Patent Publication Number: US-8125478-B2

Title: Liquid crystal display and switching voltage controlling circuit thereof for reducing occurrence of color errors

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 97116984, filed on May 8, 2008. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display (LCD). More particularly, the present invention relates to a switching voltage controlling circuit of an LCD. 
     2. Description of Related Art 
     As the photoelectric industry advances, LCDs have been applied extensively to various kinds of electronic products. It should be mentioned that the LCD can barely provide equivalent switching voltages to each pixel unit due to the increasing dimension of a display panel of the LCD, and color errors then occur between blocks of the display panel. Detailed descriptions accompanying drawings are provided hereinafter. 
       FIG. 1A  is a schematic view of a conventional LCD.  FIG. 1B  is a schematic view of a pixel unit of the conventional LCD. Referring to  FIGS. 1A and 1B , an LCD  10  has a normally white frame when liquid crystals contained in the LCD  10  are not driven. The LCD  10  is composed of a display panel  40 , a plurality of source drivers  20 , and a plurality of gate drivers  31 ,  32 , and  33 . The source drivers  20  are coupled to each pixel unit  41  through source driving lines  101 . On the other hand, the gate drivers  31 ,  32 , and  33  are coupled to each of the pixel units  41  through gate driving lines  111 , respectively. Each of the pixel units  41  includes a transistor  121 , a storage capacitor  122 , and a pixel capacitor  123 . 
       FIG. 2  is a curve illustrating a relationship between a source driving current Id and a switching voltage Vg. Referring to  FIGS. 1A ,  1 B, and  2 , the gate drivers  31 ,  32 , and  33  receive a constant voltage VGL and thereby generate the switching voltage Vg for controlling each of the pixel units  41 . It should be noted that a transmission line  131  of the display panel  40  has a line resistance, and therefore the constant voltage VGL received by the gate driver  31  is slightly greater than the constant voltage VGL received by the gate driver  32 . Besides, the constant voltage VGL received by the gate driver  32  is slightly greater than the constant voltage VGL received by the gate driver  33 . 
     Based on the above, the switching voltage Vg provided by the gate driver  33  to the pixel unit  41  is slightly less than the switching voltage Vg provided by the gate driver  32  to the pixel unit  41 , and the switching voltage Vg provided by the gate driver  32  to the pixel unit  41  is slightly less than the switching voltage Vg provided by the gate driver  31  to the pixel unit  41 . Owing to the difference in the switching voltage Vg respectively received by each block of the display panel  40 , the color errors may arise between the blocks of the display panel  40 . Specifically, when the LCD  10  displays frames at the same color level, the pixel unit  41  of the gate driver  33  of the LCD  40  is somewhat brighter than the pixel unit  41  of the gate driver  32 , and the pixel unit  41  of the gate driver  32  of the LCD  40  is somewhat brighter than the pixel unit  41  of the gate driver  31 . The above description is, for instance, exemplified in  FIG. 3  that is a schematic view of a display panel in which color errors occur between the blocks. 
       FIG. 4  is a schematic view of another conventional LCD. In order to prevent the color errors occurring between the blocks of the display panel, a solution in which a current-limiting resistor  141  is additionally disposed between the constant voltage VGL and the gate driver  31  is proposed according to the pertinent art. After the current-limiting resistor  141  is disposed, the equivalent resistance of the transmission line  131  is increased, and a current passing through the transmission line  131  is then decreased when the constant voltage VGL remains unchanged. As a result, the voltage difference in the constant voltage VGL respectively received by the gate drivers  31 ,  32 , and  33  is reduced. As such, the difference in the switching voltage Vg respectively generated by the gate drivers  31 ,  32 , and  33  is reduced as well. The color errors occurring between the blocks of the display panel  40  are then eliminated. 
     Nevertheless, the occurrence of color errors between the blocks of the display panel  40  can only be precluded by employing the current-limiting resistors  141  with different resistance values when different display panels  40  are used. That is to say, given that different types of the display panels  40  are used, the display panel manufacturers must find the proper resistance value for each type of the display panel by way of trial and error, which is time-consuming and inconvenient. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a switching voltage controlling circuit for prohibiting an occurrence of color errors between blocks of a display panel. 
     The present invention is further directed to an LCD in which the aforesaid switching voltage controlling circuit is directly configured, so as to reduce the occurrence of the color errors between the blocks of the display panel. 
     The present invention provides a switching voltage controlling circuit adapted to an LCD. The LCD includes a display panel and a plurality of gate drivers. The display panel includes a plurality of pixel units. The switching voltage controlling circuit includes a current-controlled switch, a transmission line, and a feedback circuit. The current-controlled switch has a first terminal and a second terminal, wherein the first terminal of the current-controlled switch is coupled to a constant voltage. The transmission line has a first terminal and a second terminal, wherein the first terminal of the transmission line is coupled to the second terminal of the current-controlled switch. Besides, the transmission line is serially coupled to the gate drivers. Each of the gate drivers generates a switching voltage according to a voltage provided by the transmission line and controls the pixel units of the display panel. The feedback circuit is coupled to the transmission line and the current-controlled switch. Additionally, the feedback circuit regulates an amount of a current passing through the current-controlled switch according to a voltage difference between the first terminal and the second terminal of the transmission line. 
     According to an embodiment of the present invention, the current-controlled switch is a transistor operated in an active region. 
     According to an embodiment of the present invention, the feedback circuit includes a first amplifier, a first voltage dividing resistor, a second voltage dividing resistor, and a second amplifier. A first input terminal and a second input terminal of the first amplifier are coupled to the first terminal and the second terminal of the transmission line, respectively. A first terminal of the first voltage dividing resistor is coupled to an output terminal of the first amplifier. A first terminal and a second terminal of the second voltage dividing resistor are coupled to a second terminal of the first voltage dividing resistor and a first voltage, respectively. A first input terminal and a second input terminal of the second amplifier are coupled to the second terminal of the first voltage dividing resistor and a second voltage, respectively. An output terminal of the second amplifier outputs a control voltage for regulating the amount of the current passing through the current-controlled switch. According to another embodiment of the present invention, the first voltage is a ground voltage, and the second voltage is greater than the first voltage. 
     From another perspective, the present invention provides an LCD having the aforesaid switching voltage controlling circuit, such that the color errors occurring between the blocks of the display panel can be reduced. 
     The voltage difference between the two terminals of the transmission line is monitored by the feedback circuit according to the present invention, and thereby the current passing through the transmission line is controlled. Hence, the transmission line is able to provide the voltages close to the same level for each driving circuit, such that the color errors occurring between the blocks of the display panel can be decreased. 
     To make the aforesaid features and advantages of the present invention more comprehensible, several embodiments accompanied with figures are described in detail below. 
    
    
     
       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. 1A  is a schematic view of a conventional LCD. 
         FIG. 1B  is a schematic view of a pixel unit of the conventional LCD. 
         FIG. 2  is a curve illustrating a relationship between a source driving current Id and a switching voltage Vg. 
         FIG. 3  is a schematic view of a display panel in which color errors occur between blocks. 
         FIG. 4  is a schematic view of another conventional LCD. 
         FIG. 5A  is a schematic view of an LCD and a switching voltage controlling circuit of the LCD according to a first embodiment of the present invention. 
         FIG. 5B  is a circuit diagram of a feedback circuit and a current-controlled switch according to the first embodiment of the present invention. 
         FIG. 6  is a schematic view of an LCD and a switching voltage controlling circuit of the LCD according to a second embodiment of the present invention. 
         FIG. 7  is a schematic view of another LCD and a switching voltage controlling circuit of the LCD according to the second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG. 5A  is a schematic view of an LCD and a switching voltage controlling circuit of the LCD according to a first embodiment of the present invention. Referring to  FIGS. 5A and 1B , an LCD  11  includes a display panel  40 , a plurality of gate drivers (marked as  31 ,  32 , and  33  in the present embodiment), a plurality of source drivers  20 , and a switching voltage controlling circuit  50 . The switching voltage controlling circuit  50  includes a current-controlled switch  60 , a transmission line  70 , and a feedback circuit  80 . The display panel  40  includes a plurality of pixel units  41 . The source drivers  20  and the gate drivers  31 ,  32 , and  33  can be disposed in a non-display region of the display panel  40 . The source drivers  20  are respectively coupled to each of the pixel units  41  through source driving lines  101 . On the other hand, the gate drivers  31 ,  32 , and  33  are coupled to each of the pixel units  41  through gate driving lines  111 , respectively. The source drivers  20  and the gate drivers  31 ,  32 , and  33  can be used to control the pixel units  41 . Each of the pixel units  41  includes a transistor  121 , a storage capacitor  122 , and a pixel capacitor  123 . 
     A first terminal and a second terminal of the current-controlled switch  60  are respectively coupled to a constant voltage VGL and a first terminal of the transmission line  70 . Here, the current-controlled switch  60  controls a current Ib passing through the current-controlled switch  60  based on a control voltage Vc provided by the feedback circuit  80 . The transmission line  70  is serially coupled to the gate drivers  31 ,  32 , and  33  in sequence. The transmission line  70  has a line resistance. Hence, the longer a transmission path between each of the gate drivers  31 ,  32 , and  33  and the constant voltage VGL is, the lower the voltage received by each of the gate drivers  31 ,  32 , and  33  is. Specifically, the transmission path between the gate driver  33  and the constant voltage VGL is longer than the transmission path between the gate driver  32  and the constant voltage VGL, and the transmission path between the gate driver  32  and the constant voltage VGL is longer than the transmission path between the gate driver  31  and the constant voltage VGL. As a result, the voltage received by the gate driver  33  from the transmission line  70  is less than the voltage received by the gate driver  32  from the transmission line, and the voltage received by the gate driver  32  from the transmission line  70  is also less than the voltage received by the gate driver  31  from the transmission line  70 . 
     The gate drivers  31 ,  32 , and  33  generate the switching voltage Vg according to the respective voltages received from the transmission line  70  and thereby control each of the pixel units  41 . Therefore, given that the respective voltages received by each of the gate drivers  31 ,  32 , and  33  from the transmission line  70  differ from one another to a certain degree, color errors may occur between blocks of the display panel  40 . In accordance with the Ohm&#39;s Law, the line resistance of the transmission line  70  can scarcely affect the respective voltages received by each of the gate drivers  31 ,  32 , and  33 , given that the current Ib is of a relatively small value. In other words, the respective voltages received by each of the gate drivers  31 ,  32 , and  33  from the transmission line  70  are more prone to reach similar values. (Note: the switching voltage discussed in the present invention is referred to as a turn-on voltage or a turn-off voltage.) 
     In light of the foregoing, the feedback circuit  80  is employed in the present embodiment for controlling the current Ib. A first input terminal and a second input terminal of the feedback circuit  80  are coupled to the first terminal and a second terminal of the transmission line  70 , respectively. That is to say, the feedback circuit  80  is capable of generating the control voltage Vc based on the voltage difference between the first terminal and the second terminal of the transmission line  70  and providing the same to the current-controlled switch  60 , so as to control the amount of the current Ib. In detail, as the voltage difference between the first terminal and the second terminal of the transmission line  70  exceeds a predetermined value, the feedback circuit  80  may reduce the amount of the current Ib to prevent the occurrence of the color errors between the blocks of the display panel  40 . In order to have people skilled in the art understand the present invention, the feedback circuit  80  and the current-controlled switch  60  are embodied hereinafter. 
       FIG. 5B  is a circuit diagram of the feedback circuit and the current-controlled switch according to the first embodiment of the present invention. Referring to  FIG. 5B , the current-controlled switch  60  is exemplified as a transistor  221  operated in an active region according to the present embodiment. Variations in the control voltage Vc received by a gate terminal of the transistor  221  may result in a difference in the amount of the current Ib passing through the transistor  221 . In particular, according to the present embodiment, when the control voltage Vc has a relatively large value, the amount of the current Ib is correspondingly increased. On the contrary, when the control voltage Vc has a relatively small value, the amount of the current Ib is correspondingly decreased. 
     On the other hand, the feedback circuit  80  includes two amplifiers  201  and  202  and two voltage dividing resistors  211  and  212 . A first input terminal and a second input terminal of the amplifier  210  are coupled to the first terminal and the second terminal of the transmission line  70 , respectively. Here, the amplifier  210  generates a voltage Vx 1  based on the voltage difference between the first terminal and the second terminal of the transmission line  70 . In detail, suppose the voltage at the first terminal of the transmission line  70  is Vop 1  and the voltage at the second terminal of the transmission line  70  is Vop 2 , the voltage Vx 1  is equal to Vop 1 −Vop 2  (Vx 1 =Vop 1 −Vop 2 ). Since the voltage Vop 1  is greater than the voltage Vop 2 , the voltage Vx 1  is larger than a ground voltage GND. 
     A first terminal of the voltage dividing resistor  211  is coupled to an output terminal of the amplifier  201 . A first terminal and a second terminal of the voltage dividing resistor  212  are coupled to a second terminal of the voltage dividing resistor  211  and the ground voltage GND, respectively. In view of the theorem of voltage division, the voltage dividing resistors  211  and  212  can generate a voltage Vx 2  based on the voltage Vx 1 , and the voltage Vx 1  is greater than the voltage Vx 2 . A first input terminal and a second input terminal of the amplifier  202  are coupled to the second terminal of the voltage dividing resistor  211  and a voltage Vr, respectively, and thereby the control voltage Vc is generated. Here, the control voltage Vc=Vr−Vx 2 . Besides, the amount of the current passing through the current-controlled switch  60  can be adjusted according to the control voltage Vc. It is likely for people skilled in the art to define the voltage Vr based on actual demands, while it should be taken into account that the voltage Vr of the present embodiment must be greater than the ground voltage GND, such that the control voltage Vc can be stabilized. 
     According to the present embodiment, the use of the voltage dividing resistors  211  and  212  for generating the voltage Vx 2  is conducive to adjusting the voltage Vx 2  in a flexible manner. The line resistance between terminals P 1  and P 3  is similar to that between terminals P 2  and P 3 , and thus the voltage difference between the terminals P 1  and P 3  is also similar to that between the terminals P 2  and P 3 . The voltage dividing resistors  211  and  212  having the same resistance value are used for accurately estimating the voltage difference between the terminals P 1  and P 3 . As such, the voltage Vx 2  denotes the voltage difference between the terminals P 1  and P 3 . 
     People skilled in the art should be aware that  FIG. 5B  merely depicts one embodiment of the feedback circuit  80  and the current-controlled switch  60 , which is not limited in the present invention. In other embodiments, people skilled in the art are able to modify the above embodiment based on the actual demands. For example, the voltage dividing resistors  211  and  212  having different resistance values can be used. 
     Referring to  FIG. 5B  again, as discussed hereinbefore, the voltage difference between the terminals P 1  and P 3  is increased when the amount of the current Ib is excessively large, leading to the occurrence of the color errors between the blocks of the display panel  40 . Meanwhile, the voltage Vx 1  obtained by subtracting the voltage Vop 2  from the voltage Vop 1  and the voltage Vx 2  are increased as well. As such, the control voltage Vc obtained by subtracting the voltage Vx 2  from the voltage Vr is decreased. Since the amount of the current Ib is relevant to the value of the control voltage Vc, the amount of the current Ib is reduced together with the decrease in the value of the control voltage Vc. Hence, the voltage difference between the terminals P 1  and P 3  is correspondingly reduced. Namely, the voltage difference between the terminals P 1  and P 3  is stabilized to be close to the voltage Vr. 
     From another perspective, the voltage difference between the terminals P 1  and P 3  is decreased when the amount of the current Ib is excessively small. In the meantime, the voltage Vx 1  obtained by subtracting the voltage Vop 2  from the voltage Vop 1  and the voltage Vx 2  are decreased as well. As such, the control voltage Vc obtained by subtracting the voltage Vx 2  from the voltage Vr is increased. Since the amount of the current Ib is relevant to the value of the control voltage Vc, the amount of the current Ib is increased together with the increase in the value of the control voltage Vc. Hence, the voltage difference between the terminals P 1  and P 3  is correspondingly enhanced. Namely, the voltage difference between the terminals P 1  and P 3  is stabilized to be close to the voltage Vr. As such, the occurrence of the color errors between the blocks of the display panel  40  can be reduced. 
     In comparison with the related art, the present embodiment is directed to avoiding the occurrence of the color errors between the blocks of the display panel due to the use of the current-limiting resistors having constant values. On the other hand, the feedback circuit of the present embodiment can be applied to the panels of different dimensions, and therefore it is not necessary to, by way of trial and error, place the current-limiting resistors having different values onto various panels. 
     Note that although the above embodiment has disclosed a possible type of the LCD and the switching voltage controlling circuit thereof, it is common sense to people of ordinary knowledge in this art that different manufacturers may develop different designs of the LCDs and the switching voltage controlling circuits thereof, and the application of the present invention should not be limited to this type only. Namely, using the feedback circuit for modulating the voltage difference between each of the gate drivers and for adjusting the current passing through each of the gate drivers to reduce the occurrence of the color errors between the blocks of the display panel should be deemed to have conformed to the essence of the present invention. Some other embodiments are further discussed hereinafter to allow people skilled in the art to recognize and embody the present invention. 
     Second Embodiment 
     People skilled in the art are able to change traces of the transmission line  70  introduced in the first embodiment based on the actual demands.  FIG. 6  is a schematic view of an LCD and a switching voltage controlling circuit of the LCD according to a second embodiment of the present invention. Please refer to  FIGS. 5B and 6  which are similar figures. Descriptions of the same reference numbers used to refer to the same parts in  FIGS. 6 and 5B  will be omitted. Note that the traces of the transmission line  70  in  FIG. 6  pass through the source drivers  20 . As such, the transmission line  70  of the present embodiment can be arranged in a more feasible way without sacrificing the technical effects achieved in the first embodiment.  FIG. 7  is a schematic view of another LCD and a switching voltage controlling circuit of the LCD according to the second embodiment of the present invention. Referring to  FIG. 7 , people skilled in the art can accomplish the technical effects that are similar to those provided in  FIG. 6  by proportionally adjusting the resistance value of the voltage dividing resistors  211  and  212 . 
     To sum up, the voltage difference between the two terminals of the transmission line is monitored by the feedback circuit according to the present invention, and thereby the current passing through the transmission line is controlled. As a result, the transmission line is able to provide the voltages close to the same level for each driving circuit, and thus the color errors occurring between the blocks of the display panel can be reduced. In comparison with the related art, the present invention can effectively prohibit the occurrence of the color errors between the blocks of the display panel due to the use of the current-limiting resistors having constant values. On the other hand, the feedback circuit of the present embodiment can be applied to the panels of different dimensions, and therefore it is not necessary to, by way of trial and error, place the current-limiting resistors having different values onto various panels. 
     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.