Patent Publication Number: US-8115723-B2

Title: Driving circuit for display panel

Description:
FIELD OF THE INVENTION 
     The present invention relates to a driving circuit, and particularly to a driving circuit for a display panel. 
     BACKGROUND OF THE INVENTION 
     Modern technologies develop prosperously. Information products are introduced continuously to satisfy varied demands of numerous people. Most of early displays are cathode ray tubes (CRTs). However, their size is huge and their power consumption is great. In addition, the radiation they produced may endanger the health of long-term users. Thereby, current displays in the market are gradually replaced by liquid crystal displays (LCDs). LCDs have the characteristics of lightness, thinness, shortness, and smallness. Besides, they also have the advantages of low radiation and power consumption. Hence, they have become the mainstream of the market. 
     LCDs display images by controlling the light transmittance of liquid-crystal cells according to data signals. Because active-matrix LCDs adopt active control switches, the LCDs of this sort own advantages in displaying motion pictures. Thin-film transistors (TFTs) are switches mainly used in active-matrix LCDs. 
       FIG. 1  shows a schematic diagram of the driving system for an LCD according to the prior art. As shown in the figure, the driving system comprises a display panel  10 ′, a scan driving circuit  12 ′, a data driving circuit  14 ′, a timing control circuit  16 ′, and a circuit for producing reference voltages  18 ′. The display panel  10 ′ is used for displaying images. The scan driving circuit  12 ′ is used for producing and transmitting a scan signal to the display panel  10 ′ for driving a thin-film transistor (TFT) of the display panel  10 ′. The data driving circuit  14 ′ is used for producing and transmitting a data signal to the display panel  10 ′ for displaying the images. The timing control circuit  16 ′ produces a timing control signal, and transmitting the timing control signal to the scan driving circuit  12 ′ and the data driving circuit  14 ′, respectively, for controlling the scan driving circuit  12 ′ and the data driving circuit  14 ′ to transmit the scan signal and data signal to the display panel  10 ′, respectively, and for displaying the images. In addition, the circuit for producing reference voltages  18 ′ produces a reference voltage and transmits the reference voltage to the data driving circuit  14 ′ for making the data driving circuit  14 ′ to produce the data signal according to the timing control signal and the reference voltage. 
       FIG. 2  shows a schematic diagram of a circuit for producing reference voltages according to the prior art. If the digital display data corresponding to RGB is comprised by, for example, 6 bits, the circuit for producing reference voltages  18 ′ can output 64 analog voltages V 0 ˜V 63  corresponding to 2 6 =64 grayscales. The circuit for producing reference voltage  18 ′ is comprised by resistive voltage division circuit including resistors R 0 ˜R 7  connected in series. Each of the resistors R 0 ˜R 7  is further comprised by 8 resistors connected in series. As shown in  FIG. 3 , the 8 resistors R 01 ˜R 08  are connected in series to form the resistor R 0 . Other resistors R 1 ˜R 7  are formed similarly. Thereby, the circuit for producing reference voltages  18 ′ is comprised by 64 resistors and produces voltages V 0 ˜V 63 . 
     However, because 64 resistors are needed to produce 64 different voltage levels, the area of the circuit for producing reference voltages  18 ′ is increased, and hence increasing the area of the display. Besides, in order to reduce the area of the circuit for producing reference voltages  18 ′, resistors with larger resistance have to be used, which will affect the driving capability of the data driving circuit  14 ′. Moreover, when the data driving circuit  14 ′ drives the display panel  10 ′ via the resistors, a large amount of power will be consumed on the resistors, and thus wasting power of the display. 
     Accordingly, the present invention provides a novel driving circuit for a display panel, which can reduce the amount of resistors used without sacrificing the driving capability of the data driving circuit  14 ′. Thereby, the area of the display can be reduced, and the power of the display can be saved. 
     SUMMARY 
     An objective of the present invention is to provide a driving circuit for a display panel, which uses a pre-charge power supply to charge a capacitor of the display in advance for shortening the driving time. 
     Another objective of the present invention is to provide a driving circuit for a display panel, which uses a pre-charge power supply to charge a capacitor of the display in advance for saving power of the display by avoiding power consumption on resistors. 
     The driving circuit for a display panel according to the present invention comprises a pre-charge power supply, a pre-charge switch, a buffer circuit, and a plurality of resistive devices. The pre-charge switch is coupled between the pre-charge power supply and a capacitor of the display panel. The buffer circuit is used for buffering a data signal and producing a buffer signal. The plurality of resistive devices is connected in series and coupled to the buffer circuit, and produces a plurality of driving signals therebetween according to the buffer signal. The driving circuit first closes the pre-charge switch to make the pre-charge power supply charge the capacitor. Then, one of the plurality of driving signals charges the capacitor. Thereby, the driving time can be shortened, and power of the display can be saved by avoiding power consumption on resistors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of the driving system for an LCD according to the prior art; 
         FIG. 2  shows a schematic diagram of a circuit for producing reference voltages according to the prior art; 
         FIG. 3  shows a schematic diagram of a detailed circuit for producing reference voltages according to the prior art; 
         FIG. 4  shows a schematic diagram of the driving system for an LCD according to a preferred embodiment of the present invention; 
         FIG. 5  shows a block diagram according to a preferred embodiment of the present invention; 
         FIG. 6  shows a timing diagram of driving according to a preferred embodiment of the present invention; and 
         FIG. 7  shows a block diagram according to another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In order to make the structure and characteristics as well as the effectiveness of the present invention to be further understood and recognized, the detailed description of the present invention is provided as follows along with preferred embodiments and accompanying figures. 
       FIG. 4  shows a schematic diagram of the driving system for an LCD according to a preferred embodiment of the present invention. As shown in the figure, the driving system comprises a display panel  10 , a scan driving circuit  12 , a data driving circuit  14 , a timing control circuit  16 , and a Gamma circuit  18 . The display panel  10  is used for display images. The scan driving circuit  12  is used for producing and transmitting a scan signal to the display panel  10  to drive a thin-film transistor (TFT) of the display panel  10 . The data driving circuit  14  is used for producing and transmitting a data signal to the display panel  10  to display the images according to the data signal. The timing control circuit  16  produces a timing control signal and transmits the timing control signal to the scan driving circuit  12  and the data driving circuit  14  for controlling the scan driving circuit  12  and the data driving circuit  14  to transmit the scan signal and data signal to the display panel  10 , respectively. Thereby, the images can be displayed. In addition, the Gamma circuit  18  produces a reference voltage and transmits the reference voltage to the data driving circuit  14 . Hence, the data driving circuit  14  can produce the data signal according to the timing control signal and the reference voltage. 
       FIG. 5  shows a block diagram according to a preferred embodiment of the present invention. As shown in the figure, the driving circuit for a display panel according to the present invention is applied to the data driving circuit  14  for receiving 64 voltage levels produced by the Gamma circuit  18 . Because the driving circuit according to the present invention can receive 8-bit signals, the data driving circuit  14  needs to use 8 driving circuits for receiving and processing said 64 voltage levels. According to the present preferred embodiment, only one driving circuit is used for description. The driving circuit according to the present invention comprises a first pre-charge power supply AVDD, a first pre-charge switch  140 , a buffer circuit  142 , and a plurality of resistive devices  143 ,  144 ,  146 ,  148 . The first pre-charge switch  140  is coupled between the pre-charge power supply AVDD and a capacitor  100  of the display panel. The buffer circuit  142  is used for buffering a data signal and producing a buffer signal. The plurality of resistive devices  143 ,  144 ,  146 ,  148  is connected in series and coupled to the buffer circuit  142 , and produces a plurality of driving signals therebetween according to the buffer signal. The driving circuit first closes the pre-charge switch to make the pre-charge power supply charge the capacitor  100 . Then, one of the plurality of driving signals charges the capacitor  100 .  FIG. 6  shows a timing diagram of driving according to a preferred embodiment of the present invention. As shown in the figure, the dashed line represents that the driving circuit does not pre-charge the capacitor  100 , and the solid line represents that the driving circuit pre-charges the capacitor  100 . It is known by the figure that the driving circuit according to the present invention charges the capacitor  100  using the first pre-charge power supply AVDD during the time interval T 1  to T 2 . During the time interval T 2  to T 3 , the driving signal is used to charge the capacitor  100 . Thereby, the driving circuit according to the present invention completes driving the display panel  10  at time T 3 , shortening the time the driving circuit charges the capacitor  100 . Hence, the driving time of the display panel  10  by the data driving circuit  14  is shortened, and the efficiency of the display is improved. In addition, because the time the driving circuit charges the capacitor  100  is shortened, power can be saved by avoiding power consumption on the plurality of resistive devices  143 ,  144 ,  146 ,  148 , where the resistive device is a resistor. 
     In addition, the driving circuit according to the present invention further comprises an analog-to-digital converter  15  used for converting an input signal and producing the data signal. The analog-to-digital converter  15  is coupled to the Gamma circuit  18  for receiving correction data produced by the Gamma circuit  18  as the input signal. The Gamma signal  18  produces the correction data according to a Gamma curve. Besides, the analog-to-digital converter  15  is further coupled to a memory unit  20 , which is used for storing a plurality of pixel data. The analog-to-digital converter  15  receives the plurality of pixel data and the correction data as the input signal and produces the data signal. The memory unit  20  is a random access memory (RAM). 
     Referring back to  FIG. 5 , a first switch  150 , a second switch  152 , and a third switch  154  are set between the plurality of resistive devices  143 ,  144 ,  146 ,  148 . The analog-to-digital converter can produce a control signal according to the pixel data stored in the memory unit  20  for closing/opening the first switch  150 , the second switch  152 , or the third switch  154 . Besides, the analog-to-digital converter  15  can first control the first pre-charge switch  140  or the second pre-charge switch  141  to close for charging the capacitor  100 . After a period of time, the analog-to-digital converter  15  can control the first pre-charge switch  140  or the second pre-charge switch  141  to open, and close one of the first switch  150 , second switch  152 , and third switch  154  for charging the capacitor  100  in succession. 
     Furthermore, the buffer circuit  142  includes a first buffer  1420  and a second buffer  1421 . The first buffer  1420  is used for buffering the data signal and producing a first buffer signal; the second buffer  1421  is used for buffering the data signal and producing a second buffer signal. The plurality of resistive devices  143 ,  144 ,  146 ,  148  produces the driving signal according to the voltage difference between the first buffer signal produced by the first buffer  1420  and the second buffer signal produced by the second buffer  1421 . The first buffer  1420  and the second buffer  1421  are operational amplifiers. 
     Moreover, the liquid crystal of the display panel  10  needs to perform polarity inversion for preventing charge accumulation, which will deteriorate the display quality. Thereby, the driving circuit according to the present invention further comprises a second pre-charge power supply VSS and a second pre-charge switch  141 . The driving circuit provides the first pre-charge power supply AVDD or the second pre-charge power supply VSS to the capacitor  100  via the first pre-charge switch  140  and the second pre-charge switch  141  according to the polarity inversion requirement of the liquid crystal of the display panel  10 . The first pre-charge power supply AVDD and the second pre-charge power supply VSS can be coupled to any power supply of the display. 
       FIG. 7  shows a block diagram according to another preferred embodiment of the present invention. As shown in the figure, the difference between the present preferred embodiment and the one in  FIG. 5  is that, the first pre-charge power supply AVDD and the second pre-charge power supply VSS according to the present preferred embodiment are coupled to output terminals of the first buffer  1420  and the second buffer  1421 , respectively. Thereby, during the process the driving circuit pre-charges by means of the first pre-charge power supply AVDD or the second pre-charge power supply VSS, overcharge phenomenon will not occur owing to long pro-charge time. Besides, the control of switching time can be simplified as well. 
     To sum up, the driving circuit for a display panel according to the present invention charges a capacitor of a display panel via a pre-charge switch and using a pre-charge power supply. Then, one of a plurality of driving signals produced between a plurality of resistive devices according to a buffer signal charges capacitor. Thereby, the driving time can be shortened, and the power of the display can be saved by avoiding power consumption on resistors. 
     Accordingly, the present invention conforms to the legal requirements owing to its novelty, non-obviousness, and utility. However, the foregoing description is only a preferred embodiment of the present invention, not used to limit the scope and range of the present invention. Those equivalent changes or modifications made according to the shape, structure, feature, or spirit described in the claims of the present invention are included in the appended claims of the present invention.