Patent Publication Number: US-8542171-B2

Title: Liquid crystal display and driving method thereof

Description:
This application claims the benefit of Taiwan Patent Application Serial No. 95135024, filed Sep. 21, 2006, the subject matter of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to a liquid crystal display and a driving method thereof, and more particularly to a color sequence liquid crystal display and a driving method thereof. 
     2. Description of the Related Art 
     With the rapidly developed image display technology, the liquid crystal display, which is thin and light weighted and has the low electromagnetic radiation, has become a mainstream display product. 
     A color sequence liquid crystal display sequentially displays three primary color components of one pixel to represent the color. Three light-emitting sources for respectively outputting red, green, and blue light serve as a backlight source for each pixel of this color sequence liquid crystal display. In one frame time, sub-pixels of the pixels sequentially display three sets of data, and respectively and correspondingly output the red, green, and blue light. A person can recognize the color of this pixel according to his/her persistence of vision. 
     However, the color sequence liquid crystal display has to feed one set of image data to the pixel in three times. So, the driving frequency of the pixel has to be increased from the original 60 Hz to 180 Hz. As for the color sequence liquid crystal display, the driving frequency of the pixel is increased to 180 Hz. That is, the driving voltage for the liquid crystal has to be updated every 5.56 milliseconds (ms). The time of 5.56 ms includes the time when the backlight module lights up, and the liquid crystal molecule has to finish the response before the backlight module lights up, so the allowable response time of the liquid crystal molecule is substantially shorter than 5.56 ms. 
       FIG. 1A  shows a relationship between time and a transmittance of a conventional liquid crystal display panel. As shown in  FIG. 1A , T 1  represents a transmittance of a first pixel, and Tn represents a transmittance of an N th  pixel. The first pixel is located in a first row of pixels of the pixel array, and the N th  pixel is located in an N th  row of pixels of the pixel array. First, a corresponding pixel voltage is provided to the first pixel to make a maximum transmittance of the first pixel substantially equal a predetermined transmittance T Max . Then, a corresponding pixel voltage is provided to the N th  pixel at a time point t 1  to make a maximum transmittance of the N th  pixel substantially equal the predetermined transmittance T Max . Thereafter, the backlight module is turned on between time points t 2  to t 3 . As shown in  FIG. 1A , at the time point t 2  when the backlight module is turned on, the liquid crystal molecules of the N th  pixel do not respond completely. That is, the transmittance of the N th  pixel at time point t 2  is substantially smaller than the predetermined transmittance T Max . The pixels in different rows receive the pixel voltages at different time points, and the pixel closer to the bottom of the panel receives the pixel voltage later, so the liquid crystal molecules thereof respond later. When the backlight module is turned on, the liquid crystal molecules on the bottom of the panel have not responded completely yet while the liquid crystal molecules on the top of the panel have responded completely, so the upper and lower portions of the liquid crystal display panel have different luminance. 
       FIG. 1B  shows gamma curves of the liquid crystal display panel of  FIG. 1A . In addition, as shown in  FIG. 1B , the difference between the transmittances of the first pixel P 1  and the N th  pixel Pn at the first reference gray level G 1  is ΔL 1 , and the difference between the transmittances of the first pixel P 1  and the N th  pixel Pn at the second reference gray level G 2  is ΔL 2 . As shown in  FIG. 1B , ΔL 1  and ΔL 2  are far greater than zero, which means that the gamma curve of the first pixel P 1  are not coincide with the gamma curve of the first pixel P 2 , which leads to the shifting of gamma curve. 
     It is a subject of the panel manufacturer to improve the phenomenon of the non-uniform luminance of the liquid crystal display panel caused by the fact that the pixels in different rows are scanned and enabled in different time points, and thus to reduce the shifting of the gamma curve. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a liquid crystal display and a driving method thereof to improve the phenomenon of the non-uniform luminance of a liquid crystal display panel and reduce the shifting of the gamma curve. 
     According to a first aspect of the present invention, a driving method applied to a liquid crystal display is provided. The liquid crystal display includes a backlight module and a pixel array which includes a first row of pixels and a second row of pixels. The method includes the following steps. First, a first pixel voltage is outputted to a first pixel in a first row of pixels in order to change a transmittance of the first pixel. Next, a second pixel voltage is outputted to a second pixel in a second row of pixels to change a transmittance of the second pixel. Then, a backlight module is turned on. Next, at a first predetermined time point after the first pixel voltage is outputted, a pixel electrode voltage of the first pixel is adjusted in order to lower the transmittance of the first pixel. After that, at a second predetermined time point after the second pixel voltage is outputted, a pixel electrode voltage of the second pixel is adjusted in order to lower the transmittance of the second pixel. The second predetermined time point follows the first predetermined time point. When the first pixel voltage substantially equals the second pixel voltage, an integrated value of the transmittance of the first pixel over time in a lighting period of the backlight module is a first light intensity value and an integrated value of the transmittance of the second pixel over the time in the lighting period of the backlight module is a second light intensity value. The difference between the first light intensity value and the second light intensity value is substantially smaller than 20% of the first light intensity value 
     According to a second aspect of the present invention, a liquid crystal display is provided. The liquid crystal display includes a backlight module and a pixel array. The pixel array includes a first row of pixels and a second row of pixels. A first pixel in the first row of pixels receives a first pixel voltage to change a transmittance of the first pixel. A second pixel in the second row of pixels receives a second pixel voltage to change a transmittance of the second pixel. The first row of pixels and the second row of pixels are sequentially driven. At a first predetermined time point after the first pixel receives the first pixel voltage, a pixel electrode voltage of the first pixel is adjusted in order to lower the transmittance of the first pixel. At a second predetermined time point after the second pixel receives the second pixel voltage, a pixel electrode voltage of the second pixel is adjusted in order to lower the transmittance of the second pixel. The second predetermined time point follows the first predetermined time point. When the first pixel voltage substantially equals the second pixel voltage, an integrated value of the transmittance of the first pixel over time in a lighting period of the backlight module is a first light intensity value, and an integrated value of the transmittance of the second pixel over the time in the lighting period of the backlight module is a second light intensity value. The difference between the first light intensity value and the second light intensity value is substantially smaller than 20% of the first light intensity value. 
     The invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a relationship between time and a transmittance of a conventional liquid crystal display panel. 
         FIG. 1B  shows gamma curves of the liquid crystal display panel of  FIG. 1A . 
         FIG. 2  is a schematic illustration showing a liquid crystal display according to a first embodiment of the invention. 
         FIG. 3  is a schematic illustration showing a pixel array according to the first embodiment of the invention. 
         FIG. 4  is a flow chart showing a driving method of the liquid crystal display according to the first embodiment of the invention. 
         FIG. 5A  shows a first example of a relationship between time and a transmittance of a pixel when the driving method of the liquid crystal display according to the first embodiment of the invention is applied. 
         FIGS. 5B and 5C  show second and third examples of relationships between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the first embodiment of the invention is applied. 
         FIGS. 5D and 5E  show fourth and fifth examples of relationships between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the first embodiment of the invention is applied. 
         FIG. 5F  shows a sixth example of a relationship between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the first embodiment of the invention is applied. 
         FIG. 6  shows gamma curves of the liquid crystal display according to the first embodiment of the invention. 
         FIG. 7A  is a schematic illustration showing a liquid crystal display panel according to a second embodiment of the invention. 
         FIG. 7B  is a cross-sectional view showing a portion of the liquid crystal display panel according to the second embodiment of the invention. 
         FIG. 8  is a flow chart showing a driving method according to the second embodiment of the invention. 
         FIG. 9A  shows a first example of a relationship between time and a transmittance of a pixel when the driving method of the liquid crystal display according to the second embodiment of the invention is applied. 
         FIGS. 9B and 9C  show second and third examples of relationships between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the second embodiment of the invention is applied. 
         FIGS. 9D and 9E  show fourth and fifth examples of relationships between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the second embodiment of the invention is applied. 
         FIG. 9F  shows a sixth example of a relationship between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the second embodiment of the invention is applied. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The liquid crystal display of the invention almost equalizes the light intensity values of different pixels by adjusting the time points of turning on and off the backlight module so as to improve the phenomenon of the non-uniform luminance of the liquid crystal display panel and to reduce the shifting of the gamma curve. 
     First Embodiment 
       FIG. 2  is a schematic illustration showing a liquid crystal display  200  according to a first embodiment of the invention.  FIG. 3  is a schematic illustration showing a pixel array according to the first embodiment of the invention. Referring to  FIGS. 2 and 3 , the liquid crystal display  200  includes a backlight module  210  and a liquid crystal display panel  220  having a pixel array  230 . The pixel array  230  includes many rows L 1  to Ln of pixels, which are respectively coupled to a scan driving circuit  240  through scan lines  242  and respectively coupled to a data driving circuit  250  through data lines  252 . The scan driving circuit  240  provides a scan voltage to these pixels, while the data driving circuit  250  provides a pixel voltage to the corresponding pixels in order to change the transmittances of these pixels. 
       FIG. 4  is a flow chart showing a driving method of the liquid crystal display according to the first embodiment of the invention.  FIG. 5A  shows a first example of a relationship between time and a transmittance of a pixel when the driving method of the liquid crystal display according to the first embodiment of the invention is applied. Referring to  FIGS. 4 and 5A , step  410  is performed at a first time point t 1  to enable a first pixel P 1  in the first row of pixels L 1  to receive a first pixel voltage outputted from the data driving circuit  250  in order to change the transmittance T 1  of the first pixel. Next, step  420  is performed at a second time point t 2  to enable an N th  pixel Pn in the N th  row of pixels Ln to receive an n th  pixel voltage outputted from the data driving circuit  250  in order to change the transmittance Tn of the N th  pixel. Then, step  430  is performed to turn on the backlight module  210 . Next, step  440  is performed at a first predetermined time point t 3  after the data driving circuit  250  outputs the first pixel voltage, and the data driving circuit  250  outputs a first compensation signal to the first pixel P 1  to adjust a pixel electrode voltage of the first pixel in order to lower the transmittance T 1  of the first pixel. Then, step  450  is performed at an n th  predetermined time point t 4  after the data driving circuit  250  outputs the n th  pixel voltage, and the data driving circuit  250  outputs an n th  compensation signal to the N th  pixel Pn to adjust a pixel electrode voltage of the N th  pixel in order to lower the transmittance Tn of the N th  pixel. 
     It is assumed that the first pixel voltage received by the first pixel is substantially equal to the n th  pixel voltage received by the N th  pixel in the first embodiment. Under this condition, the maximum transmittance T 1  of the first pixel substantially equals the maximum transmittance Tn of the N th  pixel, such as the maximum transmittance T Max  shown in  FIG. 5A . In a lighting period Δt ON  of the backlight module  210 , an integrated value of the transmittance T 1  of the first pixel over the time is a first light intensity value B 1 , and an integrated value of the transmittance Tn of the N th  pixel over the time is an N th  light intensity value Bn. Preferably, a difference between the first light intensity value B 1  and the N th  light intensity value Bn is smaller than 20% of the first light intensity value B 1 . More preferably, the first light intensity value B 1  substantially equals the N th  light intensity value Bn. 
     Preferably, the backlight module  210  is turned on after the transmittance T 1  of the first pixel is greater than zero to enter a lighting state, and the backlight module  210  is turned off before the transmittance Tn of the N th  pixel is substantially equal to zero to enter a darkening state. More preferably, the backlight module  210  is turned on after the transmittance T 1  of the first pixel is greater than zero and before the N th  pixel has the maximum transmittance to enter the lighting state; and the backlight module  210  is turned off after the first pixel has the maximum transmittance and before the transmittance of the N th  pixel is substantially equal to zero to enter the darkening state. 
       FIGS. 5B and 5C  show second and third examples of relationships between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the first embodiment of the invention is applied. As shown in  FIGS. 5B to 5C , what is different from the example of  FIG. 5A  is that the lighting period Δt ON  of backlight module  210  includes a first sub-period Δt b1  and a second sub-period Δt b2 . The backlight module  210  keeps a first luminance in the first sub-period Δt b1 , and keeps a second luminance in the second sub-period Δt b2 . The first luminance may be unequal to the second luminance. As shown in  FIG. 5B , the first luminance is greater than the second luminance. As shown in  FIG. 5C , the first luminance may also be smaller than the second luminance. 
       FIGS. 5D and 5E  show fourth and fifth examples of relationships between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the first embodiment of the invention is applied. As shown in  FIGS. 5D to 5E , what is different from the example of  FIG. 5B  is that the lighting period Δt ON  of backlight module  210  includes a first sub-period Δt c1 , a second sub-period Δt c2  and a third sub-period Δt c3 . The backlight module  210  keeps a first luminance in the first sub-period Δt c1 , keeps a second luminance in the second sub-period Δt c2 , and keeps a third luminance in the third sub-period Δt c3 . The first luminance is unequal to the third luminance and the second luminance almost equals zero. As shown in  FIG. 5D , the first luminance is greater than the third luminance. As shown in  FIG. 5E , the first luminance may also be smaller than the third luminance. 
       FIG. 5F  shows a sixth example of a relationship between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the first embodiment of the invention is applied. As shown in  FIG. 5F , what is the same as the example of  FIG. 5D  is that the lighting period Δt ON  of backlight module  210  includes a first sub-period Δt d1 , in which the backlight module  210  keeps a first luminance, a second sub-period Δt d2 , in which the backlight module  210  keeps a second luminance and a third sub-period Δt d3 , in which the backlight module  210  keeps a third luminance. However, what is different from the example of  FIG. 5D  is that the first luminance substantially equals the third luminance. 
       FIG. 6  shows gamma curves of the liquid crystal display according to the first embodiment of the invention. Taking the adjusting method of the backlight module of  FIG. 5A  as an example, when the liquid crystal display  200  adjusts the time points of turning on/off the backlight module  210 , the transmittance of the first pixel P 1  is almost equal to the transmittance of the N th  pixel Pn at a first reference gray level G 1 ; and the transmittance of the first pixel P 1  is almost equal to the transmittance of the N th  pixel Pn at a second reference gray level G 2 . Thus, the driving method of the liquid crystal display according to this embodiment can effectively reduce the shifting of the gamma curve comparing to the conventional liquid crystal display. 
     Second Embodiment 
       FIG. 7A  is a schematic illustration showing a liquid crystal display panel according to a second embodiment of the invention. As shown in  FIG. 7A , what is different from the first embodiment is that a display area of a pixel array  710  in a liquid crystal display panel  700  is divided into multiple sub-display areas A 1  to An. Each sub-display area includes multiple rows of pixels, which are respectively coupled to a scan driving circuit  720  and a data driving circuit  730 . In addition, first ends of storage capacitors of the rows of pixels in the first sub-display area A 1  are coupled to a first common electrode line CL 1 , first ends of storage capacitors of the rows of pixels in the second sub-display area A 2  are coupled to a second common electrode line CL 2 , and first ends of storage capacitors of the rows of pixels in the N th  sub-display area An are coupled to an N th  common electrode line CLn. A control circuit  740  is provided to adjust voltages of common electrode lines V com1  to V comN . 
       FIG. 7B  is a cross-sectional view showing a portion of the liquid crystal display panel according to the second embodiment of the invention. Referring to  FIG. 7B , the liquid crystal display panel  700  includes an upper substrate  10 , a thin film transistor substrate  20  and a liquid crystal layer  30 . The upper substrate  10  includes a reference electrode  12 , and the thin film transistor substrate  20  includes a pixel electrode  22  and a common electrode  24 . The liquid crystal layer  30  is disposed between the upper substrate  10  and the thin film transistor substrate  20 . The reference electrode  12 , the pixel electrode  22  and the liquid crystal layer  30  of the upper substrate are equivalent to a pixel capacitor, and the storage capacitor is disposed between the pixel electrode  22  and the common electrode  24 . The pixel electrode  22  is coupled to the common electrode  24 , and the common electrode  24  is coupled to the control circuit  740  through the first common electrode line CL 1 . When the voltage V com1  of the first common electrode line CL 1  outputted from the control circuit  740  changes, the change of the voltage V com1  is coupled to the pixel electrode  22  through the storage capacitor so that the voltage of the corresponding pixel electrode  22  correspondingly changes in order to change the transmittance of the pixel. 
     Next, illustrations will be made to explain how to effectively solve the non-uniform luminance of the panel, according to the second embodiment of the invention, by taking the first pixel and the N th  pixel as an example, and a method is further provided to enhance the luminance effectiveness of the liquid crystal display panel. The first pixel P 1  is located in the first sub-display area A 1  and coupled to a first scan line G 1 , and the N th  pixel Pn is located in the N th  sub-display area An and coupled to an N th  scan line Gn.  FIG. 8  is a flow chart showing a driving method according to the second embodiment of the invention.  FIG. 9A  shows a first example of a relationship between time and a transmittance of a pixel when the driving method of the liquid crystal display according to the second embodiment of the invention is applied. Referring to  FIGS. 8 and 9A , step  810  is first performed at a first time point t 1  so that the first scan line G 1  is enabled to turn on the thin film transistor of the first pixel P 1 . Next, the turned-on first pixel P 1  receives the first pixel voltage to change the transmittance T 1  of the first pixel P 1 . Thereafter, the scan lines G 2  to Gn−1 are sequentially enabled. Then, step  820  is performed at a second time point t 2  so that the N th  scan line Gn is enabled to turn on the thin film transistor of the N th  pixel. Next, the turned-on N th  pixel receives the n th  pixel voltage to change the transmittance Tn of the N th  pixel. Then, step  830  is performed to turn on a backlight module. At a third time point t 3 , the first pixel has reached the maximum transmittance T Max , that is, the response of the liquid crystal molecules in the first pixel has been completed. At a fourth time point t 4 , the N th  pixel has reached the maximum transmittance T Max , that is, the response of the liquid crystal molecules in the N th  pixel has been completed. Thereafter, step  840  is performed at a first predetermined time point t 5  after the data driving circuit  730  outputs the first pixel voltage to adjust the voltage V com1  of the first common electrode line CL 1  through the control circuit  740  and to adjust the pixel electrode voltage V p1  of the first pixel P 1  in order to lower the transmittance T 1  of the first pixel. After time point t 6 , the thin film transistor of the first pixel P 1  is again turned on to receive a next pixel voltage so that the transmittance of the first pixel P 1  changes again. Thereafter, the control circuit  740  sequentially adjusts the voltages of the other common electrode lines so as to adjust the pixel electrode voltages of the corresponding pixels and thus to lower the transmittances of the corresponding pixels. Then, step  850  is performed at a second predetermined time point t 7  after the data driving circuit  730  outputs the N th  pixel voltage so as to adjust the voltage V comN  of the N th  common electrode line CLn by the control circuit  740 , and to adjust the pixel electrode voltage V pN  of the N th  pixel to lower the transmittance TN of the N th  pixel. The time points of turning on/off the backlight module of this embodiment may be selected according to the rule the same as that for the backlight module of the first embodiment, so detailed descriptions thereof will be omitted. 
     Similar to the first embodiment, it is assumed that the first pixel voltage received by the first pixel substantially equals the second pixel voltage received by the N th  pixel in the second embodiment. Under this condition, the maximum transmittance T 1  of the first pixel substantially equals the maximum transmittance Tn of the N th  pixel, as shown in the maximum transmittance T Max  of  FIG. 9A . In the lighting period Δt ON  of backlight module  210 , the integrated value of the transmittance T 1  of the first pixel over the time is the first light intensity value B 1 , and the integrated value of the transmittance Tn of the N th  pixel over the time is the N th  light intensity value Bn. Preferably, the difference between the first light intensity value B 1  and the N th  light intensity value Bn is smaller than 20% of the first light intensity value B 1 . More preferably, the first light intensity value B 1  substantially equals the N th  light intensity value Bn. 
       FIGS. 9B and 9C  show second and third examples of relationships between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the second embodiment of the invention is applied. As shown in  FIGS. 9B to 9C , what is different from the example of  FIG. 9A  is that the lighting period Δt ON  of backlight module  210  includes a first sub-period Δt b1  and a second sub-period Δt b2 . The backlight module keeps the first luminance in the first sub-period Δt b1 , and keeps the second luminance in the second sub-period Δt b2 . The first luminance is unequal to the second luminance. As shown in  FIG. 9B , the first luminance is greater than the second luminance. As shown in  FIG. 9C , the first luminance may also be smaller than the second luminance. 
       FIGS. 9D and 9E  show fourth and fifth examples of relationships between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the second embodiment of the invention is applied. As shown in  FIGS. 9D to 9E , what is different from the example of  FIG. 9B  is that the lighting period Δt ON  of backlight module  210  includes a first sub-period Δt c1 , a second sub-period Δt c2  and a third sub-period Δt c3 . The backlight module keeps a first luminance in the first sub-period Δt c1 , keeps a second luminance in the second sub-period Δt c2 , and keeps a third luminance in the third sub-period Δt c3 . The first luminance is unequal to the third luminance, and the second luminance almost approaches zero. As shown in  FIG. 9D , the first luminance is greater than the third luminance. As shown in  FIG. 9E , the first luminance may also be smaller than the third luminance. 
       FIG. 9F  shows a sixth example of a relationship between the time and the transmittance of the pixel when the driving method of the liquid crystal display according to the second embodiment of the invention is applied. Referring to  FIG. 9F , what is the same as the example of  FIG. 9D  is that the lighting period Δt ON  of backlight module  210  includes a first sub-period Δt d1 , in which the backlight module keeps a first luminance, a second sub-period Δt d2 , in which the backlight module keeps a second luminance, and a third sub-period Δt d3 , in which the backlight module keeps a third luminance. However, what is different form the example of  FIG. 9D  is that the first luminance substantially equals the third luminance. 
     Compared  FIG. 9A  with  FIG. 5A , the following conditions may be obtained. In  FIG. 5A , before a next frame display signal is driven, the scan lines cannot be enabled to write the next frame display signal until the compensation signals of all pixels are sequentially written. In  FIG. 9A , the first pixel P 1  of this embodiment does not have to wait for the finished signal compensating operations of the pixels in other areas (e.g., the N th  pixel Pn), and the first scan line G 1  can be enabled to drive the first pixel P 1  to receive the next pixel voltage. Consequently, the next pixel voltage signal could be provided while the signal compensating operations are performed, so that the time of keeping the maximum transmittance of the pixels can be lengthened and the light intensity value of each pixel can be enhanced to avoid the luminance loss of the liquid crystal display panel. Similarly, the driving method of the liquid crystal display in  FIGS. 9B to 9D  may also avoid the luminance loss of the liquid crystal display panel, and detailed description thereof will be omitted. 
     The conventional color sequence liquid crystal display has the liquid crystal molecules with the shorter response time (about 5.56 ms) so that not both of the liquid crystal molecules in the upper and lower rows of pixels of the liquid crystal display panel response completely when the backlight module are turned on, the liquid crystal display panel has the phenomenon of the non-uniform luminance, and shifting of the gamma curve occurs. In the liquid crystal displays according to the embodiment of the invention, adjusting the time points of turning on and off the backlight module can substantially equalize the integrated values of the transmittances of the pixels over the time during the period when the backlight module lights up. So, the problems of the non-uniform luminance of the liquid crystal display panel and the shifting of the gamma curve can be effectively solved. In addition, adjusting the voltage of the common electrode line can lengthen the response time of the liquid crystal molecules so that the luminance of all the pixels can be further increased to avoid the luminance loss of the liquid crystal display. 
     While the invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.