Abstract:
An exemplary liquid crystal display includes a plurality of pixel units each including a pixel electrode for receiving data voltages and a common electrode for receiving a common voltage having a constant value. The data voltages applied to each pixel electrodes are equal a sum of a main data voltage having a square waveform and an auxiliary voltage that is periodically changed at intervals each formed by four continuous frames. An absolute value of the auxiliary voltage is less than a voltage difference between the main data voltage and the common voltage. A sum of the auxiliary voltage is zero in a minimum period.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 12/001,704, filed Dec. 11, 2007 and entitled “LIQUID CRYSTAL DISPLAY WITH PERIODICAL CHANGED VOLTAGE DIFFERENCE BETWEEN DATA VOLTAGE AND COMMON VOLTAGE AND DRIVING METHOD THEREOF.” The disclosure of such parent application is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to liquid crystal displays (LCDs), and more particularly to an LCD with a periodical changed voltage difference between a data voltage and a common voltage. The present invention also relates to a driving method of the LCD. 
         [0004]    2. Description of Related Art 
         [0005]    A liquid crystal display (LCD) utilizes liquid crystal molecules to control light transmissivity of each of pixels of the LCD. The liquid crystal molecules are driven according to external video signals received by the LCD. A conventional LCD generally employs an inversion driving method to drive the liquid crystal molecules to protect the liquid crystal molecules from decay or damage. 
         [0006]      FIG. 11  is a side view of a conventional LCD. The LCD  10  includes a first substrate  11 , a common electrode  12 , a first alignment film  13 , a liquid crystal layer  14 , a second alignment film  15 , a plurality of pixel electrodes  16 , and a second substrate  17 . The first substrate  11  is opposite to the second substrate  17 . The common electrode  12  is disposed on an inner surface of the first substrate  11 . The plurality of pixel electrodes  16  are disposed on an inner surface of the second substrate  17  and arranged in a matrix. The first alignment film  13  is coated on the common electrode  12 , and the second alignment film  15  is coated on the plurality of pixel electrodes  16 . The liquid crystal layer  14  is sandwiched between the first alignment film  13  and the second alignment film  15 . Each of the pixel electrodes  16 , part of the common electrode  12  opposite to the corresponding pixel electrode  16 , and liquid crystal molecules (not labeled) sandwiched therebetween cooperatively define a pixel unit (not labeled). 
         [0007]    Data voltages generated by a data driving circuit (not shown) are provided to the plurality of pixel electrodes  16 , and a common voltage generated by a common voltage generating circuit (not shown) is provided to the common electrode  12 . In each pixel unit, an electric field is generated between the pixel electrode  16  and the common electrode  12 . The electric field controls rotating angles of the liquid crystal molecules of the pixel unit, whereby the rotating angles determine the light transmissivity of the pixel unit. The light transmissivity of the pixel unit determines a brightness of the pixel unit. The LCD  10  displays images via controlling the brightness of each of the pixel units. 
         [0008]    A waveform diagram of the data voltage and the common voltage of one of the pixel units is shown in  FIG. 12 . In frame N−1, a value of the data voltage is Vdata 1 , a value of the common voltage is Vcom, where Vdata 1 &gt;0, Vcom&gt;0, Vdata 1 &lt;Vcom. A value of the electric field of the pixel unit is (Vcom−Vdata 1 )/d, where d is a vertical distance between the common electrode  12  and the pixel electrode  16 . A direction of the electric field of the pixel unit is from the common electrode  12  to the pixel electrode  16 . In frame N, the value of the data voltage is Vdata 2 , the value of the common voltage is Vcom, where Vdata 2 &gt;Vcom, Vdata 2 −Vcom=Vcom−Vdata 1 . The value of the electric field of the pixel unit is (Vdata 2 −Vcom)/d. The direction of the electric field of the pixel unit is from the pixel electrode  16  to the common electrode  12 . In frame N+1, the value of the data voltage is Vdata 1 , and the value of the common voltage is Vcom. The value of the electric field of the pixel unit is (Vcom−Vdata 1 )/d. The direction of the electric field of the pixel unit is from the common electrode  12  to the pixel electrode  16 . The value and the direction of the electric field of the pixel unit in frame N+1 are the same as that in frame N−1. That is, frame N−1 and frame N define a minimum period. The value and the direction of the electric field of the pixel unit in the following frames repeat that in frame N−1 or frame N. 
         [0009]    The direction of the electric field of each pixel unit is alternate in each two continuous frames, but the value of the electric field of each pixel unit is constant in each frame. The rotating angles of the liquid crystal molecules of each pixel unit are merely determined by the value of the electric field of each pixel unit. That is, when the value of the electric field of the pixel unit is constant, the rotating angles of the liquid crystal molecules of the pixel unit are constant. 
         [0010]    In fact, the liquid crystal layer  14  is not pure and has a plurality of impurity ions (not shown). The alignment films  13  and  15  are made of organic materials and easily capture the impurity ions. When the value of the electric field of each pixel unit keeps constant for a long time, the rotating angles of the liquid crystal molecules of each pixel unit are constant, correspondingly. That is, each liquid crystal molecule stays in the same position in the liquid crystal layer  14 . A moving resistance stressed by the liquid crystal molecules to the impurity ions has little effect on random motions of the impurity ions. Thus, part of the impurity ions are captured by the alignment films  13  and  15  and a residual direct current electric field (not shown) is generated between the first alignment film  13  and the second alignment film  15 . Even if the value of the electric field of each pixel unit changes, the residual direct current electric field may still exist. The residual direct current electric field also controls the liquid crystal molecules to rotate, and an extra rotating angle of each liquid crystal molecule exists. If the value of the electric field of each pixel unit changes in a small range, the liquid crystal molecules may stay in the same position as in previous frames. Thus, images of the previous frames still can be watched, which is so-called image residue phenomenon. 
         [0011]    It is desired to provide an LCD which overcomes the above-described deficiencies. It is also desired to provide a related driving method for an LCD. 
       SUMMARY 
       [0012]    In one aspect, a liquid crystal display includes a plurality of pixel units each including a pixel electrode for receiving data voltages and a common electrode for receiving a common voltage with a constant value. The data voltages applied to each pixel electrodes are equal a sum of a main data voltage having a square waveform and an auxiliary voltage that is periodically changed at intervals each formed by four continuous frames. The auxiliary voltage is less than a voltage difference between the main data voltage and the common voltage. In two frames of the four continuous frames, the voltage differences between the data voltages and the common voltage are substantially equal to an absolute value of Vdata−Vcom, and in remaining two frames of the two continuous frames, the voltage difference between the data voltages and common voltage in one of the remaining two frames is substantially equal to an absolute value of Vdata−Vcom−Vn; and in other one of the remaining two frames is substantially equal to an absolute value of Vdata−Vcom+Vn, where Vcom denotes the constant value of the common voltage, Vdata denotes the main data voltage, and Vn denotes an absolute value of the auxiliary voltage. 
         [0013]    In another aspect, a liquid crystal display includes a plurality of pixel units each including a pixel electrode for receiving data voltages and a common electrode for receiving a common voltage with a constant value. The data voltages applied to each pixel electrodes are equal a sum of a main data voltage having a square waveform and a first auxiliary voltage. The common voltage is equal to a main common voltage with a constant value and a second auxiliary voltage. The first and second auxiliary voltage is periodically changed at intervals each formed by four continuous frames. Each of the first and second auxiliary voltages is less than a voltage difference between the main data voltage and the main common voltage, and one of the first and second auxiliary voltages is equal to zero in each frames. In two frames of the four continuous frames, the voltage differences between the data voltages and the common voltage are substantially equal to an absolute value of Vdata−Vcom. In remaining two frames of the two continuous frames, the voltage difference between the data voltages and common voltage in one of the remaining two frames is substantially equal to an absolute value of Vdata−Vcom−Vn; and in other one of the remaining two frames is substantially equal to an absolute value of Vdata−Vcom+Vn, where Vcom denotes the constant value of the main common voltage, Vdata denotes the main data voltage, and Vn denotes an absolute value of the other one of the first and second auxiliary voltages. 
         [0014]    Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. In the drawings, all the views are schematic. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a side view of an LCD according to a first embodiment of the present invention. 
           [0016]      FIG. 2  is an abbreviated circuit diagram of the LCD of  FIG. 1 , the LCD having a common voltage generating circuit and a plurality of pixel units. 
           [0017]      FIG. 3  is a circuit diagram of the common voltage generating circuit of  FIG. 2 , the common voltage generating circuit having a second input terminal and a third input terminal. 
           [0018]      FIG. 4  is a waveform diagram of a first control signal received by the second input terminal and a second control signal received by the third input terminal of  FIG. 3 . 
           [0019]      FIG. 5  is a waveform diagram of a data voltage and a common voltage of one of the pixel units of  FIG. 2 . 
           [0020]      FIG. 6  is a waveform diagram of a data voltage and a common voltage of one of pixel units of an LCD according to a second embodiment of the present invention. 
           [0021]      FIG. 7  is an abbreviate circuit diagram of a gamma voltage generating circuit of an LCD according to a third embodiment of the present invention, the gamma voltage generating circuit having an input terminal. 
           [0022]      FIG. 8  is a waveform diagram of a DC voltage received by the input terminal of  FIG. 7 . 
           [0023]      FIG. 9  is a waveform diagram of a data voltage and a common voltage of one of pixel units of the LCD according to the third embodiment of the present invention. 
           [0024]      FIG. 10  is a waveform diagram of a data voltage and a common voltage of one of pixel units of an LCD according to a fourth embodiment of the present invention. 
           [0025]      FIG. 11  is a side view of a conventional LCD, the LCD having a plurality of pixel units. 
           [0026]      FIG. 12  is a waveform diagram of a data voltage and a common voltage of one of the pixel units of  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0027]    Reference will now be made to the drawings to describe various embodiments of the present invention in detail. 
         [0028]      FIG. 1  is a side view of an LCD according to a first embodiment of the present invention. The LCD  20  includes a first substrate  21 , a common electrode  22 , a first alignment film  23 , a liquid crystal layer  24 , a second alignment film  25 , a plurality of pixel electrodes  26 , and a second substrate  27 . The first substrate  21  is opposite to the second substrate  27 . The common electrode  22  is disposed on an inner surface of the first substrate  21 . The plurality of pixel electrodes  26  are disposed on an inner surface of the second substrate  27  and arranged in a matrix. The first alignment film  23  is coated on the common electrode  22 , and the second alignment film  25  is coated on the plurality of pixel electrodes  26 . The liquid crystal layer  24  is sandwiched between the first alignment film  23  and the second alignment film  25 . 
         [0029]      FIG. 2  is an abbreviated circuit diagram of the LCD of  FIG. 1 . The LCD  20  further includes a control circuit  31 , a gate driving circuit  32 , a data driving circuit  33 , a common voltage generating circuit  34 , and a gamma voltage generating circuit  35 . The second substrate  27  includes a plurality of gate lines  201 , a plurality of data lines  202 , and a plurality of thin film transistors (TFTs)  206 . The plurality of gate lines  201  are parallel to each other and each gate line  201  extends along a first direction. The plurality of data lines  202  are parallel to each other and each data line  202  extends along a second direction vertical to the first direction. Each TFT  206  is positioned near a crossing of one of the gate lines  201  and one of the corresponding data lines  202 . Each pixel electrodes  26 , part of the common electrode  22  opposite to the pixel electrode  26 , and liquid crystal molecules sandwiched therebetween cooperatively define a pixel unit  240 . 
         [0030]    Each TFT  206  includes a gate electrode, a source electrode, and a drain electrode. The gate electrode of each TFT  206  is connected to a corresponding gate line  201 , and the source electrode of each TFT  206  is connected to a corresponding data line  202 . Further, the drain electrode of each TFT  206  is connected to a corresponding pixel electrode  26 . 
         [0031]    The control circuit  31  receives and processes external video signals. Timing signals generated in the control circuit  31  are transmitted to the gate driving circuit  32  and the data driving circuit  33 , and the processed video signals are transmitted into the data driving circuit  33 . The gamma voltage generating circuit  35  generates gamma voltages and the gamma voltages are transmitted to the data driving circuit  33 . The gate driving circuit  32  generates corresponding scanning signals according to the timing signals. The data driving circuit  33  latches up the processed video signals according to the timing signals. The data driving circuit  33  receives corresponding gamma voltages according to the processed video signals and generates corresponding data voltages. The gate driving circuit  32  provides the scanning signals to the gate lines  201 , and the data driving circuit  33  provides the data voltages to the data lines  202  when the gate lines  201  are scanned. In each pixel unit  240 , an electric field is generated between the pixel electrode  26  and the common electrode  22 . The electric field controls rotating angles of the liquid crystal molecules of the pixel unit  240  and the rotating angles determine a light transmissivity of the pixel unit  240 . The light transmissivity of the pixel unit  240  determines a brightness of the pixel unit  240 . The LCD  20  displays images via controlling the brightness of each pixel unit  240 . 
         [0032]      FIG. 3  is a circuit diagram of the common voltage generating circuit of  FIG. 2 . The common voltage generating circuit  34  includes a first input terminal  301 , a second input terminal  302 , a third input terminal  303 , an output terminal  304 , an operational amplifier  306 , a first transistor  311 , a second transistor  312 , a first resistor  321 , a second resistor  322 , a third resistor  323 , a fourth resistor  324 , and a variable resistor  320 . The first input terminal  301  is used for receiving a direct current (DC) voltage and a value of the DC voltage is Vdd. The second input terminal  302  is used for receiving a first control signal and the third input terminal  303  is used for receiving a second control signal. The output terminal  304  is used for outputting the common voltage. A resistance of the first resistor  321  is R 1 , a resistance of the second resistor  322  is R 2 , a resistance of the third resistor  323  is R 3 , a resistance of the fourth resistor  324  is R 4 , and a resistance of the variable resistor  320  is R 0 . The resistance of the third resistor  323  is equal to that of the fourth resistor  324 , i.e. R 3 =R 4 . The first resistor  321 , the second resistor  322 , the variable resistor  320 , the third resistor  323 , and the fourth resistor  324  are connected in series between the first input terminal  301  and ground. That is, the resistors  321 ,  322 ,  320 ,  323 , and  324  cooperatively form a voltage dividing circuit. A gate electrode of the first transistor  302  is connected to the second input terminal  302 , and a drain electrode of the first transistor  311  is connected to a node between the variable resistor  320  and the third resistor  323 . Further, a source electrode of the first transistor  311  is connected to a node between the third resistor  323  and the fourth resistor  324 . A gate electrode of the second transistor  312  is connected to the third input terminal  303 , and a drain electrode of the second transistor  312  is connected to the node between the third resistor  323  and the fourth resistor  324 . Further, a source electrode of the second transistor  312  is connected to ground. A non-inverting input terminal of the operational amplifier  306  is connected to a node between the first resistor  321  and the second resistor  322 , and an inverting input terminal of the operational amplifier  306  is connected to an output terminal of the operational amplifier  306 . The output terminal  304  is connected to the output terminal of the operational amplifier  306 . The first input terminal  301  is connected to ground via a capacitor (not labeled) and the non-inverting input terminal of the operational amplifier  306  is connected to ground via a capacitor (not labeled). 
         [0033]      FIG. 4  is a waveform diagram of the first control signal received by the second input terminal and the second control signal received by the third input terminal of  FIG. 3 . In frame N−2, the first control signal is a high level voltage, and the second control signal is a low level voltage. The first transistor  311  is turned on and the second transistor  312  is turned off. The third resistor  323  is in short circuit state, and the value of the common voltage is (R 2 +R 0 +R 4 )*Vdd/(R 1 +R 2 +R 0 +R 4 ). In frame N−1, the first control signal is a high level voltage, and the second control signal is a high level voltage. The first transistor  311  is turned on and the second transistor  312  is turned on. The third resistor  323  and the fourth resistor  324  are in short circuit state, and the value of the common voltage is (R 2 +R 0 )*Vdd/(R 1 +R 2 +R 0 ). In frame N, the first control signal is a low level voltage, and the second control signal is a high level voltage. The first transistor  311  is turned off and the second transistor  312  is turned on. The fourth resistor  324  is in short circuit state, and the value of the common voltage is (R 2 +R 0 +R 3 )*Vdd/(R 1 +R 2 +R 0 +R 3 ). In frame N+1, the first control signal is a low level voltage, and the second control signal is a low level voltage. The first transistor  311  is turned off and the second transistor  312  is turned off. The value of the common voltage is (R 2 +R 0 +R 3 +R 4 )*Vdd/(R 1 +R 2 +R 0 +R 3 +R 4 ). In frame N+2, the first control signal is a high level voltage, and the second control signal is a low level voltage. The first transistor  311  is turned on and the second transistor  312  is turned off. The third resistor  323  is in short circuit state, and the value of the common voltage is (R 2 +R 0 +R 4 )*Vdd/(R 1 +R 2 +R 0 +R 4 ). That is, the first control signal and the second control signal in frame N+2 are the same as that in frame N−2. Therefore, frame N−2, frame N−1, frame N, and frame N+1 define a minimum period. The first control signal and the second control signal in the following frames repeat that in one of frame N−2, frame N−1, frame N, and frame N+1. 
         [0034]      FIG. 5  is a waveform diagram of the data voltage and the common voltage of one of the pixel units of  FIG. 2 . In frame N−2, a value of the data voltage is Vdata 1  and the value of the common voltage is Vcom, where Vdata 1 &gt;0, Vcom&gt;0, Vdata 1 &lt;Vcom, Vcom=(R 2 +R 0 +R 4 )*Vdd/(R 1 +R 2 +R 0 +R 4 ). A voltage difference between the pixel electrode  26  and the common electrode  26  is Vcom−Vdata 1 . A value of the electric field E 1  of the pixel unit  240  is (Vcom−Vdata 1 )/d, where d is a vertical distance of the pixel electrode  26  and the common electrode  22 . A direction of the electric field E 1  of the pixel unit  240  is from the common electrode  22  to the pixel electrode  26 . The liquid crystal molecules are polar molecules and are polarized in the electric field E 1 . Each liquid crystal molecule can be regarded as an electric dipole. A value of an angle between the direction of the electric field E 1  and a direction of an electric dipole moment of the liquid crystal molecule is θ. 
         [0035]    In frame N−1, the value of the data voltage is Vdata 2  and the value of the common voltage is Vcom−Va, where Vdata 2 &gt;Vcom, Va&lt;Vdata 2 −Vcom, Va=R 1 *R 4 *Vdd/[(R 1 +R 2 +R 0 )*(R 1 +R 2 +R 0 +R 4 )], Vdata 2 −Vcom=Vcom−Vdata 1 . The voltage difference between the pixel electrode  26  and the common electrode  22  is Vdata 2 −Vcom+Va. The value of the electric field E 1  is (Vdata 2 −Vcom+Va)/d and the direction of the electric field E 1  is from the pixel electrode  26  to the common electrode  22 . The value of the angle between the direction of the electric field E 1  and the direction of the electric dipole moment of the liquid crystal molecule is θ−ψ. 
         [0036]    In frame N, the value of the data voltage is Vdata 1  and the value of the common voltage is Vcom. The voltage difference between the pixel electrode  26  and the common electrode  22  is Vcom−Vdata 1 . The value of the electric field E 1  is (Vcom−Vdata 1 )/d and the direction of the electric field E 1  is from the common electrode  22  to the pixel electrode  26 . The value of the angle between the direction of the electric field E 1  and the direction of the electric dipole moment of the liquid crystal molecule is θ. 
         [0037]    In frame N+1, the value of the data voltage is Vdata 2  and the value of the common voltage is Vcom+Va. The voltage difference between the pixel electrode  26  and the common electrode  22  is Vdata 2 −Vcom−Va. The value of the electric field E 1  is (Vdata 2 −Vcom−Va)/d and the direction of the electric field E 1  is from the pixel electrode  26  to the common electrode  22 . The value of the angle between the direction of the electric field E 1  and the direction of the electric dipole moment of the liquid crystal molecule is θ+ψ. 
         [0038]    In frame N+2, the value of the data voltage is Vdata 1  and the value of the common voltage is Vcom. The voltage difference between the pixel electrode  26  and the common electrode  22  is Vcom−Vdata 1 . The value of the electric field E 1  is (Vcom−Vdata 1 )/d and the direction of the electric field E 1  is from the common electrode  22  to the pixel electrode  26 . The value of the angle between the direction of the electric field E 1  and the direction of the electric dipole moment of the liquid crystal molecule is θ. 
         [0039]    The value and the direction of the electric field E 1  in frame N+2 are the same as that in frame N−2. That is, frame N−2, frame N−1, frame N, and frame N+1 define a minimum period. The value and the direction of the electric field E 1  in the following frames repeat that in one of frame N−2, frame N−1, frame N, and frame N+1. 
         [0040]    The value of the electric field of each pixel unit  240  increases or decreases by a value of Va/d in any two continuous frames, and the value of the angle between the direction of the electric field and the direction of the electric dipole moment of the liquid crystal molecule correspondingly increases or decreases by a value of ψ. The ψ is far less than the θ. The little changes of the angle between the direction of the electric field E 1  and the direction of the electric dipole moment of the liquid crystal molecule can not be perceived by human eyes. Thus, an influence of the little changes of the value of the electric field can be ignored. 
         [0041]    Because the value of the angle between the direction of the electric field and the direction of the electric dipole moment of the liquid crystal molecule has a little change in any two continuous frames, the liquid crystal molecule will not stay in the same position in the liquid crystal layer  24 , correspondingly. A random collision probability between the liquid crystal molecule and the impurity ion increases, and a random collision probability among the impurity ions correspondingly increases. A probability that the impurity ions captured by the alignment films  23  and  25  decreases and a value of a residual DC electric field between the first alignment film  23  and the second alignment film  25  correspondingly decreases. The image residue phenomenon of the LCD  20  can be improved effectively. 
         [0042]      FIG. 6  is a waveform diagram of a data voltage and a common voltage of one of pixel units of an LCD according to a second embodiment of the present invention. In frame N−2, a value of the data voltage is Vdata 1  and a value of the common voltage is Vcom−Vb, where Vdata 1 &lt;Vcom, Vdata 1 &gt;0, Vcom&gt;0, Vb&lt;Vcom−Vdata 1 . A voltage difference between a pixel electrode (not shown) and a common electrode (not shown) of the pixel unit (not shown) is Vcom−Vdata 1 −Vb. In frame N−1, the value of the data voltage is Vdata 2  and the value of the common voltage is Vcom−Vb, where Vdata 2 &gt;Vcom, Vdata 2 −Vcom=Vcom−Vdata 1 . The voltage difference between the data voltage and the common voltage is Vdata 2 −Vcom+Vb. In frame N, the value of the data voltage is Vdata 1  and the value of the common voltage is Vcom+Vb. The voltage difference between the data voltage and the common voltage is Vcom−Vdtal+Vb. In frame N+1, the value of the data voltage is Vdata 2  and the value of the common voltage is Vcom+Vb. The voltage difference between the data voltage and the common voltage is Vdata 2 −Vcom−Vb. In frame N+2, the value of the data voltage is Vdata 1  and the value of the common voltage is Vcom−Vb. The voltage difference between the data voltage and the common voltage is Vcom−Vdata 1 −Vb. 
         [0043]    The values of the data voltage and the common voltage in frame N+2 are the same as that in frame N−2. That is, frame N−2, frame N−1, frame N, and frame N+1 define a minimum period. The values of the data voltage and the common voltage in the following frames repeat that in one of frame N−2, frame N−1, frame N, and frame N+1. 
         [0044]    The common voltage is generated by a common voltage generating circuit (not shown), and the common voltage generating circuit is the same as the common voltage generating circuit  34  of  FIG. 3 . However, waveforms of a first control signal received by a second input terminal of the common voltage generating circuit and a second control signal received by a third input terminal of the common voltage generating circuit need to change correspondingly. 
         [0045]      FIG. 7  is an abbreviate circuit diagram of a gamma voltage generating circuit of an LCD according to a third embodiment of the present invention. The gamma voltage generating circuit  75  includes an input terminal  750 , fourteen output terminals  760 , and fifteen resistors (not labeled). The input terminal  750  is used for receiving a DC voltage, and the fourteen output terminals  760  are used for outputting gamma voltages. The fifteen resistors are connected in series between the input terminal  750  and ground. That is, the fifteen resistors cooperatively form a voltage dividing circuit. A node between each two resistors is connected to one of the fourteen output terminals  760 . 
         [0046]      FIG. 8  is a waveform diagram of the DC voltage received by the input terminal of  FIG. 7 . In frame N−2, a value of the DC voltage is AVDD, where AVDD&gt;0. In frame N−1, the value of the DC voltage is AVDD−Vd, where Vd is less than five percent of AVDD. In frame N, the value of the DC voltage is AVDD. In frame N+1, the value of the DC voltage is AVDD+Vd. In frame N+2, the value of the DC voltage is AVDD. That is, the value of the DC voltage in frame N+2 is the same as that in frame N−2. Therefore, frame N−2, frame N−1, frame N, and frame N+1 define a minimum period. The value of the DC voltage in the following frames repeat that in one of frame N−2, frame N−1, frame N, and frame N+1. 
         [0047]      FIG. 9  is a waveform diagram of a data voltage and a common voltage of one of the pixel units of the LCD according to the third embodiment of the present invention. In frame N−2, a value of the data voltage is Vdata 1  and a value of the common voltage is Vcom, where Vdata 1 &lt;Vcom, Vdata 1 &gt;0, Vcom&gt;0. A voltage difference between a pixel electrode  96  and a common electrode  92  of the pixel unit (not labeled) is Vcom−Vdata 1 . A value of an electric field E 2  of the pixel unit is (Vcom−Vdata 1 )/d, where d is a vertical distance of the pixel electrode  96  and the common electrode  92 . A direction of the electric field E 2  of the pixel unit is from the common electrode  92  to the pixel electrode  96 . A value of an angle between the direction of the electric field E 2  and a direction of an electric dipole moment of the liquid crystal molecule is α. 
         [0048]    In frame N−1, the value of the data voltage is Vdata 2 −Vm and the value of the common voltage is Vcom, where Vdata 2 &gt;Vcom, Vm&lt;Vdata 2 −Vcom, Vdata 2 −Vcom=Vcom−Vdata 1 . The voltage difference between the pixel electrode  96  and the common electrode  92  of the pixel unit is Vdata 2 −Vcom−Vm. The value of the electric field E 2  of the pixel unit is (Vdata 2 −Vcom−Vm)/d and the direction of the electric field E 2  of the pixel unit is from the pixel electrode  96  to the common electrode  92 . The value of the angle between the direction of the electric field E 2  and the direction of the electric dipole moment of the liquid crystal molecule is α+β. 
         [0049]    In frame N, the value of the data voltage is Vdata 1  and the value of the common voltage is Vcom. The voltage difference between the pixel electrode  96  and the common electrode  92  of the pixel unit is Vcom−Vdata 1 . The value of the electric field E 2  is (Vcom−Vdata 1 )/d and the direction of the electric field E 2  is from the common electrode  92  to the pixel electrode  96 . The value of the angle between the direction of the electric field E 2  and the direction of the electric dipole moment of the liquid crystal molecule is α. 
         [0050]    In frame N+1, the value of the data voltage is Vdata 2 +Vm and the value of the common voltage is Vcom. The voltage difference between the pixel electrode  96  and the common electrode  92  of the pixel unit is Vdata 2 −Vcom+Vm. The value of the electric field E 2  is (Vdata 2 −Vcom+Vm)/d and the direction of the electric field E 2  is from the pixel electrode  96  to the common electrode  92 . The value of the angle between the direction of the electric field E 2  and the direction of the electric dipole moment of the liquid crystal molecule is α−β. 
         [0051]    In frame N+2, the value of the data voltage is Vdata 1 , a value of the common voltage is Vcom. The voltage difference between the pixel electrode  96  and the common electrode  92  of the pixel unit is Vcom−Vdata 1 . The value of the electric field E 2  is (Vcom−Vdata 1 )/d and the direction of the electric field E 2  is from the common electrode  92  to the pixel electrode  96 . The value of the angle between the direction of the electric field E 2  and the direction of the electric dipole moment of the liquid crystal molecule is α. 
         [0052]    The value and the direction of the electric field E 2  in frame N+2 are the same as that in frame N−2. That is, frame N−2, frame N−1, frame N, and frame N+1 define a minimum period. The value and the direction of the electric field E 2  in the following frames repeat that in one of frame N−2, frame N−1, frame N, and frame N+1. 
         [0053]    The value of Vm/d is approximately equal to the value of Va/d, and the value of β is approximately equal to the value of ψ. Thus, the LCD of the third embodiment has the same advantages with the LCD  20  of the first embodiment. 
         [0054]      FIG. 10  is a waveform diagram of a data voltage and a common voltage of one of pixel units of an LCD according to a fourth embodiment of the present invention. In frame N−2, a value of the data voltage is Vdata 1  and a value of the common voltage is Vcom. The voltage difference between a pixel electrode (not shown) and a common electrode (not shown) of the pixel unit is Vcom−Vdata 1 . In frame N−1, the value of the data voltage is Vdata 2 +Vn and the value of the common voltage is Vcom, where Vn&lt;Vdata 2 −Vcom. The voltage difference between the data voltage and the common voltage of the pixel unit is Vdata 2 −Vcom+Vn. In frame N, the value of the data voltage is Vdata 1 +Vn and the value of the common voltage is Vcom. The voltage difference between the data voltage and the common voltage of the pixel unit is Vcom−Vdata 1 -Vn. In frame N+1, the value of the data voltage is Vdata 2  and the value of the common voltage is Vcom. The voltage difference between the data voltage and the common voltage of the pixel unit is Vdata 2 −Vcom. In frame N+2, the value of the data voltage is Vdata 1 , the value of the common voltage is Vcom. The voltage difference between the data voltage and the common voltage of the pixel unit is Vcom−Vdata 1 . 
         [0055]    The value of the data voltage and the common voltage in frame N+2 are the same as that in frame N−2. That is, frame N−2, frame N−1, frame N, and frame N+1 define a minimum period. The value of the data voltage and the common voltage in the following frames repeat that in one of frame N−2, frame N−1, frame N, and frame N+1. 
         [0056]    The gamma voltage is generated by a gamma voltage generating circuit (not shown), and the gamma voltage generating circuit is the same as the gamma voltage generating circuit  75  of  FIG. 7 . However, a waveform of a DC voltage received by an input terminal of the gamma voltage generating circuit needs to change correspondingly. 
         [0057]    According to the above descriptions, a change law of the voltage difference between the data voltage and the common voltage of the pixel unit is as follows: 
         [0058]    The voltage difference between the data voltage and the common voltage of each pixel unit is a sum of a main voltage and an auxiliary voltage with periodical change. An absolute value of the main voltage is constant. An absolute value of the auxiliary voltage is less than the absolute value of the main voltage. In a minimum period, a sum of the auxiliary voltage is zero. For example, the value of the main voltage is Vcom−Vdata 1  or Vdata 2 −Vcom and the value of the auxiliary voltage is 0, ±Va, ±Vb, ±Vm, or ±Vn. The minimum period is frame N−2, frame N−1, frame N, and frame N+1. 
         [0059]    The value of the auxiliary voltage is 0 in frame N−2, the value of the auxiliary voltage is Va in frame N−1, the value of the auxiliary voltage is 0 in frame N, and the value of the auxiliary voltage is −Va in frame N+1. 
         [0060]    It is to be further understood that even though numerous characteristics and advantages of preferred and exemplary embodiments have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only; and that changes may be made in detail within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.