Abstract:
A thin-film-transistor liquid crystal display comprises a display unit which contains a plurality of scanning lines, a plurality of data lines arranged to cross the plurality of scanning lines and defining a plurality of pixels, and a data driving circuit providing pixel data signals to the plurality of data lines. The pixels of each scanning line are divided into groups of N successive pixels, where N is an integer greater than 1. A polarity of the respective pixel data signals for the data lines within each group is the same as each other. The polarity of the respective pixel data signals for each successive group along at least one of the scanning lines alternates between a first polarity and a second polarity.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a liquid crystal display (LCD), and more particularly to a driving method for an LCD.  
       BACKGROUND  
       [0002]     In general, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell according to a video signal to display a picture. In other words, a liquid crystal displays contains a plurality of picture elements, or pixels, formed by liquid crystal cells that change the polarization direction of light in response to an electrical voltage of the video signal. By controlling a voltage applied to a liquid crystal cell, the amount of light coming out of the LCD changes. Among various driving methods, active matrix liquid crystal displays, which have a respective switching element such as a thin film transistor for each of the pixels so as to control a voltage to be applied to the liquid crystal, are superior in display quality. Thus, active matrix LCDs have been intensively developed and have come to be widely used as monitors in personal computers.  
         [0003]      FIG. 1  shows a perspective view of a conventional LCD which comprises an upper panel  110 , a lower panel  120 , and liquid crystal materials  130  inserted therebetween. The upper panel  110  contains an upper substrate  112 , an upper polarization plate  114 , a color filter  116 , and a common electrode  118 . The lower panel includes a lower substrate  122  and a lower polarization plate  124 . The layout of the lower substrate  122  includes a plurality of scanning lines  140 , a plurality of data lines  142  which perpendicularly cross the scanning lines, a plurality of thin film transistors  144  (TFTs), and a plurality of pixel electrodes  146 .  
         [0004]     In  FIG. 2 , a data driving circuit  210  receives video data signals  212  and polarity control signals  214  and applies pixel data signals to data lines D 1 -DN. The pixel data signals represent the gray level of red, green, and blue pixels. A scan driving circuit  220  receives scanning control signals  222  and is electrically connected to scanning lines S 1 -SN. When a voltage is applied to a scanning line, all the TFTs connected to the scanning line are turned on. As a result, the pixel data signals are sent to the pixel electrodes for that scanning line through the TFTs and a voltage is applied to pixel electrodes. On the other hand, a constant voltage Vcom is applied to the common electrode. The difference of voltages between the common electrode Vcom and the pixel electrode creates an electric field resulting in the rotation of liquid crystal molecules and a specific gray level.  
         [0005]     Typically, a pixel data signal has either positive polarity or negative polarity depending on whether the voltage of the pixel data signal is higher or lower than a common electrode voltage Vcom. A pixel data signal has positive polarity when its voltage level is higher than the common electrode voltage Vcom. Also, a pixel data signal has negative polarity when its voltage is lower than the common electrode voltage Vcom. The light transmission from the liquid crystal materials (and, therefore, the gray level presented by a pixel,) is related to the difference between the voltages of the pixel data signal and the common electrode voltage Vcom, regardless of the polarity of the pixel data signal. However, a pixel data signal having positive polarity causes liquid crystal molecules to turn to a direction opposite to that caused by a pixel data signal having negative polarity. In order to prolong the lifetime of an LCD, some conventional driving methods such as dot inversion, line inversion, and column inversion are designed to change the polarity of pixel data signals.  
         [0006]      FIGS. 3A and 3B  are tables showing the polarity of pixel data signals driven by the line inversion method, in which the polarity of pixel data signals is reversed at every scanning line (row). In the column inversion method as shown in  FIGS. 4A and 4B , the polarity of pixel data signals is reversed at every data line (column). In the dot inversion as shown in  FIGS. 5A and 5B , the polarity is reversed at every row and column. Also,  FIGS. 3A, 4A , and  5 A represent the polarity status at a specific time frame and  FIGS. 3B, 4B , and  5 B represent the polarity status at the next time frame. Thus, for any given pixel, the polarity changes each time the pixel is scanned.  
         [0007]     At a specific time frame, different polarities of pixel data signals for two adjacent pixels may cause light leakage because of the edge electric field effect resulting from either one of the adjacent pixel electrodes.  FIG. 6A  shows two adjacent pixels with pixel electrodes  632  and  634 , and the data line  625 .  FIG. 6B  is a schematic drawing of a cross-sectional view taken along the section line  6 B- 6 B of  FIG. 6A . A TFT layer  620  with a data line  625  is disposed on a substrate  610 . The pixel electrodes  632  and  634  are disposed on the TFT layer  620 . The liquid crystal material  630  is filled underneath a common electrode  640 . A color filter  650  is disposed on the common electrode  640 . An edge electric field is generated to effect the rotation of liquid crystal molecules because the polarity of pixel electrode  632  is different from that of pixel electrode  634 . As a result, light leakage  660  may occur if the width of data line  625  is not large enough to block the light. If wider data lines are used to prevent light leakage, the aperture ratio of the LCD is sacrificed.  
         [0008]     The dot inversion driving method has the serious disadvantage of lower aperture ratio or light leakage. The line inversion driving method has a high system load, because the total voltage level of pixel electrodes connected to a scanning line is high. The column inversion method has the same disadvantage as the dot inversion driving method. Thus, a driving method to resolve these difficulties is desired.  
       SUMMARY OF THE INVENTION  
       [0009]     A thin-film-transistor liquid crystal display comprises a display unit which contains a plurality of scanning lines, a plurality of data lines arranged to cross the plurality of scanning lines and defining a plurality of pixels, and a data driving circuit providing data signals to the plurality of data lines. The pixels of each scanning line are divided into groups of N successive pixels, where N is an integer greater than 1. A polarity of the respective pixel data signals for the data lines within each group is the same as each other. The polarity of the respective pixel data signals for each successive group along at least one of the scanning lines alternates between a first polarity and a second polarity. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     A more complete understanding of the present invention can be obtained by reference to the detailed description of embodiments in conjunction with the accompanying drawings, in which:  
         [0011]      FIG. 1  illustrates a perspective view of a conventional liquid crystal display;  
         [0012]      FIG. 2  is a schematic diagram showing the structure of a lower panel in  FIG. 1 ;  
         [0013]      FIGS. 3A and 3  B are tables showing the polarity of pixel data signals driven by a line inversion method;  
         [0014]      FIGS. 4A and 4B  are tables showing the polarity of pixel data signals driven by a column inversion method;  
         [0015]      FIGS. 5A and 5B  are tables showing the polarity of pixel data signals driven by a dot inversion method;  
         [0016]      FIG. 6A  is a schematic diagram showing two adjacent pixels on a scanning line;  
         [0017]      FIG. 6B  illustrates a cross-sectional view along the line  6 B- 6 B shown in  FIG. 6A ;  
         [0018]      FIG. 7  is a schematic diagram showing the structure of liquid crystal display using an inventive driving method;  
         [0019]      FIGS. 8A and 8B  are tables showing the polarity of pixel data signals driven by an inversion method with groups of three pixels;  
         [0020]      FIGS. 9A and 9B  are tables showing the polarity of pixel data signals driven by an inversion method with groups of six pixels;  
         [0021]     FIGS.  10  is a table showing the polarity of pixel data signals driven by an inversion method with groups of nine pixels;  
         [0022]      FIG. 11  is a table showing the polarity of pixel data signals driven by an inversion method with groups of two pixels;  
         [0023]      FIG. 12  illustrates an exemplary embodiment of employing priority control signals to generate an inversion driving pattern;  
         [0024]      FIG. 13  illustrates an exemplary embodiment of an LCD with a wider data line between two successive pixel groups than data lines within each pixel group. 
     
    
     DETAILED DESCRIPTION  
       [0025]     As shown in  FIG. 7 , an exemplary embodiment of an LCD comprises a plurality of scanning lines S 1 -SN, a plurality of data lines D 1 -DN arranged to cross the plurality of scanning lines S 1 -SN and to define a plurality of pixels, a data inversion driving circuit  710 , and a scan driving circuit  720 . The data inversion driving circuit  710  receives video data signal  712  and priority control signal  714  to generate pixel data signals transmitted to the plurality of data lines D 1 -DN.  
         [0026]     The video data signal  712  indicates the gray level of red, green, and blue pixels. The data inversion driving circuit  710  employs priority control signal  714  to convert the video data signal  712  into pixel data signal with a desired inversion driving pattern. A pixel data signal has either positive polarity or negative polarity depending on whether the voltage of the pixel data signal is higher or lower than a common electrode voltage Vcom. A pixel data signal has positive polarity when its voltage level is higher than the common electrode voltage Vcom. Likewise, a pixel data signal has negative polarity when its voltage is lower than the common electrode voltage Vcom. The light transmission from liquid crystal materials (the gray level presented by a pixel) is related to the difference between the voltage of the pixel data signal and the common electrode voltage Vcom, regardless of the polarity of the pixel data signal. However, a pixel data signal having the positive polarity causes liquid crystal molecules to turn to a direction opposite to that caused by a pixel data signal having the negative polarity.  
         [0027]      FIG. 8A  shows an exemplary embodiment of an inversion driving pattern in a specific time frame. The pixels of each scanning line are divided into groups of three (3) successive pixels, which are respectively red, green, and blue color pixels. The polarities of the respective pixel data signals for the data lines within each group are the same as each other. For example, the polarity of pixel data signals for pixels R 1 , G 1 , B 1  in the first scanning line is the same, i.e. all are positive. The polarity of the respective pixel data signals for each successive group along the scanning lines alternates between a first polarity and a second polarity. For example, the polarities of pixel data signals for pixels R 2 , G 2 , B 2  in the first scanning line are the same as each other, but the polarity of R 2 , G 2 , B 2  is negative which is different from that of the adjacent pixel group (R 1 , G 1 , B 1 ). The polarities of pixel data signals for pixels R 3 , G 3 , B 3  in the first scanning line are the same as each other, but the polarity of R 3 , G 3 , B 3  alternates back to the positive.  
         [0028]     In one embodiment, the inversion driving pattern can be generated by assigning a polarity of the respective pixel data signals for the data lines within each group to be the same as each other and assigning the polarity of the respective pixel data signals for each successive group along the same scanning line to alternate between a first polarity and a second polarity. The data inversion driving circuit  710  then provides pixel data signals to the data lines.  
         [0029]     In a given time frame, the polarity of the respective pixel data signals for each successive group in successive scanning lines and within the same data lines alternates between the first polarity and the second polarity. For example, the polarity of pixel data signals for the pixel group (R 1 , G 1 , B 1 ) in the first scanning line is positive. The polarity of pixel data signal for the successive pixel group (R 1 , G 1 , B 1 ) in the second scanning line is negative which is different from that of the first scanning line. The polarity of pixel data signal for the next successive pixel group (R 1 , G 1 , B 1 ) in the third scanning line alternates back to the positive. In one embodiment, the polarity of the respective pixel data signals for each successive group in successive scanning lines and within the same data lines is assigned by the data inversion driving circuit  710  to alternate between the first polarity and the second polarity.  
         [0030]      FIG. 8B  shows an exemplary embodiment of a inversion driving pattern in a time frame succeeding that shown in  FIG. 8A . The polarity of the respective pixel data signals for each group in successive frames alternates between the first polarity and the second polarity. For example, in  FIG. 8A  the polarity of pixel data signal for pixel group (R 1 , G 1 , B 1 ) in the first scanning line is positive. In the next successive time frame as shown in  FIG. 8B , the polarity of pixel data signal for the same pixel group (R 1 , G 1 , B 1 ) in the first scanning line is negative, which is different from that in the immediately preceding frame shown in  FIG. 8A . In one embodiment, the data inversion driving circuit  710  assigns the polarity of the respective pixel data signals for any given group in successive frames to alternate between the first polarity and the second polarity.  
         [0031]      FIG. 9A  shows another embodiment of an inversion driving pattern in a specific time frame where the pixels of each scanning line are divided into groups of six (6) successive pixels. Polarity of the respective pixel data signals for pixels within any given pixel group are the same as each other. For example, the polarities of pixel data signal for pixels R 1 , G 1 , B 1 , R 2 , G 2 , B 2  (the first pixel group) in the first scanning line are the same as each other; all are positive. The polarities of the respective pixel data signals for each successive group along the same scanning line alternate between a first polarity and a second polarity. For example, the polarities of pixel data signal for pixels R 3 , G 3 , B 3 , R 4 , G 4 , B 4  (the second pixel group) in the first scanning line are the same as each other, but the polarity is negative, which is different from that of the first pixel group (R 1 , G 1 , B 1 , R 2 , G 2 , B 2 ).  
         [0032]     In any given time frame, the polarity of the respective pixel data signals for each successive group in successive scanning line and within the same data lines alternates between the first polarity and the second polarity. For example, the polarity of pixel data signals for the pixel group (R 1 , G 1 , B 1 , R 2 , G 2 , B 2 ) in the first scanning line is positive. The polarity of pixel data signals for the successive pixel group (R 1 , G 1 , B 1 , R 2 , G 2 , B 2 ) in the second scanning line is negative which is different from that of the first scanning line. The polarity of pixel data signals for the next successive pixel group (R 1 , G 1 , B 1 , R 2 , G 2 , B 2 ) in the third scanning line alternates back to the positive.  
         [0033]      FIG. 9B  shows an inversion driving pattern in a time frame immediately succeeding that in  FIG. 9A . The polarity of the respective pixel data signals for any given group in successive frames alternates between the first polarity and the second polarity. For example, in  FIG. 9A  the polarity of pixel data signals for pixel group (R 1 , G 1 , B 1 , R 2 , G 2 , B 2 ) in the first scanning line is positive. In the next successive time frame as shown in  FIG. 9B , the polarity of pixel data signals for the same pixel group (R 1 , G 1 , B 1 , R 2 , G 2 , B 2 ) in the same scanning line is negative which is different from that in the previous successive frame shown in  FIG. 8A .  
         [0034]     As shown in  FIG. 10 , another embodiment of an inversion driving pattern divides the pixels in a scanning line into groups of nine (9) successive pixels. Similarly,  FIG. 11  shows another embodiment which divides the pixels in a scanning line into groups of two (2) successive pixels. Although the pixels in a scanning line can be divided into groups of N successive pixels as long as N is an integer greater than one (1), the number of total pixels in a scanning line does not have to be a multiple of N.  
         [0035]     In  FIG. 12 , an exemplary embodiment employs signals POL 1  and POL 2  as priority control signal  714  to generate the inversion driving pattern. Skilled artisans will appreciate many other ways to generate an inversion driving pattern.  
         [0036]      FIG. 13  shows an exemplary embodiment of an LCD with a data line between two successive pixel groups that is wider than data lines within each pixel group. This embodiment is driven by an inversion driving pattern as shown in  FIG. 8A . A TFT layer  1320  with data lines  1330 ,  1332 ,  1334 ,  1336 , and  1338  is disposed on a substrate  1310 . The pixel electrodes  1350 ,  1352 ,  1354 ,  1356 , and  1358  are disposed on the TFT layer  1320 . The liquid crystal material  1340  is filled underneath a common electrode  1360 . A color filter  1370  is disposed on the common electrode  1360 . Because the pixels are divided into groups of three (3) successive pixels, the pixel electrodes  1350 ,  1352 , and  1354  have positive polarity; the pixel electrodes  1356 ,  1358 , and  1360  have negative polarity. Although there is no edge electric field between pixel electrodes  1350  and  1352 , or between pixels  1352  and  1354 , an edge electric field between pixel electrodes  1354  and  1356  could cause light leakage. As a result, the data line  1334  is wider to eliminate the light leakage.  
         [0037]     In the embodiment of  FIG. 13 , every third data line is wider to accommodate having groups of three pixels as shown in  FIGS. 8A and 8B . One of ordinary skill will understand that for any group size N, where N pixels within each group have the same polarity and successive groups alternate in polarity, every Nth data line can be made wider to eliminate light leakage. Thus, by making N greater than one, light leakage can be eliminated without a severe reduction in aperture ratio.  
         [0038]     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.