Patent Publication Number: US-2009231253-A1

Title: Lcd with the function of eliminating the power-off residual images

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an LCD, and more particularly, to an LCD with the function of eliminating the power-off residual images. 
     2. Description of the Prior Art 
     Currently the main reason causing the residual image on the LCD is that the pixel electrodes of the display panel discharge too slow when the power supply of the LCD is turned off. The electric charges cannot release immediately and remain in the liquid capacitors after the LCD is turned off so as to generate the residual image on the LCD, called power-off residual images. 
     Please refer to  FIG. 1 .  FIG. 1  is a timing diagram of driving voltages of the LCD. A TFT LCD according to the prior art comprises a display panel and a backlight module. When the TFT LCD is turned on, a power supply of the TFT LCD is turned on at time t 1  (waveform A), and voltages are applied to the common electrode and the pixel electrodes of the TFT LCD. Then, the image signals are inputted to the pixel array of the TFT LCD at time t 2  (waveform B). Afterwards the backlight module is turned on at time t 3  (waveform C) for providing the light to the display panel to display the image. When the LCD is turned off, operation is in reverse order, the backlight module is turned off at time t 4 , and the image signals end at time t 5 , and finally the power supply of the TFT LCD is turned off at time t 6 . 
     According to the above, during the period after the backlight is turned off and before the image signals end, that is, from time t 4  to time t 5 , the image signals are still transmitted in the pixel array and the electric charges remain in the pixel electrodes, so it will take a long time to release the residual charges because of lack of effective discharge paths. Thus, the residual image is generated after time t 6  when the TFT LCD is turned off. 
     Please refer to  FIG. 2 .  FIG. 2  is a schematic diagram of an LCD with the function of eliminating the power-off residual images according to the prior art. The LCD  10  comprises a power supply  11 , a voltage detector  12 , a display panel  13 , a gate driver  14 , and a source driver  15 . The power supply  11  provides a source voltage VCC to the source driver  15  and the gate driver  14 . In addition, the power supply  11  provides the source voltage VCC to the voltage detector  12 . The voltage detector  12  can compare the source voltage VCC with a reference voltage. When the LCD  10  is turned off, the source voltage VCC goes down to a level below the reference voltage. At this time, the voltage detector  12  sends a control signal XAO to the gate driver  14 . When receiving the control signal XAO, the gate driver  14  turns on all thin film transistors so that the residual charges can be released effectively to improve the power-off residual images. 
     In conclusion, the LCD according to the prior art utilizes an external voltage detector to detect the level of the source voltage after time t 6 . When detecting the level of the source voltage being lower than the reference voltage, the voltage detector outputs a control signal to the gate driver to start the mechanism of eliminating the power-off residual images of the LCD. At this time, the gate driver turns on all thin film transistors of the display panel to release the residual charges to improve the power-off residual images. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, an LCD with a function of eliminating power-off residual images comprises a display panel comprising a plurality of scan lines and a plurality of data lines, a power supply for outputting a supply voltage, a source driver, and a gate driver coupled to the plurality of scan lines. The source driver comprises a plurality of output channels coupled to the plurality of data lines, a hysteresis comparator, and a level translator coupled to an output end of the hysteresis comparator for generating a control signal. The gate driver turns on the plurality of scan lines according to the control signal. 
     According to another embodiment of the present invention, an LCD with a function of eliminating power-off residual images comprises a display panel comprising a plurality of scan lines and a plurality of data lines, a power supply for outputting a supply voltage, a gate driver coupled to the plurality of scan lines, and a source driver coupled to the plurality of data lines. The gate driver comprises a first detection circuit for comparing the supply voltage and a reference voltage to generate a first control signal. The gate driver turns on the plurality of scan lines according to the first control signal. The source driver comprises a second detection circuit for comparing the supply voltage and the reference voltage to generate a second control signal. The source driver outputs the same image signals to the plurality of data lines according to the second control signal. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a timing diagram of driving voltages of an LCD according to the prior art. 
         FIG. 2  is a schematic diagram of an LCD with the function of eliminating the power-off residual images according to the prior art. 
         FIG. 3  is a schematic diagram of the first embodiment of an LCD according to the present invention. 
         FIG. 4  is a schematic diagram of the source driver in  FIG. 3 . 
         FIG. 5  is a waveform diagram of the signals of the source driver in  FIG. 4 . 
         FIG. 6  is a schematic diagram of the second embodiment of an LCD according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 3 .  FIG. 3  is a schematic diagram of the first embodiment of an LCD according to the present invention. The LCD  20  comprises a display panel  22 , a power supply  24 , a source driver  26 , and a gate driver  28 . The power supply  24  provides a source voltage VCC to the source driver  26  and the gate driver  28 . The display panel  22  comprises a plurality of scan lines and a plurality of data lines for controlling a plurality of thin film transistors. The LCD  20  has a function of eliminating the power-off residual images. The source driver  26  comprises a detection circuit  30  for detecting the source voltage VCC provided by the power supply  24 , so that when the LCD  20  is turned off, the source driver  26  sends a control signal XAO to the gate driver  28 . When receiving the control signal XAO, the gate driver  28  turns on the plurality of scan lines so as to turn on all thin film transistors of the display panel  22 , which enables the release of the residual charges of the display panel  22 . According to the embodiment of the present invention, the LCD  20  can utilize the source driver  26  to determine if the power-off of the LCD  20  is in action. After the power-off of the LCD  20  is confirmed, the source driver  26  outputs the control signal XAO to the gate driver  28 . When receiving the control signal XAO, the gate driver  28  turns on all the thin film transistors of the display panel  22  to release the residual charges, and the power-off residual images of the LCD  20  will be removed. 
     Please refer to  FIG. 4 .  FIG. 4  is a schematic diagram of the source driver  26  in  FIG. 3 . The source driver  26  comprises a plurality of output channels (Y 1 ˜Yn), a plurality of switches SW, a hysteresis comparator  32 , a level translator  34 , a first inverter  36 , and a second inverter  38 . A negative input end of the hysteresis comparator  32  receives a reference voltage VR, so the hysteresis comparator  32  can generate a first threshold voltage VTL and a second threshold voltage VTH according to the reference voltage VR. When the source voltage VCC is smaller than the first threshold voltage VTL, the hysteresis comparator  32  outputs a low level voltage. When the source voltage VCC is greater than the second threshold voltage VTH, the hysteresis comparator  32  outputs a high level voltage. The level translator  34  is coupled to an output end of the hysteresis comparator  32  for adjusting the level of the output voltage of the hysteresis comparator  32  so as to generate a control signal XAO. The first inverter  36  and the second inverter  38  are coupled in series to an output end of the level translator  34  for enhancing the driving capability of the control signal XAO. Each of the plurality of switches SW is coupled between two adjacent output channels (Y 1 ˜Yn) respectively. The control signal XAO controls the plurality of switches SW to turn on or turn off. When the control signal XAO starts the function of eliminating the power-off residual images, the gate driver  28  turns on the plurality of scan lines of the display panel  22  so that the residual charges of the display panel  22  can be released. In addition, the plurality of switches SW of the source driver  26  is turned on to perform the charge sharing of the plurality of output channels (Y 1 ˜Yn). Thus, even if the display panel  22  has the residual charges, it can hardly be noticed because of the uniform image of the display panel  22 . 
     Please refer to  FIG. 5 .  FIG. 5  is a waveform diagram of the signals of the source driver  26  in  FIG. 4 . When the source voltage VCC is smaller than the first threshold voltage VTL, the voltage level of the control signal XAO goes down to the ground voltage GND in a short time so as to start the function of eliminating the power-off residual images. However, after the function of eliminating the power-off residual images is started, the source voltage VCC may have a variation because the gate driver  28  turns on all scan lines of the display panel  22 . Thus, a hysteresis function is added. The control signal XAO will not stop the function of eliminating the power-off residual images until the source voltage VCC is greater than the second threshold voltage VTH, thus the execution of the function of eliminating the power-off residual images can be assured while turning off the LCD  20 . 
     Please refer to  FIG. 6 .  FIG. 6  is a schematic diagram of the second embodiment of an LCD according to the present invention. The LCD  40  comprises a power supply  24 , a display panel  22 , a gate driver  28 , and a source driver  26 . The display panel  22  comprises a plurality of scan lines and a plurality of data lines for controlling a plurality of thin film transistors. The gate driver  28  is coupled to the plurality of scan lines. The gate driver  28  comprises a first detection circuit  43  for comparing the source voltage VCC with a reference voltage to generate a first control signal. When the LCD  40  is turned off, the first detection circuit  43  can detect the source voltage VCC. When the source voltage VCC goes down to a level lower than the reference voltage, the gate driver  28  turns on the plurality of scan lines to turn on all thin film transistors of the display panel  22 , so that the residual charges of the display panel  22  can be released. The source driver  26  is coupled to the plurality of data lines. The source driver  26  comprises a second detection circuit  44  for comparing the source voltage VCC with the reference voltage to generate a second control signal. Thus, the source driver  26  can detect the level of the source voltage VCC provided by the power supply  24 , so that the accuracy of determining power-off of the LCD  40  can be increased. In addition, when the source voltage VCC goes down to a level lower than the reference voltage, the source driver  26  outputs the same image signals to the plurality of data lines. Thus, the residual charges of the LCD  40  can be released before the LCD  40  is powered off and the display panel  22  will display a uniform image. According to this embodiment, the LCD  40  utilizes the first detection circuit  43  of the gate driver  28  to determine the power-off. When the LCD  40  is turned off, the gate driver  28  turns on all scan lines. In addition, the LCD  40  utilizes the second detection circuit  44  of the source driver  26  to determine the power-off at the same time. When the LCD  40  is turned off, the source driver  26  outputs the same image signals to all data lines. 
     In conclusion, the LCD according to the present invention has the function of eliminating power-off residual images. The LCD comprises a display panel, a power supply, a source driver, and a gate driver. According to the embodiment of the present invention, the LCD can utilize the source driver to detect the power-off or utilize the source driver and the gate driver at the same time to detect the power-off. In the first embodiment, the source driver comprises a hysteresis comparator and a level translator. The hysteresis comparator compares a supply voltage provided by the power supply with a reference voltage. The level translator is coupled to an output end of the hysteresis comparator for generating a control signal. The gate driver turns on a plurality of scan line of the display panel according to the control signal. In the second embodiment, the gate driver comprises a first detection circuit for comparing a source voltage VCC with a reference voltage to generate a first control signal. The gate driver turns on all scan lines of the display panel according to the first control signal. In addition, the source driver comprises a second detection circuit for comparing the source voltage VCC with the reference voltage to generate a second control signal. The source driver outputs the same image signals to all data lines of the display panel. Thus, the LCD can release the residual charges in a short time and display a uniform image. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.