matrix liquid crystal display device, wherein a liquid crystal is held in a gap between an array substrate having a signal line for inputting a signal line potential and a counter substrate having a transparent electrode formed thereon. Is a drive system for applying the voltage to the potential applied to the transparent electrode, and the scanning line potential is applied during a holding period in which the polarity of the signal line potential is positive and the polarity of the pixel electrode potential is positive. A method for driving an active matrix type liquid crystal display device, which is set to be higher than that in other holding periods. 6. A potential difference between a scanning line potential set in a holding period and a scanning line potential set in another holding period in which the polarity of the signal line potential is positive and the polarity of the potential of the pixel electrode is positive. 6. The method for driving an active matrix liquid crystal display device according to claim 5, wherein the signal line voltage amplitude is ± 1V. 7. The driving method is a frame inversion driving method in which the polarities of pixel electrodes adjacent to each other in the signal line direction are the same, and the polarities are inverted every frame period. Driving method for active matrix liquid crystal display device. 8. The driving method is a gate line inversion driving method in which the polarities of the pixel electrodes adjacent in the scanning line direction are the same and the polarities of the pixel electrodes adjacent in the signal line direction are alternately opposite. 7. The method for driving an active matrix type liquid crystal display device according to claim 5 or 6.