Patent Publication Number: US-2006012551-A1

Title: Liquid crystal display with an image flicker eliminaiton function applied when power-on and an operation method of the same

Description:
FIELD OF THE INVENTION  
      The present invention relates to a method of improving displaying quality of a liquid crystal display. More particularly, the present invention relates to a method of avoiding flickers and residual images generating at the time of turning on and turning off the liquid crystal display.  
     BACKGROUND OF THE INVENTION  
      A liquid crystal display takes the place of a cathode ray tube (CRT) due to its features of compact, light, thin, small and radiation-free. The LCD technology is widely applied on high-resolution digital televisions, laptops, personal digital assistants, notebooks, digital cameras and mobile phones.  
      Referring to  FIG. 1 , a single pixel  1  includes a thin film transistor (TFT)  2  and a liquid crystal (LC) capacitor  3 . The source of the TFT  2  is electrically connected to the data line  4 , and the gate of the TFT  2  is electrically connected to the scan line  5 . An electrode layer of the LC capacitor  3  is connected with the drain of the TFT  2 . The other electrode layer of the LC capacitor  3  is supplied with a common voltage (V com ).  
      The LC capacitor  3  comprises a common electrode (not shown herein), a pixel electrode (not shown herein) and an LC layer (not shown herein), wherein the LC layer is sandwiched between the common electrode and the pixel electrode. The common electrode is supplied with the V com , and the pixel electrode is fed with an operation voltage (V o ). The voltage difference between the V com  and the V o  generates an electrical field capable of driving LC molecules within the LC layer to align. Generally speaking, certain properties of the LC molecules, e.g. alignments responsive to the electrical field, will be ruined when the polarity of the voltage is fixed. To avoid the phenomenon, accordingly, different polarities of the V o , i.e. the alternating current (AC), are applied to drive the LC molecules.  
      As shown in  FIG. 2 , when an LCD displays a static image, the V o  applied onto the pixel electrode exhibits positive cycle and negative cycle by turns. The positive cycle indicates the V o  is greater than the V com , whereas the negative cycle means the V o  is smaller than the V com . To display the static image, the absolute value of the voltage difference exists between the V o  and V com  has to remain constant. That is, although alignments of the LC molecules alter responsively to the positive cycle and the negative cycle, the transparency of the LC layer will be consistent if the intensity of the electric field generated from the voltage difference is fixed.  
      Referring to  FIG. 3 , when the real common voltage (V com′ ) shifts from the ideal common voltage (V com ), the absolute value of the voltage difference between the V com′  and V o  will change correspondingly. For example, when the positive bias is occurring, the absolute value of the voltage difference between the V com′  and V o  respectively reduces and increase in the positive cycle region and the negative cycle region. In this case, the intensity of the electric field within the LC layer varies with fluctuations of the absolute value. As a result, the transparency of the LC layer cannot maintain consistent. Image flickers thus generate.  
      It is noted that electrical charges usually remains within the LC capacitor while powering off the display. Accordingly, residual images are generated on the display at the time of turning the display off. Referring to  FIG. 1 , When the electrical charges within the LC capacitor is not released completely, the bias, e.g. DC-bias, will occur and the V com  will be influenced, resulting in the image flickers described above on the LCD panel. In other words, if the electrical charges cannot be eliminated before restarting the display, the image flickers will generate at the time of restarting the LCD panel.  
      As concluded, the residual charge within the LC layer is needed to be released to avoid the residual images and flickers.  
     SUMMARY OF THE INVENTION  
      The present invention provides an LCD and a method to rapidly charge the LC capacitor when turning on the LCD.  
      The LCD with functions of eliminating flickers and residual images comprises an LCD panel, a power supply, an image control unit and a timing control unit. The LCD panel having a plurality of LC capacitors therein is used to display image. The power supply provides power to the LCD panel. The image control unit is used to control image signals inputted into the LCD panel. The timing control unit connected between the LCD panel and the image control unit controls the operational timing of the image control unit and the LCD panel. While powering on the LCD, the timing control unit notifies the LCD panel at a first predetermined timing to activate, and then notifies the image control unit at a second predetermined timing to provide a designed pattern to the LCD panel for rapidly charging the LC capacitor.  
      The method of preventing the generation of the flickers and the residual images comprises steps of turning on the power supply of the LCD, starting the LCD panel at the first predetermined timing after turning the power supply on, and providing a designed pattern to the LCD panel at the second predetermined timing for rapidly charging the LC capacitor. The LC layer within the LC capacitor is activated through the rapid charging. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated and understood by referencing the following detailed description in conjunction with the accompanying drawings, wherein:  
       FIG. 1  is a typically circuit diagram of a single pixel of an LCD.  
       FIG. 2  shows an oscillogram of operation voltage when a static image is displayed on the LCD.  
       FIG. 3  shows an oscillogram of operation voltage when flickers occurs on the LCD.  
       FIG. 4  shows the LCD in accordance with the present invention.  
       FIG. 5  shows a flowchart of a preferred method to avoid flickers occurring at the time of restarting the LCD in accordance with the present invention.  
       FIG. 6  shows an oscillogram regarding panel power and image signal corresponding to  FIG. 5 .  
       FIG. 7  shows a flowchart of another method to avoid flickers occurring at the time of restarting the LCD in accordance with the present invention.  
       FIG. 8  shows an oscillogram regarding panel power and image signal corresponding to  FIG. 7 .  
       FIG. 9A  is an oscillogram showing the variation of operation voltage when not providing a designed pattern to an LCD panel in accordance with the present invention.  
       FIG. 9B  is an oscillogram showing the variation of operation voltage when providing a designed pattern to the LCD panel in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PERFERRED EMBODIMENTS  
      The invention discloses a liquid crystal display (LCD) and a method of improving displaying quality of the LCD. The present invention is now described in detail below.  
      Referring to  FIG. 4 , a diagram of a preferred embodiment of the LCD  10  is shown. The LCD  10  comprises an LCD panel  20 , an image control unit  40  and a timing control unit  50 . The LCD panel  20  having a plurality of LC capacitors (not shown herein) is used to display images. The image control unit  40  connecting with the LCD panel  20  controls signals of images and transmits the signals to drivers of the LCD panel  20 , such as source drive and gate driver, for generating images on the LCD panel  20 . The timing control unit  50  connecting with the LCD panel  20  and image control unit  40  controls the operational sequence and timing of charging and discharging the LC capacitors.  
      As shown in the figure, the image control unit  40  includes an image generator  42 , a multiplexer  44  and a timing signal generator  46 . The image generator  42  is used to produce a designed pattern F. The multiplexer  44  connects with the image generator  42  and the timing control unit  50 , receiving the designed pattern F and an inputted normal image A. Subsequently, the multiplexer  44  selectively transmitting one of the designed pattern F and the normal image A to the LCD panel  20 . The timing signal generator  46  connecting with the image generator  42  provides a timing signal S of the designed pattern F, wherein the timing signal S is used to control scanning speed and displaying duration of the designed pattern F on the LCD panel  20 .  
      At the time of turning off the LCD  10 , the electrical charges usually remain within the LC capacitor of the LC panel  20 . The remaining electrical charges result in temporary flickers while restarting the LCD  10 . Accordingly, if the LC capacitor can be rapidly charged for activating the LC layer when restarting the LCD  10 , the flickers will be effectively eliminated.  
      To charge the LC capacitor, the image control unit  40  transmits signals of the designed pattern F to the LCD panel  20  after powering on the LCD  10 . The designed pattern F provides adequate electrical field to the LC capacitor of the LCD panel  20 , thus charging the LC capacitor in a short period of time. In one preferred embodiment of the present invention, the intensity of the electrical field provided by the designed pattern F is at least 90% of the intensity of maximal electrical field originally set to drive the LC molecules within the LC capacitor. In another embodiment, the intensity of the electrical field provided by the designed patterns for driving LC molecules within the LC layer is equal to maximal intensity of the electrical field originally set to drive the LC molecules. For example, in a 8-bits-per-pixel conventional LCD panel, the maximal electrical field indicates a driving voltage at the value of 255. Yet the intensity of the electrical field provided by the designed pattern F usually varies according to the types of LCDs.  
      For ensuring the complete activation of the LC molecules within the LC capacitor, the length of the designed image F ranges from 1 to 100 predetermined frames or about 10 ms to 1000 ms. It is noted that the designed pattern F varies with types of LCD panels. For example, when the LCD panel  20  is a twisted nematic (TN) LCD panel, the designed pattern F used to rapidly charge the LC capacitors is substantially a white image. While the LCD panel  20  is a multi-domain vertical alignment (MVA) LCD panel or an in-plane switching (IPS) LCD panel, the designed pattern F is substantially a black image.  
       FIG. 5  shows a flowchart of a preferred method to rapidly charge the LC capacitor. Descriptions of  FIG. 5  are simultaneously referred to  FIG. 4  and  FIG. 6 . As shown in  FIG. 5 , the power of the LCD is turned on firstly (operation  100 ). Thereafter, the timing control unit  50  notifies the LCD panel  20  at a first predetermined timing t 1  to actuate (operation  120 ). The timing control unit  50  then notifies the multiplexer  44  of the image control unit  40  at a second predetermined timing t 2  to transmit the signals the designed pattern F to the LCD panel  20  for rapidly charging the LC layer (operation  140 ). After displaying the designed pattern F on the LCD panel  20 , the timing control unit  50  notifies the multiplexer  44  of the image control unit  40  at a third predetermined timing t 3  to transmit signals of the normal image A to the LCD panel  20  (operation  160 ), thus initiating the usual operation of the LCD panel  20 . In a preferred embodiment of the present invention, the first predetermined timing t 1  is set at 100 ms at latest after turning on the LCD. In addition, the length of the designed pattern F (d 1 ) is between 20 ms and 100 ms. Further, the duration between the second predetermined timing t 2  and the third predetermined timing t 3  is equal to the length of the designed pattern F (d 1 ). In other embodiments, these parameters, i.e. t 1 , t 2 , t 3  and d 1 , vary with different designs of LCDs.  
      In the embodiment, it is the timing control unit  50  that controls and notifies the multiplexer  44  to transmit the signals of the normal image A to the LCD panel  20 . In another embodiment of the present invention, the multiplexer  44  can directly detect an ending signal generated when the display of the designed pattern F has been accomplished. Once the ending signal is detected, the multiplexer  44  switches the signals of the designed pattern F to the signals of the normal image A, and then transmits the signals of the normal image A to the LCD panel  20 .  
      The embodiments illustrated above are applied to a reflective LCD. Referring to  FIG. 7  and  FIG. 8 , other embodiments applied to a transmissive or a transflective LCD are disclosed herein. After powering on the LCD (operation  200 ), the timing control unit  50  notifies the LCD panel  20  at a first predetermined timing t 1  to actuate (operation  220 ). The timing control unit  50  then notifies the multiplexer  44  of the image control unit  40  at a second predetermined timing t 2  to provide the signals of the designed pattern F to the LCD panel  20  for rapidly charging the LC layer (operation  240 ). After displaying the designed pattern F on the LCD panel  20 , the timing control unit  50  notifies the multiplexer  44  at a third predetermined timing t 3  to transmit the signals of the normal image A to the LCD panel  20  (operation  260 ), thus initiating the usual operation of the LCD panel  20 . Subsequently, a backlight unit of the transmissive or the transflective LCD is turned on (operation  280 ). The timing of turning on the backlight unit is preferably set at 20 ms at earliest to 50 ms at latest after transmitting the signals of the normal image A to the LCD panel  20 .  
      Referring to  FIGS. 9A and 9B , the improvement on DC-bias via the application of the designed pattern F is shown. V o  means the operation voltage. The V com  indicates predetermined common voltage and V com″  is real common voltage.  FIG. 9A  is an oscillogram which shows the variation of V com″  while not providing the designed pattern F to the LCD panel  20  after powering on the LCD, whereas  FIG. 9B  is an oscillogram that shows the variation of V com″  when providing the designed pattern F to the LCD panel  20  after powering on the LCD. As shown in the figure, the designed pattern F can rapidly activate the LC molecules, thus accelerating the elimination of the DC-bias and shortening the appearance of the flickers on the LCD panel after powering on the LCD.  
      The present invention discloses methods of using a designed pattern F to rapidly charge LC capacitors when turning on the LCD panel. Therefore, the flickers generating at the time of turning on the LCD is eliminated and the displaying quality of the LCD is improved. Because the interference from the remaining electrical charges can be prevented, the flickers produced at the time of restarting the LCD within a short period of time are perfectly eliminated. Moreover, it is unnecessary to use other devices to have the image control unit provide a designed pattern to the LCD panel at a predetermined timing. As a result, the cost does not increase.  
      While the preferred embodiment of the invention has been illustrated and described, it is appreciated that modifications and variations can be made therein without departing from the spirit and scope of the invention.