JP2005309282A - Display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a display device which reduces influence of coupling of a pixel transistor when a video signal is inverted in polarity, field by field, and written in a pixel. <P>SOLUTION: In a one-field period, a video signal is written in respective pixels by a 1F inversion driving system having the same polarity with a signal line potential and in a horizontal blanking period wherein the video signal is not written in the respective pixels, a precharge gray signal having the same polarity with the signal line potential is supplied to respective pixel electrodes successively to a precharge black signal having the opposite polarity. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a display device, and more particularly to an active matrix type display device of a so-called dot sequential drive system.

  In a display device in which pixels are arranged in a matrix, for example, an active matrix liquid crystal display (LCD), as a driving method thereof, each pixel is sequentially driven in units of pixels for each line (one row). A sequential drive system is known. Further, as the dot sequential driving method, there are a 1H inversion driving method and a 1F inversion driving method.

FIG. 7 shows a pixel circuit of the above-described dot-sequential driving type active matrix liquid crystal display device. The liquid crystal display device is configured by connecting the pixel circuits shown in FIG. 7 in a matrix.
As shown in FIG. 7, the pixel circuit includes a thin film transistor (TFT) that is a pixel transistor, a liquid crystal cell LC in which a pixel electrode is connected to the drain electrode of the TFT, and one electrode on the drain electrode of the TFT. Has a storage capacitor Cs connected thereto.
To this pixel circuit, a signal line SIGL1 driven by a horizontal drive circuit (not shown) and a scanning line SCL driven by a vertical drive circuit (not shown) are connected.
The counter electrode of the liquid crystal cell LC and the other electrode of the storage capacitor Cs are given as a common voltage Vcom through a common Cs line.

In FIG. 7, when a gate pulse is applied to the scanning line SCL and the TFT is turned on, that is, when the illustrated scanning line is selected, a video signal is written to the pixel electrode of the liquid crystal cell LC through the signal line SIGL1. .
However, in reality, there is a resistance between the left and right pixels with respect to the Cs line, and there are further parasitic capacitances (Ccp1 and Ccp2 in the figure) between the Cs line and the signal line. As a result, a phenomenon (coupling) occurs in which the video signal jumps into the Cs line or the scanning line SCL (gate line) and the potential of the Cs line fluctuates in the same polarity direction as the video signal. Due to the influence of this coupling, lateral crosstalk becomes noticeable or shading failure occurs, and the image quality is greatly impaired.

8A and 8B are diagrams showing the influence of coupling in the case of the 1H driving method. FIG. 8A shows the signal line potential VSIG supplied through the signal line every 1H, and FIG. 8B shows the pixel electrode potential. Each pixel potential VP is shown.
As shown in FIG. 8, in the case of the 1H inversion driving method, the polarity of the signal line potential VSIG is inverted every 1H, so that the black side coupling (black side coupling) and the white side coupling (white side) Coupling) occurs in the 1H cycle with opposite polarities, so that these couplings cancel each other. Therefore, in the case of the 1H inversion driving method, the influence of coupling is relatively small.

By the way, when the video signal to be written to each pixel is inverted every 1H when performing dot sequential driving, if the charge / discharge current due to the writing of the video signal to the signal line wired for each column of the pixel portion is large, They appear on the display screen as vertical stripes.
In order to suppress the charging / discharging current due to the writing of the video signal as much as possible, a precharge driving method in which a precharge signal is written in advance prior to the writing of the video signal is employed.

Here, the gray level is the most visible as a vertical stripe. Therefore, the precharge signal level is usually set to the gray level at which vertical stripes are most visible.
However, when the precharge signal level is set to the gray level, when a window pattern or the like is displayed, the amount of light leakage between the source and drain of the pixel transistor differs depending on the location of the image. Talk will occur and the image quality will be impaired.

  In order to prevent this vertical crosstalk from occurring, the precharge signal level may be set to the black level, and thereby the leak current between the source and drain of the pixel transistor is made uniform over the entire screen. can do. However, when the precharge signal level is set to the black level, the above-described vertical stripe is easily seen. That is, there is a trade-off relationship between vertical crosstalk and vertical stripes.

For this reason, a dot sequential two-step precharge method for precharging the black level and the gray level in two steps has been proposed.
JP 2001-356740 A

By the way, in the case of the 1F inversion driving method which is another driving method of the dot sequential driving method, unlike the 1H inversion driving method, there is a problem in that it is easily affected by coupling and vertical crosstalk is likely to occur. .
That is, while the pixel holds the pixel information for one field period, the potential of the signal line fluctuates every 1H (H is a horizontal scanning period). Since the polarity of the video signal is the same, the fluctuation of the potential of the signal line becomes large, and this fluctuation of the potential jumps into the pixel by the source / drain coupling of the pixel transistor, so that the vertical crosstalk becomes remarkable, This is a cause of poor image quality.

FIG. 9 is a diagram showing the influence of coupling in the case of the 1F driving method, where (a) shows the signal line potential VSIG supplied through the signal line every 1H, and (b) shows the pixel electrode potential. Each pixel potential VP is shown.
As shown in FIG. 9, in the case of the 1H inversion driving method, the polarity of the signal line potential VSIG does not change during the 1F (field) period, so that the black side coupling is not canceled and affected by the coupling. It becomes easy.

  On the other hand, according to the 1F inversion driving method, since the video signals have the same polarity in each field, no vertical electric field is applied between the pixels, and liquid crystal alignment is not disturbed. As a result, high contrast is achieved. There is an advantage that it can be achieved, and in order to widely apply the 1F inversion driving method, it is desired to eliminate the disadvantages due to the influence of the coupling described above.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a display device that reduces the influence of coupling of a pixel transistor when the polarity of a video signal is inverted for each field and written to a pixel. There is to do.

  In order to achieve the above object, an aspect of the present invention is to provide a pixel portion in which pixel circuits are arranged in a matrix, a signal line is wired for each column, and a scanning line is wired for each row, and each scanning of the pixel portion The polarity is inverted for each field through the signal line with respect to a first driving unit that applies a scanning pulse to the line and a pixel circuit connected to the scanning line to which the scanning pulse is applied from the first driving unit. A second driving means for sequentially supplying video signals, and a black level precharge signal having a polarity opposite to that of the video signals within a horizontal blanking period before the second driving means supplies the video signals; And a third driving means.

  Preferably, when the video signal has a negative polarity, the positive polarity precharge signal is supplied at least once in at least two horizontal blanking periods.

  Preferably, the third driving means supplies a second precharge signal having a gray level having the same polarity as that of the video signal following the precharge signal within a horizontal blanking period.

  According to the present invention, when a predetermined scanning line is selected by applying a scanning pulse to each scanning line of the pixel portion by the first driving unit, the second driving circuit is connected to the pixel circuit connected to the scanning line. The driving means sequentially supplies the video signals whose polarity is inverted for each field through the signal lines. At this time, the third driving means performs horizontal blanking before the second driving means supplies the video signals. Since the black level precharge signal having the opposite polarity to the video signal is supplied within the period, the influence of the coupling in the pixel circuit is canceled.

  According to the present invention, since the influence of coupling of pixel transistors included in a pixel circuit for displaying an image is reduced, there is an advantage that the image quality of the display image is improved.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a block diagram showing a configuration example of an active matrix type liquid crystal display device 1 according to an embodiment of the present invention. As is clear from the figure, the liquid crystal display device 1 includes a video signal processing unit 2, a timing generator (TG) 3, a liquid crystal driver 4, and an LCD panel module 5, and allows light to pass therethrough without applying an electric field. The panel configuration is normally white mode.
Hereinafter, each component of the liquid crystal display device 1 having the above configuration will be described.

The video signal processing unit 2 inputs RGB digital video signals and executes signal processing for adjusting image quality such as white balance adjustment and gamma adjustment.
In general, the display output corresponding to the input signal of the video display device does not appear to be linearly changing for the human eye, so gamma correction is performed to adjust this. That is, RGB video signals input from the outside are output after being subjected to gamma correction that matches the potential-transmittance characteristics (VT characteristics) of the liquid crystal mounted on the LCD panel module 5.

  The timing generator 3 generates a control signal (CLK) for 1F inversion based on the horizontal synchronization signal HSYNC and the vertical synchronization signal VSYNC, outputs the control signal (CLK) to the liquid crystal driver 4 and also instructs the start of vertical scanning. Various timing signals such as VST, a vertical clock pulse VCK serving as a reference for vertical scanning, a horizontal start pulse HST for instructing the start of horizontal scanning, and a horizontal clock pulse HCK serving as a reference for horizontal scanning are generated to the LCD panel module 5. Supply.

  The liquid crystal driver 4 performs control for 1F inversion on the RGB video signal output from the video signal processing unit 2 based on the 1F inversion control signal (CLK) supplied from the timing generator 3. As a result, the liquid crystal driver 4 outputs an analog video signal that is AC driven in a 1F cycle.

  The liquid crystal driver 4 includes a D / A conversion circuit and a sample hold circuit, converts the digital video signal from the video signal processing unit 2 into an analog signal, and is based on the sample hold pulse (SHP) supplied from the timing generator 3. Then, a signal sampled at a constant interval is generated, and the AC processing as described above is performed.

The LCD panel module 5 is driven for video display in synchronization with a predetermined timing signal from the timing generator 3 by an analog video signal supplied from the liquid crystal driver 4.
FIG. 2 shows an example of the circuit configuration of the LCD panel module 5.
Here, for simplification of the drawing, a pixel arrangement of 3 rows (n−1 rows to n + 1 rows) and 4 columns (m−1 columns to m + 2 columns) is shown as an example.

  In FIG. 2, in a display area (effective pixel area) 51, unit pixels 52 each including a thin film transistor TFT, a liquid crystal cell LC, and a storage capacitor Cs as pixel transistors are arranged in a matrix. Here, the liquid crystal cell LC means a capacitance generated between a pixel electrode formed by a thin film transistor TFT and a counter electrode formed opposite to the pixel electrode.

In the pixel structure described above, the thin film transistor TFT has a gate electrode connected to the scanning lines 53n-1, 53n, 53n + 1, and a source electrode connected to the signal lines 54m-1, 54m, 54m + 1, 54m + 2.
In the liquid crystal cell LC, the pixel electrode is connected to the drain electrode of the thin film transistor TFT, and the counter electrode is connected to the common line 55. The storage capacitor Cs is connected between the drain electrode of the thin film transistor TFT and the common line 55. A common potential Vcom that is a reference potential is applied to the common line 55.

  One end of each of the scanning lines 53n−1, 53n, 53n + 1 is connected to each output end of the corresponding row of the vertical drive circuit 56. One end of each of the signal lines 54m-1, 54m, 54m + 1, 54m + 2 is connected to each output end of the corresponding row of the horizontal drive circuit 57.

A vertical start pulse VST and a vertical clock pulse VCK are supplied as timing signals from the timing generator 3 shown in FIG. 1 to the vertical drive circuit 56 as the first drive means.
The vertical driving circuit 56 starts vertical driving (vertical scanning) in response to the vertical start pulse VST, sends a gate pulse to the scanning line, turns on the TFT, selects the scanning line, and selects the selected scanning line. On the other hand, a video signal from a horizontal drive circuit 57 described later is written.

An analog video signal from the liquid crystal driver 4 shown in FIG. 1 is supplied to a horizontal drive circuit 57 as second and third drive means, and a horizontal start pulse HST and a horizontal clock are used as timing signals from the timing generator 3. A pulse HCK is provided.
The horizontal drive circuit 57 starts horizontal drive in response to the horizontal start pulse HST, and sequentially samples the analog video signal every 1H in synchronization with the horizontal clock pulse HCK.

  As a driving method of the horizontal driving circuit 57, for example, in the case of the dot sequential driving method, analog video signals for 1H are sequentially sampled and output to the signal lines 54m-1, 54m, 54m + 1, 54m + 2 in order. As a result, video signals are sequentially written to the pixels 52 in the line (row) selected by the vertical drive circuit 56.

  Next, a driving method by the horizontal driving circuit 57 will be described with reference to FIGS. FIG. 3 is a timing chart showing the signal line potential VSIG in the driving method according to the present invention. FIG. 4 is a diagram showing a precharge signal when the signal line potential is positive, and FIG. 5 shows a precharge signal when the signal line potential is negative.

  In FIG. 3, a positive signal line potential VSIG (11 V) is supplied in the first field, and a negative signal line potential VSIG (4 V) inverted from the first field is supplied in the second field. In each field, a precharge signal is supplied prior to the signal line potential VSIG within a horizontal blanking period in which no video signal is written to each pixel (indicated as PP in the figure).

In addition, as shown in FIG. 4, when the signal line potential VSIG is positive, the precharge is performed after the N-th stage signal line potential VSIG is supplied after the N-th stage signal line potential VSIG is supplied. The signals are supplied in the order of the Nth stage signal line potential VSIG → the precharge black signal → the precharge gray signal → the (N + 1) th stage signal line potential VSIG.
That is, a precharge gray signal (10 V) having the same polarity as the signal line potential VSIG is supplied to the positive signal line potential VSIG (11 V) following the precharge black signal (2.5 V) having the opposite polarity. The

On the other hand, as shown in FIG. 5, when the signal line potential VSIG is negative, the precharge is performed after the N-th stage signal line potential VSIG is supplied after the N-th stage signal line potential VSIG is supplied. The signal is N-th stage signal line potential VSIG → precharge black signal (negative polarity or positive polarity) → precharge gray signal (same polarity) → N + 1 stage signal line potential VSIG → precharge black signal (positive polarity or Negative polarity) → precharge gray signal (same polarity) → N + second stage signal line potential VSIG.
That is, a precharge gray signal (5V) having the same polarity as the signal line potential VSIG is supplied to the negative signal line potential VSIG (4V) following the precharge black signal (2.5V) having the same polarity. Further, in the next 1H period, a precharge gray signal (5 V) having the same polarity as the signal line potential VSIG is supplied following the precharge black signal (12.5 V) having the reverse polarity.

In FIG. 4, the precharge black signal having the reverse polarity is supplied in order to cancel the influence of the coupling as will be described later. In FIG. 5, when the signal line potential VSIG is negative, the precharge black signal is adjacent. The polarity of the precharge black signal supplied to the scanning lines (Nth and N + 1th stages in the figure) is inverted.
This is because, when the pixel potential VP is low, it is desirable to apply a precharge black signal of reverse polarity (high potential) within each horizontal blanking period from the viewpoint of canceling the influence of coupling. However, in such a case, the influence of the leaky crosstalk caused by the high potential difference between the source / drain of the pixel transistor cannot be ignored. Therefore, the precharge black signal is applied every horizontal blanking period. This is because it is limited to at least once for the 2H period.
That is, since the coupling and leakage crosstalk are in a trade-off relationship, when the signal line potential VSIG is negative, it is supplied as described above in order to achieve both of these performances. The polarity of the precharge black signal is reversed once every two times.

  Further, as shown in FIGS. 4 and 5, the precharge gray signal supplied within the horizontal blanking period following the precharge black signal is applied to suppress vertical stripes. The signal line potential VSIG has the same polarity so that no talk occurs.

6A and 6B are timing charts showing the influence of coupling in the driving method by the horizontal driving circuit 57 described above. FIG. 6A shows the signal line potential VSIG, and FIG. 6B shows the pixel potential VP. In FIG. 6, the precharge gray signal is not shown in order to pay attention to the effect of the coupling of the precharge black signal having the reverse polarity.
Further, in the figure, the influences of the black side coupling and the white side coupling are shown.

As described with reference to FIG. 7, the coupling occurs due to the parasitic capacitance (Ccp1 and Ccp2) generated between the pixel electrode and the signal line, but the potential difference generated between these parasitic capacitances, that is, In the case of the normal 1F inversion driving method in which the polarity of the pixel potential VP with respect to the signal line potential VSIG does not change between one field, as shown in FIG. 8, the influence of the black side coupling cannot be reduced.
On the other hand, as described above, the horizontal drive circuit 57 supplies the precharge black signal having the opposite polarity to the signal line potential VSIG in each 1H horizontal blanking period in one field period. In the meantime, the polarity of the pixel potential VP with respect to the signal line potential VSIG is reversed to cause white side coupling. As a result, the white side coupling and the black side coupling occur in opposite directions as shown in FIG. Are mutually canceled, so that the influence of coupling is suppressed.

As described above, according to the liquid crystal display device 1 according to the present embodiment, the video signal is written to each pixel by the 1F inversion driving method in which the signal line potential has the same polarity within one field period. In the horizontal blanking period during which no video signal is written, a precharge gray signal having the same polarity as the signal line potential is supplied to each pixel electrode following the precharge black signal having the opposite polarity to the signal line potential. As a result of mutual cancellation of the ring and the black side coupling, the crosstalk in the vertical direction is reduced.
Further, according to the liquid crystal display device 1 according to the present embodiment, when the signal line potential is negative, the application of the precharge black signal is not 1 every horizontal blanking period but 1 for at least 2H period. Limit to times. As a result, even when the signal line potential is low and the influence of leakage due to the pixel transistor cannot be ignored, it is possible to achieve both performances with respect to coupling effects and leakage crosstalk that are in a trade-off relationship. Become.

It is a block diagram which shows the structural example of the liquid crystal display device which concerns on embodiment. It is a figure which illustrates the circuit structure of a LCD panel module. 4 is a timing chart showing a signal line potential VSIG in the driving method according to the present invention. It is a figure which shows the precharge signal in case a signal line electric potential is positive polarity. It is a figure which shows the precharge signal in case a signal line electric potential is negative polarity. 5 is a timing chart showing the influence of coupling, where (a) shows a signal line potential VSIG and (b) shows a pixel potential VP. It is a circuit diagram which shows the structure of one pixel circuit. 4 is a timing chart showing the influence of coupling in the 1H inversion driving method, where (a) shows a signal line potential VSIG and (b) shows a pixel potential VP. 4 is a timing chart showing the influence of coupling in the 1F inversion driving method, where (a) shows a signal line potential VSIG and (b) shows a pixel potential VP.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Liquid crystal display device, 2 ... Video signal processing part, 3 ... Timing generator, 4 ... Liquid crystal driver, 5 ... LCD panel module, 56 ... Vertical drive circuit, 57 ... Horizontal drive circuit.

Claims (4)

A pixel portion in which pixel circuits are arranged in a matrix, a signal line is wired for each column, and a scanning line is wired for each row;
First driving means for applying a scanning pulse to each scanning line of the pixel portion;
Second driving means for sequentially supplying a video signal whose polarity is reversed for each field through the signal line to the pixel circuit connected to the scanning line to which the scanning pulse is given from the first driving means;
And a third driving means for supplying a black level precharge signal having a polarity opposite to that of the video signal before the second driving means supplies the video signal. The display device according to claim 1, wherein when the video signal is negative, the positive precharge signal is supplied at least once in a horizontal blanking period. The display device according to claim 1, wherein the third driving unit supplies a second precharge signal having a gray level having the same polarity as that of the video signal following the precharge signal in a horizontal blanking period. The display device according to claim 1, wherein the pixel circuit includes a liquid crystal cell.

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