Patent Publication Number: US-2007115274-A1

Title: Circuit for setting up common voltage and method therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the priority benefit of Taiwan application serial no. 94140910, filed on Nov. 22, 2005. All disclosure of the Taiwan application is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of Invention  
      The present invention relates to a circuit for setting up voltage and the method therefor. More particularly, the present invention relates to a circuit for setting up the common voltage of a display panel and the method therefor.  
      2. Description of Related Art  
      A flat display panel generally comprises an active matrix substrate, an opposite substrate, and a liquid crystal layer between the two substrates. Wherein, the active matrix substrate includes a substrate, a plurality of pixel structures arranged in an array on the substrate, a plurality of scan lines, and a plurality of data lines. The foregoing pixel structure mainly comprises a thin film transistor, a pixel electrode, and a storage capacitance. Generally speaking, scan lines and data lines respectively transfer scan signal voltages and data voltages to corresponding pixel structures, so that the liquid crystal display panel can achieve the display function.  
      In a display panel, each intersection of the scan lines and data lines is disposed with a thin film transistor. Wherein, the source of the thin film transistor is coupled to the corresponding data line, the gate thereof is coupled to the corresponding scan line, and the drain thereof is coupled to a common voltage through a storage capacitance and a pixel electrode connected in parallel.  
      Generally speaking, when the display panel performs column-wise scanning, the parasitic capacitance Cgd between the gate and the drain of the thin film transistor affects the voltage at the drain, which would suddenly cause the driving voltage of the pixel electrode to drop a voltage level, and the abruptly dropped voltage is called a kickback voltage. The frame quality of the display panel will be affected due to the kickback voltage. In the conventional technology, to reduce the kickback voltage, images are viewed manually to adjust the common voltage level to the kickback voltage level.  
      However, human&#39;s visual perception is different, and the manual adjustment is less efficient, so errors may vary from people to people.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention is directed to provide a method for measuring kickback voltage and setting up common voltage, so as to improve efficiency and reduce errors caused by manual adjustment.  
      The present invention provides a circuit for setting up common voltage and the device is applicable to display panels. According to the present invention, the circuit includes: a voltage-dividing circuit used for receiving a DC bias and dividing the DC bias according to a voltage-dividing proportion to produce a common voltage; a coupler having a positive input terminal receiving the common voltage output by the voltage-dividing circuit and its output terminal and negative input terminal being coupled with each other; a switch determining whether to transfer the common voltage output by the coupler to the display panel according to a control signal; and a testing module transferring a constant voltage to all the data lines in the display panel and fixing the scan signal period and the frame switching ratio of the display panel to measure the kickback voltage of the display panel.  
      Wherein, the testing module further adjusts the voltage-dividing proportion according to the kickback voltage. When the common voltage level is equal to the kickback voltage level, the testing module produces a control signal to turn on the switch and further to transfer the common voltage to the display panel.  
      According to another embodiment of the present invention, in the circuit of the present invention, the testing module is a field programmable gate array (FPGA) chip and a display module.  
      In addition, the voltage-dividing circuit includes: a first resistor having one terminal grounded; a variable resistor having one terminal coupled to the other terminal of the first resistor, and having a central terminal coupled to the other terminal of the variable resistor and to the testing module and the positive input terminal of the coupler; a second resistor having one terminal coupled to the other terminal of the variable resistor and one terminal of the third resistor, and the other terminal of the third resistor receiving DC bias −15V, wherein the testing module determines the resistance of the variable resistor according to the output of the display panel.  
      In an embodiment of the present invention, the constant voltage may be ground voltage. In addition, the scan signal period of the display panel can be set at 32 μs, and the frame switching ratio of the display panel can be set at 20 ms. Here in the setup circuit of the display panel, the display panel is an active matrix display panel.  
      According to another aspect of the present invention, a method for setting up common voltage is provided, which is also applicable to display panels. The method includes: first transferring a constant voltage to all the data lines in the display panel, and fixing the scan signal period and the frame switching ratio of the display panel; next, measuring the kickback voltage of the display panel and automatically adjusting the common voltage level to be equal to the kickback voltage level; finally, transferring the adjusted common voltage to the display panel.  
      In the present invention, the common voltage level can be automatically adjusted to be equal to the kickback voltage level, thus, compared with the conventional technology, the efficiency is improved considerably and the difference due to visual perception is avoided.  
      In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, a preferred embodiment accompanied with figures is described in detail below.  
      It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     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 diagram illustrating a circuit for setting up common voltage according to an embodiment of the present invention.  
       FIG. 2  is a flowchart illustrating a method for setting up common voltage according to an embodiment of the present invention.  
       FIG. 3  is a timing diagram illustrating the timing produced by a timing controller. 
    
    
     DESCRIPTION OF EMBODIMENTS  
       FIG. 1  is a diagram illustrating a circuit for setting up common voltage according to an embodiment of the present invention. Referring to  FIG. 1 , the circuit of the present invention includes a voltage-dividing circuit  120 , a coupler  109 , a switch  111 , and a testing module  117 . Wherein, the voltage-dividing circuit  120  is coupled to the positive input terminal of the coupler  109  and the testing module  117 . In addition, the output terminal and the negative input terminal of the coupler  109  are coupled with each other and to the display panel  113  through the switch  111 , wherein the switch  111  determines whether to be turned on according to the output of the testing module  117 .  
      In the present embodiment, the voltage-dividing circuit  120  includes a resistor  101 , a variable resistor  103 , a resistor  105 , and a resistor  107 . Wherein, one terminal of the resistor  101  is grounded, the other terminal thereof is coupled to one terminal of the variable resistor  103 . The variable resistor is coupled to the resistors  101  and  105  respectively, and the variable resistor  103  has a central terminal also coupled to the resistor  105  and the positive input terminal of the coupler  109 . Moreover, one terminal of the resistor  105  is coupled to the variable resistor  103 , and the other terminal thereof is coupled to a DC bias through the resistor  107 . In the present embodiment, the DC bias is −15V.  
      In the present embodiment, the resistance of the variable resistor  103  is determined by the testing module  117 . Thus, the testing module  117  can change the voltage-dividing proportion of the voltage-dividing circuit  120  by adjusting the resistance of the variable resistor  103 . Accordingly, the voltage-dividing circuit  120  divides the DC bias based on the voltage-dividing proportion to produce the common voltage required by the common voltage line of the display panel  113 , and then transfers the common voltage to the positive input terminal of the coupler  109 .  
      It is known that the display panel  113  has a plurality of scan lines, data lines, and common voltage lines. Wherein, the scan lines are coupled to the gate driver  119 , the data lines are coupled to the source driver  121 , and the common voltage lines transfer the common voltage to each pixel in the display panel  113 . In addition, the operations of the gate driver  119  and the source driver  121  are controlled by the timing controller  115 . Wherein, the timing controller  115  controls the gate driver  119  to produce scan signals and the scan signals are respectively transferred to the display panel  113  through the scan lines, and the source driver  121  transfers the data voltage to the corresponding pixel through one of the data lines (not shown) in the display panel  113  according to the output of the timing controller  115 . Since the structure of the display panel is not the focus of the present invention, and the driving technology of display panel is known to those with ordinary skill in the art, so they are not described here.  
      Referring to  FIG. 1  again, in the present invention, when the manufacturer is about to adjust the common voltage of the display panel, the testing module  117  can be made to control the timing controller  115  to transfer a constant voltage to all the data lines of the display panel. The constant voltage may be set as 0V, i.e. ground voltage. This is to keep the output of all the data lines as 0V to be transferred to the display panel, and to further keep all the scan lines scanning. Next, the testing module  117  controls the timing controller  115  to fix the scan signal period and frame switching ratio of the display panel  113 . For example, as shown in  FIG. 3 , the scan signal period can be set at 32 μs, and the frame switching ratio can be set at 20 ms.  
       FIG. 3  is a timing diagram illustrating the timing produced by a timing controller. Referring to both  FIG. 1  and  FIG. 3 , the testing module  117  keeps scanning for 2 seconds after it has completed the foregoing operations, and then a signal for measuring the kickback voltage of the display panel  113  is sent by the display panel  113  to the testing module  117 , so as to obtain the kickback voltage level of the display panel  113 , as the Δ V shown in  FIG. 3 . In some selected embodiments, the testing module  117  can be implemented with a field programmable gate array (FPGA). While in the present embodiment, the testing module  117  further includes a display module  118  for displaying the kickback voltage level.  
      When the testing module  117  has measured the kickback voltage Δ V, it generates a control signal for the voltage-dividing circuit  120  to adjust the resistance of the variable resistor  103 , so as to make the common voltage level produced by the voltage-dividing circuit  120  to be equal to the kickback voltage level.  
      When the testing module  117  detects that the voltage level at the output terminal of the coupler  109  is equal to the kickback voltage, the testing module  117  sends a control signal to the switch  111  to turn on the switch  111 , so that the common voltage output by the coupler  109  can be transferred to the common voltage lines of the display panel  113 . In the present invention, because the common voltage level is adjusted to be equal to the kickback voltage level, change in the image quality of the display panel  113  can be avoided.  
       FIG. 2  is a flowchart illustrating a method for setting up common voltage according to an embodiment of the present invention. First, as described in step S 201 , a constant voltage is transferred to all the data lines of the display panel, and as described in step S 203 , the scan signal period of the display panel is fixed, and as described in step S 205 , the frame switching ratio of the display panel is fixed. Next, in step S 207 , the kickback voltage level of the display panel is measured, then in step S 209 , the common voltage level is automatically adjusted to be equal to the kickback voltage level. Finally, as described in step S 211 , the common voltage is transferred to the display panel.  
      In overview, the present invention provides a device for setting up common voltage and the device is applicable to display panels. Since the device for setting up common voltage provided by the present invention can automatically adjust the common voltage level to be equal to the kickback voltage level, the quality of the displayed images of the display panel can be improved. And since manual correction is not necessary in the present invention, the panel testing efficiency can be improved, and the labor cost can be reduced.  
      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.