Abstract:
A pixel circuit has a light emitting diode, a first driving transistor, a second driving transistor, a capacitor, and a switch unit. When a scan signal is asserted, the switch unit couples sources/drains of the second driving transistor respectively to a first and a second source/drain of the first driving transistor, and couples a gate and second source/drain of the first driving transistor together. When the scan signal is de-asserted, the switch unit decouples one of the sources/drains of the second driving transistor from the first/second source/drain of the first driving transistor, and decouples the gate from the second source/drain of the first driving transistor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of U.S. application Ser. No. 11/692,318, filed Mar. 28, 2007, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a display, and more particularly relates to a source driver of the display. 
         [0004]    2. Description of Related Art 
         [0005]    In order to avoid image sticking, the polarity of each pixel of the display should not be consistent for a long time. There are many kinds of polarity distribution, for example the one-dot-line inversion shown in  FIG. 1A . The display operates according to data lines S 1 ˜S 8  and gate lines G 1 ˜G 8 . The symbol ‘+’ represents the pixel has a positive polarity, and the symbol ‘−’ represents the pixel has a negative polarity. 
         [0006]      FIG. 1B  is a block diagram of a source driver for the display. The source driver has a sample/hold circuit  110 , multiplexers (MUX)  150   a ,  150   b  and  160 , a low voltage operational amplifier (LV OPA)  130 , and a high voltage operational amplifier (HV OPA)  140 , for driving data lines, for example S 1  and S 2 . 
         [0007]    The sample/hold circuit  110  receives a positive polarity voltage VA +  and a negative polarity voltage VA −  of a first signal for outputting a first sampled-held voltage SH 1  and a second sampled-held voltage SH 2 . And, the sample/hold circuit  110  receives a positive polarity voltage VB +  and a second polarity voltage VB −  of a second signal for outputting a third sampled-held voltage SH 3  and a fourth sampled-held voltage SH 4 . 
         [0008]    The low voltage operational amplifier  130  amplifies the first sampled-held voltage SH 1  or the third sampled-held voltage SH 3  selectively output by the multiplexer  150   a  and outputs a low pixel voltage LP with a negative polarity. The high voltage operational amplifier  140  amplifies the second sampled-held voltage SH 2  or the fourth sampled-held voltage SH 4  selectively output by the multiplexer  150   b  to output a high pixel voltage HP with a positive polarity. The multiplexer  160  output the low pixel voltage LP and the high pixel voltage HP to data lines S 1  and S 2  of the display according to the polarity signal POL. 
         [0009]    The sample/hold circuit  110  has a first capacitor device  114   a  and a second capacitor device  118   a  to respectively deal with the positive polarity voltage VA+ and the negative polarity voltage VA− of the first signal. Moreover, the sample/hold circuit  110  has a third capacitor device  114   b  and a fourth capacitor device  118   b  to respectively deal with the positive polarity voltage VB+ and the negative polarity voltage VB− of the second signal. That is, the source driver needs at least four capacitor devices to drive two data lines. 
       SUMMARY 
       [0010]    According to one embodiment of the present invention, the source driver for driving a display panel has a sample/hold circuit, a first multiplexer, a first low voltage amplifier, a high voltage amplifier device, and a second multiplexer. The sample/hold circuit has two inputs for receiving a first voltage and a second voltage and two outputs for outputting a first sampled-held voltage and a second sampled-held voltage. The first multiplexer has two inputs respectively connected to the outputs of the sample/hold circuit, and has two outputs for respectively outputting the first sampled-held voltage and the second sampled-held voltage selectively according to a polarity signal. The first low voltage amplifier connects to one output of the first multiplexer to output a low pixel voltage. The high voltage amplifier device connects to the other output of the first multiplexer to output a high pixel voltage. The second multiplexer respectively outputs the low pixel voltage and the high pixel voltage to two data lines of the display panel according to the polarity signal. 
         [0011]    According to another embodiment of the present invention, the source driver for driving a display panel has a sample/hold circuit, a first low voltage amplifier, a second low voltage amplifier, a first multiplexer, a high voltage amplifier, a second multiplexer, and a third multiplexer. The sample/hold circuit receives a first voltage and a second voltage and outputs a first sampled-held voltage and a second sampled-held voltage. The first low voltage amplifier receives the first sampled-held voltage and generates a first low pixel voltage within a low-voltage range. The second low voltage amplifier receives the second sampled-held voltage and generates a second low pixel voltage within the low-voltage range. The first multiplexer has one output for outputting one of the first low pixel voltage and the second low pixel voltage according to a polarity signal. The high voltage amplifier connects to the output of the first multiplexer and generates a high pixel voltage within a high-voltage range. The second multiplexer outputs one of the first low pixel voltage and the high pixel voltage to a first data line of the display panel according to the polarity signal. The third multiplexer outputs one of the second low pixel voltage and the high pixel voltage to a second data line of the display panel according to the polarity signal. 
         [0012]    According to another embodiment of the present invention, the display has a display panel and a source driver for driving a display panel. The source driver has a sample/hold circuit and an amplifier device. The sample/hold circuit has a first capacitor device and a second capacitor device. The first capacitor device generates a first sampled-held voltage based on a first voltage. The second capacitor device generates a second sampled-held voltage based on a second voltage. The amplifier device selects one of the first sampled-held voltage and the second sampled-held voltage to be high-voltage amplified as a high pixel voltage, and selects the other to be low-voltage amplified as a low pixel voltage according to a polarity signal. The amplifier device further respectively outputs the low pixel voltage and the high pixel voltage to two data lines of the display panel according to the polarity signal. 
         [0013]    It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0015]      FIG. 1A  shows a polarity distribution of a pixel array; 
           [0016]      FIG. 1B  shows a source driver of the prior art; 
           [0017]      FIG. 2  shows a source driver according to an embodiment of the invention; and 
           [0018]      FIG. 3  shows a source driver according to another embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
         [0020]      FIG. 2  is a block diagram of a source driver according to an embodiment of the invention. The source driver has a sample/hold circuit  210 , a first multiplexer (MUX)  250 , a first low voltage operational amplifier (LV OPA)  230 , a high voltage amplifier device  240 , and a second multiplexer (MUX)  260 . The sample/hold circuit  210  has two inputs for receiving a first signal VA and a second signal VB and two outputs for outputting a first sampled-held voltage SH 1  and a second sampled-held voltage SH 2 . The first multiplexer (MUX)  250  has two inputs respectively connected to the outputs of the sample/hold circuit  210 , and has two outputs for respectively outputting the first sampled-held voltage SH 1  and the second sampled-held voltage SH 2  according to a polarity signal POL. 
         [0021]    The first low voltage operational amplifier  230  connects to one output of the first multiplexer  250  to output a low pixel voltage LP with a negative polarity. The high voltage amplifier device  240  connects to the other output of the first multiplexer  250  to output a high pixel voltage HP with a positive polarity. The second multiplexer  260  output the low pixel voltage LP and the high pixel voltage HP selectively to data lines S 1  and S 2  of the display according to the polarity signal POL. The polarity signal POL is determined by the polarity distribution. 
         [0022]    Moreover, the low pixel voltage LP is lower than a common voltage VCOMREF, and the high pixel voltage HP is higher than the common voltage VCOMREF. The common voltage VCOMREF is the reference voltage to determine the polarity of the pixel voltages. 
         [0023]    The sample/hold circuit  210  has a first capacitor device  214  and a second capacitor device  218 . The first capacitor device  214  generates the first sampled-held voltage SH 1  based on the first signal VA. The second capacitor device  218  generates the second sampled-held voltage SH 2  based on the second signal VB. 
         [0024]    When the polarity signal POL is a first value (such as a value represents a positive polarity), the first multiplexer  250  outputs the first sampled-held voltage SH 1  to the first low voltage operational amplifier  230  and outputs the second sampled-held voltage SH 2  to the high voltage amplifier device  240 . When the polarity signal POL is a second value (such as a value represents a negative polarity), the first multiplexer  250  outputs the first sampled-held voltage SH 1  to the high voltage amplifier device  240  and outputs the second sampled-held SH 2  voltage to the first low voltage operational amplifier  230 . 
         [0025]    The high voltage amplifier device  240  has a second low voltage operational amplifier (LV OPA)  244  receiving the output from the first multiplexer  250 , and a high voltage operational amplifier (HV OPA)  248  connecting to the second low voltage operational amplifier  244  in series to generate the high pixel voltage HP. 
         [0026]    The high voltage operational amplifier  248  generates the high pixel voltage HP based on an output from the second low voltage operational amplifier  244  and the common voltage VCOMREF. Namely, the high pixel voltage HP is generated from the output of the second low voltage operational amplifier  244  and is higher than the common voltage VCOMREF. 
         [0027]      FIG. 3  is a block diagram of a source driver according to another embodiment of the invention. The source driver for driving a display has a sample/hold circuit  310 , a first low voltage operational amplifier (LV OPA)  330 , a second low voltage operational amplifier (LV OPA)  344 , a first multiplexer (MUX)  350 , a high voltage operational amplifier (HV OPA)  348 , a second multiplexer (MUX)  360 , and a third multiplexer (MUX)  370 . The sample/hold circuit  310  receives a first signal VA and a second signal VB, and outputs a first sampled-held voltage SH 1  and a second sampled-held voltage SH 2 . 
         [0028]    The first low voltage operational amplifier  330  receives the first sampled-held voltage SH 1  and generates a first low pixel voltage LP 1 , which is within a low-voltage range. The second low voltage operational amplifier  344  receives the second sampled-held voltage SH 2  and generates a second low pixel voltage LP 2 , which is within the low-voltage range. The first multiplexer  350  outputs one of the first low pixel voltage LP 1  and the second low pixel voltage LP 2  selectively according to a polarity signal POL. 
         [0029]    The high voltage operational amplifier  348  connects to the output of the first multiplexer  350  and generates a high pixel voltage HP, which is within a high-voltage range. The second multiplexer  360  selectively outputs one of the first low pixel voltage LP 1  and the high pixel voltage HP to a first data line S 1  of the display panel according to the polarity signal POL. The third multiplexer  370  selectively outputs one of the second low pixel voltage LP 2  and the high pixel voltage HP to a second data line S 2  of the display panel according to the polarity signal POL. The polarity signal POL here is determined by the polarity distribution. 
         [0030]    Moreover, the first and second low pixel voltages LP 1  and LP 2  are within the low-voltage range that is lower than a common voltage VCOMREF. The high pixel voltage HP is within the high-voltage range that is higher than the common voltage VCOMREF. The common voltage VCOMREF is the reference voltage to determine the polarity of the pixel voltages. 
         [0031]    The sample/hold circuit  310  has a first capacitor device  314  and a second capacitor device  318 . The first capacitor device  314  generates the first sampled-held voltage SH 1  based on the first signal VA; and the second capacitor device  318  generates the second sampled-held voltage SH 2  based on the second signal VB. 
         [0032]    The high voltage operational amplifier  348  generates the high pixel voltage HP based on the output from the first multiplexer  350  and the common voltage VCOMREF. Namely, the high pixel voltage HP is generated from one of the first and second low pixel voltages LP 1  and LP 2 , and is higher than the common voltage VCOMREF. 
         [0033]    The source driver described above is arranged to drive a display panel. Generally speaking, the source driver has a sample/hold circuit  310  and an amplifier device  390 . The sample/hold circuit  310  has the first capacitor device  314  and the second capacitor device  318  described above. The amplifier device  390  selects one of the first sampled-held voltage SH 1  and the second sampled-held voltage SH 2  to be high-voltage amplified as a high pixel voltage, and selects the other to be low-voltage amplified as a low pixel voltage according to a polarity signal POL. The amplifier device  390  further respectively outputs the low pixel voltage and the high pixel voltage to two data lines S 1  and S 2  of the display panel according to the polarity signal POL. 
         [0034]    The amplifier device  390  has a first multiplexer  350  to select one of the first sampled-held voltage SH 1  and the second sampled-held voltage SH 2  according to the polarity signal POL. Moreover, the amplifier device  390  also has a high voltage operational amplifier  348  to high-voltage amplify the selected one of the first sampled-held voltage SH 1  and the second sampled-held voltage SH 2  after selection. 
         [0035]    The amplifier device  390  also has low voltage operational amplifiers  330  and  345  to low-voltage amplify the selected other one of the first sampled-held voltage SH 1  and the second sampled-held voltage SH 2  after selection. The amplifier device  390  has other multiplexers  360  and  370  to output the low pixel voltage and the high pixel voltage to the two data lines S 1  and S 2  of the display panel selectively according to the polarity signal POL. 
         [0036]    These two sample/hold circuits  310  and  310   a , and two amplifier devices  390  and  390   a  can be used to supply the high pixel voltage and low pixel voltage from three signals VA, VB and VC for four data lines S 1 , S 2 , S 3  and S 4 . The three signals VA, VB, and VC are data voltages respectively represent red, green, and blue color. The data lines S 1 , S 2 , S 3 , and S 4  are respectively arranged to transmit the data voltages for red, green, blue, and red pixels. The sample/hold circuit  310   a  has the corresponding configuration to the sample/hold circuits  310 , and the amplifier devices  390   a  has the corresponding configuration to the amplifier devices  390 . 
         [0037]    When the data line S 1  needs to transmit the data voltage of red color with positive polarity to a pixel, the first signal VA is transmitted to the first low voltage operational amplifier  330  through the sample/hold circuit  310 . Since the polarity signal POL is positive, the first multiplexer  350  selects the first sampled-held voltage SH 1  for inputting to the high voltage operational amplifier  348 . Then, the second multiplexer  360  selects the high pixel voltage HP generated by the high voltage operational amplifier  348  for inputting to the data line S 1 . 
         [0038]    When the data line S 3  needs to transmit the data voltage of blue color with negative polarity to a pixel. The third signal VC is transmitted to the first low voltage operational amplifier  330   a  through the sample/hold circuit  310   a . Since the polarity signal POL is negative, the second multiplexer  360   a  directly selects the low pixel voltage LP 1  generated by the first low voltage operational amplifier  330   a  for inputting to the data line S 3 . 
         [0039]    Compared with the conventional source driver in  FIG. 1B , the embodiments of this invention each just needs two capacitor devices (first and second capacitor devices) to drive two data lines. Moreover, the embodiments of this invention operate with fewer and lower signals than the conventional source driver. Namely, the conventional source driver operates with three high signals and three low signals, but the embodiments of this invention operate with only three low signals. Therefore, the source drivers of the embodiments can reduce the cost and the power consumption. 
         [0040]    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.