Patent Publication Number: US-2006012552-A1

Title: Liquid crystal display with image flicker and shadow elimination functions applied when power-off 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  
      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 completely and rapidly release the charge within the LC layer when powering off the LCD, thus preventing residual images and flickers generating at the time of turning off and restarting the liquid crystal display.  
      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 power supply provides power to the LCD panel. The image control unit is used to control images input into the LCD panel. The timing control unit connected between the LCD panel and the image control unit. While powering off the LCD, the timing control unit notifies the image control unit to transmit a designed pattern to the LCD panel for rapidly releasing the charge within LC capacitors at a first predetermined timing, and then notifies the power supply to stop providing power to the LCD panel at a second predetermined timing.  
      The method of preventing the generation of the flickers and the residual images comprises steps of turning off the LCD, providing a signal of a designed pattern to the LCD panel at a first predetermined timing to rapidly release the residual charge within the LC capacitor, and cutting off power supplied to the LCD panel. 
    
    
     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. 8A  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. 8B  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 .  
      The electrical charges usually remain within the LC capacitor of the LC panel  20  at the time of turning off the LCD  10 . Therefore, a residual image generates on the LC panel  20  after powering the LCD  10  off. In another aspect, the remaining charges result in flickers when immediately restarting the LCD  10 . Accordingly, if the remaining charge can be rapidly and effectively released from the LC capacitor during turning off the LCD  10 , the residual image and flickers will be effectively eliminated.  
      To release the remaining charges within the LC capacitor, the image control unit  40  provides the designed image F to the LCD panel  20  after powering off the LCD  10 . The designed pattern F is used to charge and discharge the LCD layer of the LCD panel  20  for rapidly releasing the electrical charges that remains within the LC capacitor. In one preferred embodiment of the present invention, the intensity of the electrical field provided by the designed pattern F is at most 10% of the maximal intensity of original electrical field set to drive the LC molecules within the LC capacitor. In another embodiment, the intensity of the electrical field provided by the designed pattern F is substantially equal to the minimal intensity of original electrical field set to drive the LC molecules within the LC layer. For example, the maximal electrical field indicates a driving voltage at the value of 255 in a 8-bits-per-pixel conventional LCD panel. Yet the intensity of the electrical field provided by the designed pattern F usually varies according to the types of LCDs.  
      For ensuring the remaining charges within the LC capacitor are completely released, 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 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 release the remaining charges within 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 off at first (operation  100 ). Thereafter, the timing control unit  50  controls the multiplexer  44  of the image control unit  40  to stop sending the normal image A to the LCD panel  20  (operation  120 ). The timing control unit  50  then notifies the multiplexer  44  of the image control unit  40  at a first predetermined timing t 1  to transmit the signal of the designed pattern F generated by the image generator  42  to the LCD panel  20  (operation  140 ) for rapidly releasing the electrical charges remaining within the LC capacitor. It is noted that the multiplexer  44  will not send the normal image A to the LCD panel  20  again after displaying the designed pattern F on the LCD panel  20 . Next, the timing control unit  50  notifies a panel power supply of the LCD panel  20  at a second predetermined timing t 2  to stop providing the power (operation  160 ). In a preferred embodiment of the present invention, the first predetermined timing t 1  is set at 100 ms at latest after turning off the LCD; the length of the designed pattern F (d 1 ) is about 10 ms to 1000 ms; the duration between the first predetermined timing t 1  and the second predetermined timing t 2  is greater than the length of the designed pattern F (d 1 ). Moreover, the second predetermined timing t 2  is set at 50 ms at latest after stopping providing the designed pattern F. In other embodiments, these parameters vary with different types of LCDs.  
      The embodiments illustrated above are applied to a reflective LCD. Referring to  FIG. 6  and  FIG. 7 , other embodiments applied to a transmissive or a transflective LCD are disclosed herein. After powering off the LCD (operation  200 ), a backlight unit of the LCD is turned off (operation  220 ). Subsequently, the timing control unit  50  controls the multiplexer  44  of the image control unit  40  to stop sending the normal image A to the LCD panel  20  (operation  240 ). The timing control unit  50  then notifies the multiplexer  44  of the image control unit  40  at a first predetermined timing t 1  to transmit the signal of the designed pattern F generated by the image generator  42  to the LCD panel  20  (operation  260 ) for rapidly releasing the electrical charges remaining within the LC capacitor. Lastly, the timing control unit  50  notifies a panel power supply of the LCD panel  20  at a second predetermined timing t 2  to stop providing the power (operation  280 ).  
      Referring to  FIGS. 8A and 8B , an induction of a bias voltage induced by the remaining charges within the LC capacitor and an improvement of the bias voltage are shown respectively. It is clear that the remaining charges (V rem′ ) within the LC capacitor are rapidly released by displaying the designed pattern F on the LCD panel. That is, the designed pattern F displaying on the LCD panel rapidly charges and discharges the LC capacitor, accelerating the release of the remaining charges within the LC capacitor. In this case, the flickers and residual images generating at the time of turning on and turning off the LCD are effectively eliminated.  
      The present invention incorporates an additional step, i.e. displaying the designed pattern F on the LCD panel before stopping the power supply to the LCD panel, into the conventionally procedure of turning off the LCD. The step of displaying the designed pattern F on LCD panel can rapidly release the charge remaining within the LC capacitor. Accordingly, the flickers generating at the time of turning on the LCD and residual images generating at the time of turning off the LCD are eliminated, thus improving displaying quality of the LCD. The present invention uses a step of displaying the designed pattern F on LCD panel to improve displaying quality of the LCD. Consequently, the flickers, especially produced at the time of restart the LCD, can be perfectly eliminated. Moreover, the timing control unit of the present invention controls the image control unit and the LCD panel. 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.