Patent Publication Number: US-2009231259-A1

Title: Apparatus and method for eliminating image sticking of liquid crystal display

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 97108691, filed Mar. 12, 2008. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and a method for eliminating an image sticking of a liquid crystal display (LCD) when the LCD is turned off, and more particularly, to a method and an apparatus that provide a common voltage as a driving voltage of a data driver for eliminating an image sticking of an LCD when the LCD is turned off. 
     2. Description of Related Art 
     As optoelectronic and semiconductor technologies advance, thin film transistor liquid crystal displays (TFT-LCDs) with advantages of great space utilization efficiency, low power consumption, free radiation, and low electrical field interference have become the mainstream in the display device market. The TFT-LCDs have been widely applied to electronic products closely relevant to our daily lives, such as laptop computers, mobile phones, televisions, and so forth. Therefore, research efforts aiming to improve a display quality of the TFT-LCDs have been continuously made. 
     The TFT-LCD has no capacity of emitting light by itself so that a backlight module providing a light source is normally disposed within the TFT-LCD.  FIG. 1  is a schematic circuit diagram of a TFT-LCD. Referring to  FIG. 1 , a TFT-LCD  100  includes a scan driver  101 , a data driver  105 , and a plurality of pixel units  104 . Each of the pixel units  104  includes a TFT  106 , a liquid crystal capacitor  107 , and a storage capacitor  108 . The liquid crystal capacitor  107  and the storage capacitor  108  are commonly coupled to a common voltage Vcom. 
     As the TFT-LCD  100  displays frames, the scan driver  101  turns on scan lines  102  in sequence, such that the TFTs  106  on the same scan line  102  are conducted. In the meantime, the data driver  105  converts video signals into data driving voltages and output the data driving voltages to data lines  103 . Thereby, the liquid crystal capacitors  107  and the storage capacitors  108  on the turn-on scan lines  102  can receive and store the data driving voltages through the data lines  103 , such that the frame can be retained until the frame is updated to the next frame. 
     However, when the TFT-LCD  100  is turned off, electrical charges stored in the liquid crystal capacitors  107  and the storage capacitors  108  should be released after a period of time. Here, the residual charges may make an impact on the TFT-LCD  100 , such that liquid crystals disposed on a panel of the TFT-LCD  100  are rotated. At this time, a problem of an image sticking often occurs if the backlight is not completely off. To solve said problem of the image sticking, a reset circuit is disposed in the TFT-LCD  100  according to the pertinent art. At the moment that the TFT-LCD  100  is turned off, the scan driver  101  is controlled by the reset circuit for conducting all the TFTs  106  on each of the scan lines  102 . As such, the liquid crystal capacitors  107  and the storage capacitors  108  are discharged to the data lines  103  through the conducted TFTs  106 , so as to reduce the time during which the image sticking occurs. 
     Nevertheless, said solution still has the following drawbacks:
     1. The conducted TFTs  106  return to a semi-conductive state after a short period of time, and thus a speed of discharging the charges is significantly reduced.   2. Not all of the panels have the common voltages Vcom set as a ground voltage. As the common voltage Vcom is half of a voltage VDD, a relatively long period of time is required for discharging the common voltage to the ground voltage. The unstable voltage difference has an influence on the liquid crystal capacitors  107  and the storage capacitors  108 , and results in the rotation of the liquid crystals, thus giving rise to the image sticking.   

     Hence, how to effectively eliminate the image sticking of the LCD when the LCD is turned off has become one of the most important issues at this current stage. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and a method for eliminating an image sticking of an LCD when the LCD is turned off, so as to accelerate a discharging speed of residual charges and prevent a panel of the LCD from being affected by an unstable voltage difference. Thereby, the image sticking of the LCD can be avoided when the LCD is turned off. 
     The present invention provides an apparatus for eliminating an image sticking of an LCD when the LCD is turned off. The LCD includes a data driver and a plurality of pixels coupled to a common voltage. Said apparatus includes a voltage detector and a voltage switching device. The voltage detector detects a system voltage used to operate the LCD and outputs a control signal according to the system voltage. The voltage switching device is coupled to the voltage detector for selectively providing a data voltage or the common voltage as a driving voltage of the data driver according to the control signal. The voltage switching device is controlled by the control signal to provide the common voltage to the data driver when the system voltage is less than a reference voltage. 
     The present invention further provides a method for eliminating an image sticking of an LCD when the LCD is turned off. The LCD includes a data driver and a plurality of pixels coupled to a common voltage. In the method, a system voltage for operating the LCD is first detected. Next, the common voltage is provided as a driving voltage of the data driver when the system voltage is less than a reference voltage. 
     The present invention provides the common voltage serving as the driving voltage to the data driver when the LCD is detected to have been turned off. Here, the pixels are collectively coupled to the common voltage. At the moment that the LCD is turned off, an electrical field for controlling a rotation of liquid crystals is reduced to the lowest extent, and the residual charges in the storage capacitor are minimized. As such, discharging the residual charges can be expedited, and the image sticking of the LCD can be avoided when the LCD is turned off. 
     In order to make the aforementioned and other objects, features and advantages of the present invention more comprehensible, several embodiments accompanied with figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a schematic circuit diagram of a TFT-LCD. 
         FIG. 2  is a circuit diagram of a display apparatus according to an embodiment of the present invention. 
         FIG. 3  is a flowchart illustrating a method for eliminating an image sticking of a display apparatus when the display apparatus is turned off according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In general, storage capacitors and liquid crystal capacitors in an LCD are used to store charges, such that a frame can be retained until the frame is updated to the next frame. As the LCD is turned off, a period of time is required for completely releasing the stored charges, and thus the residual charges may result in an image sticking on a panel of the LCD. On the other hand, at the moment that the LCD is turned off, a common voltage coupled to both the storage capacitors and the liquid crystal capacitors may give rise to the image sticking on the panel of the LCD due to an unstable voltage, difference. In light of the foregoing, the present invention is exemplified for resolving the issue of the image sticking of the LCD when the LCD is turned off. Before the present invention is elaborated by the following embodiments, it is first assumed that a display apparatus discussed herein is a transmissive LCD. 
       FIG. 2  is a schematic circuit diagram of a display apparatus according to an embodiment of the present invention. Referring to  FIG. 2 , a display apparatus  200  includes a plurality of pixels  201 , a voltage detector  202 , a voltage switching device  203 , a data driver  204 , and a scan driver  205 . Each of the pixels  201  includes a TFT T 1  and a capacitor unit  206  composed of a liquid crystal capacitor C LC  and a storage capacitor C st . 
     The voltage detector  202  detects a system voltage Vsys, which is used to operate the display apparatus  200 , and outputs a control signal CON according to a comparison result of the system voltage Vsys and a reference voltage. The voltage switching device  203  is coupled to the voltage detector  202  for selectively providing a data voltage VDDA or the common voltage Vcom to the data driver  204  according to the control signal CON. The data driver  204  is coupled between the voltage switching device  203  and the pixels  201  for receiving the common voltage Vcom or the data voltage VDDA that is provided by the voltage switching device  203  as a driving voltage of the pixels  201 . In addition, a gate of the TFT T 1  and a first source/drain of the TFT T 1  are coupled to the scan driver  205  and the data driver  204  through scan lines  207  and data lines  208 , respectively. The liquid crystal capacitor C LC  and the storage capacitor C st  of the capacitor unit  206  are coupled between a second source/drain of the TFT T 1  and the common voltage Vcom. 
       FIG. 3  is a flowchart illustrating a method for eliminating an image sticking of a display apparatus when the display apparatus is turned off according to an embodiment of the present invention. Referring to  FIG. 3 , the voltage detector  202  first detects the system voltage Vsys used for operating the display apparatus  200  in step S 301  and then determines if the system voltage Vsys is less than a reference voltage in step S 302 . 
     When the system voltage Vsys is not less than the reference voltage, the voltage switching device  203  is controlled by the control signal CON for providing the data voltage VDDA to the data driver  204  in step S 303 . Therefore, the data driver  204  outputs the data voltage VDDA as the driving voltage for driving the pixels  201  in step S 305 . At this time, the data driver  204  is operated as usual and displays frames in a normal manner. 
     On the contrary, when the system voltage Vsys is less than the reference voltage, i.e., the voltage detector  202  detects that the display apparatus  200  is turned off, the voltage switching device  203  is controlled by the control signal CON to provide the common voltage Vcom to the data driver  204  in step S 304 . Therefore, the data driver  204  outputs the common voltage Vcom as the driving voltage for driving the pixels  201  in step S 305 . 
     It is noted that a voltage difference causing a rotation of liquid crystals is equal to the driving voltage subtracting the common voltage Vcom. As the driving voltage output by the data driver  204  is the common voltage Vcom, the voltage difference between the liquid crystal capacitor C LC  and the storage capacitor C st  is zero theoretically, and thus the liquid crystals are not rotated and no frame is displayed by the panel. Practically, when the common voltage Vcom is provided as the driving voltage, the residual charges in the capacitor unit  206  are minimized. Accordingly, when the display apparatus  200  is turned off, not only the panel is not affected by the unstable voltage difference, but also discharging the residual charges stored in the capacitor unit  206  is accelerated. The image sticking of the display apparatus  200  is further avoided when the LCD is turned off. 
     In addition, when the system voltage Vsys is less than the reference voltage, the scan driver  205  is also controlled by the control signal CON to generate a plurality of driving signals at a logic-high level and transmit the driving signals to the TFT T 1  in each of the pixels  201  through the corresponding scan line  207 , such that the TFTs T 1  are conducted. At this time, each of the pixels  201  are turned on and a discharge path is formed for expediting the discharging operation performed on the residual charges in the capacitor unit  206 . 
     In summary, when the LCD is detected to have been turned off, the data voltage inputted to the data driver is switched to the common voltage according to the embodiments of the present invention, such that the data driver can output the voltage level of the common voltage to the pixels in the panel. Since the pixels are coupled to the common voltage, the voltage across the capacitor unit  206 , which controls the rotation of the liquid crystals, is reduced to the greatest extent at the moment that the LCD is turned off. Besides, the residual charges in the liquid crystal capacitors and the storage capacitors are minimized. As such, the speed of discharging the residual charges can be relatively increased when the LCD is turned off. In conclusion, the above embodiments of the present invention are provided to significantly reduce the image sticking of the LCD when the LCD is turned off. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.