Patent Publication Number: US-8531445-B2

Title: Device for controlling the gate drive voltage in liquid crystal display and influencing the turn-on voltage to have a similar ripple to a turn-off voltage

Description:
BACKGROUND 
     The present invention relates to a device for controlling the gate drive voltage in a liquid crystal display. 
     A liquid crystal display comprises a liquid crystal panel and a backlight module. The backlight module provides planar light to the liquid crystal panel. The liquid crystal panel comprises an array substrate, a color filter substrate and a liquid crystal layer, and the liquid crystal layer is formed through injecting liquid crystal into the space between the array substrate and the color filter substrate that face each other. 
     The array substrate comprises a plurality of pixel units, and each of the pixel units may be formed with a plurality of gate lines, a plurality of data lines, a plurality of thin film transistors (TFTs), a plurality of pixel electrodes, a plurality of common electrode lines and the like. The gate lines, the data lines, and the common electrode lines can be collectively referred to as the signal lines. For example, the gate lines and the common electrodes line are laterally provided on the array substrate, the data lines are longitudinally provided on the array substrate, and the TFTs are provided at the intersections of the gate lines and the data lines. The TFTs are active switching elements and each may be formed with a gate electrode, a gate insulating layer, an active layer, a TFT channel, a source electrode, a drain electrode, a passivation layer and the like. The gate electrode is connected or integrally formed with one of the gate lines, the source electrode is connected or integrally formed with one of the data lines, and the drain electrode is normally connected with one of the pixel electrodes through a passivation layer via hole. When a turn-on (“ON”) voltage is input into one of the gate lines, the active layer of the TFT that is connected with the gate line becomes conductive, and the data signal over the data line connected with the TFT travels, through the TFT channel region, from the source electrode to the drain electrode, and ultimately into the pixel electrode. After receiving the signal input, the pixel electrode, together with a common electrode provided on the color filter substrate, forms an electric field to drive the liquid crystal to rotate. 
     The drive devices for driving the liquid crystal display include a BLU controller, a timing controller, a gate drive circuit, a data driver and the like. 
     Liquid crystal displays tend to have a thinner appearance and a lower manufacturing cost in recent years. With the development of liquid crystal displays, there emerges a liquid crystal display without an individual gate drive print circuit board (referred to as GATE PCB-less LCD). In a GATE PCB-less LCD, signals originally transmitted by the gate drive integrated circuit board are transmitted by circuits that are directly formed on the glass substrate used to form the array substrate, that is, the gate drive circuits are formed on the array substrate, and thus it is not necessary to form the individual gate drive integrated circuit board. The liquid crystal display, therefore, has a reduced thickness and a lowered manufacturing cost. 
     In the GATE PCB-less LCD, a source drive circuit board is used to output the turn-off (“OFF”) voltage (referred to as V off ) and the turn-on voltage (referred to as V on ), which are used to drive the gate lines, to the gate drive circuits, and to output the common voltage (referred to as V com ) used by the common electrode lines. Because of the resistance of the wirings on the array substrate that connect the source drive circuit board with the gate drive circuits, there exists a variation of V off  among the gate drive circuits (one gate drive circuit is used to transmit signal to one gate line) and a variation of V com  among the common electrode lines. In addition, for each pixel unit, in the case that parasitic capacitance is formed between the data line and the common electrode line on the array substrate and parasitic capacitance is formed between the gate line and the data line, a ripple of V com  and a ripple of V off  occur due to the influence of the data signal, and the magnitude and the waveform of the ripples of V com  and V off  are similar. The variation and ripple of V com  and V off  are especially significant between the first gate drive circuit and the second gate drive circuit, because the amount of the current flowing through the first gate drive circuit and the second gate drive circuit is the largest and thereby the voltage drop is also the largest. In contrast, V on , of the gate lines are almost not subject to the aforementioned variation and ripple, due to its short duration, and thus V on  at one gate drive circuit is almost the same as that at another gate drive circuit, i.e., V on  is relatively uniform among the gate drive circuits. 
       FIG. 1  is a waveform diagram showing V off  in a conventional GATE PCB-less LCD. 
     The solid lines a and a′ in  FIG. 1  are V off  of the first gate drive circuit, and the dashed lines b and b′ are V off  of the second gate drive circuit. The lines a and b represent the waveforms of V off  under positive data signal (Positive DATA), and the lines a′ and b′ represent the waveforms of V off  under negative data signal (Negative DATA). As shown in  FIG. 1 , in the GATE PCB-less LCD, the variation and ripple of V off  become more significant with the gate drive circuit of a bigger serial number. 
     Since V on  is relatively uniform among the gate drive circuits and V off  is subject to the above variation and ripple, the difference of the V on  and V off  at one gate drive circuit differs from that at another gate drive circuit. The difference of V on  and V off  is referred to as ΔV g , which is an important factor influencing the charging characteristic of a pixel electrode. 
       FIG. 2  is a diagram showing the variation of ΔV g  between the first gate drive circuit and the second gate drive circuit. In  FIG. 2 , line a represents the waveform of the V off  at the first gate drive circuit, line b represents the waveform of V off  at the second gate drive circuit, and line d represents the waveform of V on  at the first and second gate drive circuits; ΔV g-1  represents ΔV g  at the first gate drive circuit; and ΔV g-2  represents ΔV g  at the second gate drive circuit. As sown in  FIG. 2 , ΔV g-1  is larger than ΔV g-2 , that is, ΔV g-1  and ΔV g-2  differ from each other. Because of the variation of ΔV g , the ripple of the charging amount (referred to as ΔV p ) of a pixel electrode varies among different gate line circuits. ΔV p  is caused by the parasitic capacitance formed by the pixel electrode and the gate line. When the gate line switches between V off  and V on , the ripple of the charging amount is generated due to the parasitic capacitance. 
     The relationship between ΔV p  and ΔV g  is expressed as follows:
 
Δ V   p   =C   gd   *ΔV   g   /C   tot   (1)
 
 C   tot   =C   gd   +C   1c   +C   s   (2),
 
where C gd  is the parasitic capacitance between the gate line and the drain electrode, C 1c  stands for the liquid crystal capacitance, and C s  is the storage capacitance in parallel with the liquid crystal capacitance. Since C gd , C 1c , and C, are constants, ΔV p  is proportional to ΔV g .
 
     The variation of ΔV p  among the pixel electrodes corresponding to the gate drive circuits causes an abnormal block image in the lateral direction (also referred to as Y-Block phenomenon). The so-called Y-Block phenomenon is referred to the phenomenon that a variation of the gray level occurs among the driving regions of the gate drive circuits. The Y-Block phenomenon occurs when there exists a variation of ΔV g  among the gate drive circuits but does not occur when there is no variation of ΔV g . The Y-block phenomenon is an important factor that reduces the display quality of the liquid crystal display. The Y-Block phenomenon occurs when the variation of ΔV g  exists, irrespective of the polarities of the data signals. 
     SUMMARY 
     A device for controlling the gate drive voltage in the liquid crystal display is provided in an embodiment of the invention. The device for controlling the gate drive voltage in the liquid crystal display includes a turn-on voltage output terminal and a turn-off voltage output terminal for outputting a turn-on voltage and a turn-off voltage to a gate drive circuit, respectively, and a control circuit. The control circuit is coupled with the turn-on voltage output terminal and exerts an influence on the turn-on voltage, so that the turn-on voltage has a ripple similar to that of the turn-off voltage. 
     A liquid crystal display is provided in another embodiment of the invention. The liquid crystal display includes the above-described device for controlling the gate drive voltage. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: 
         FIG. 1  is a waveform diagram showing the V off  in a conventional GATE PCB-less LCD; 
         FIG. 2  is a diagram showing the variation of the ΔV g  between the first gate drive circuit and the second gate drive circuit; 
         FIG. 3  is a schematic view showing a device for controlling the gate drive voltage in the liquid crystal display according to a first embodiment of the invention; 
         FIG. 4  is a schematic view showing a device for controlling the gate drive voltage in the liquid crystal display according to a second embodiment of the invention; and 
         FIG. 5  is a waveform diagram showing the voltages when the device for controlling the gate drive voltage according to the first and second embodiments is employed in the liquid crystal display. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The device for controlling the gate drive voltage in the liquid crystal display according to the embodiments of the invention can apply to a thin film transistor liquid crystal display (TFT-LCD) such as a GATE PCB-less LCD. The device includes a turn-on voltage output terminal, a turn-off voltage output terminal, a common voltage output terminal, and a control circuit. Through employing the control circuit, the V on  of the turn-on voltage output terminal is rendered under the influence of the ripple of V com  of the common voltage output terminal or the ripple of V off  of the turn-off voltage output terminal. For example, the control circuit may be disposed between the turn-on voltage output terminal and the turn-off voltage output terminal so as to couple them together, which renders the ripple of V on  similar to that of V off . If V com  and V off  have similar ripples, the control circuit may be provided between the turn-on voltage output terminal and the common voltage output terminal so as to couple them together, and thus V on  is rendered under the influence of the ripple of V com  and thus the ripple of V on  is rendered similar to that of V off . In this way, ΔV g  (the difference between the V on  and the V off ) does not substantially vary with the ripple of V off  and can be kept relatively uniform among the gate drive circuits on the array substrate of the liquid crystal display, and accordingly the ripple of the charging amount ΔV E , of the pixel electrode becomes relatively uniform among the gate drive circuits. Therefore, the Y-Block phenomenon occurring among the gate drive circuit regions can substantially reduced. 
     Hereinafter, the embodiments of the invention will be described in detail with reference to the drawings. 
       FIG. 3  is a diagram showing the structure of a device for controlling the gate drive voltage in the liquid crystal display according to a first embodiment of the invention. As shown in  FIG. 3 , the device for controlling the gate drive voltage in the liquid crystal according to this embodiment includes a turn-on voltage output terminal  11 , a common voltage output terminal  12 , and a control circuit  13 . The control circuit in this embodiment is a capacitor  13 , for example. The turn-on voltage output terminal  11  and the common voltage output terminal  12  are coupled with each other through the capacitor  13 . Specifically, one end of the capacitor  13  is connected with both the turn-on voltage output terminal  11  and a gate drive circuit  100 , and the other end of the capacitor  13  is connected with both the common voltage output terminal  12  and the gate drive circuit  100 . In this embodiment, parasitic capacitance is formed between the data line on the array substrate and the common electrode and parasitic capacitance is formed between the gate line and the data line. Therefore, V com  and V off  have ripples with similar magnitudes and waveforms. The common voltage output terminal  12  is used to provide a common voltage for a liquid crystal display panel, and in this embodiment, the common voltage output terminal  12  provides the common voltage for a liquid display panel via the gate drive circuit  100 , but in another embodiment, the common voltage output terminal  12  may directly provide the common voltage for a liquid display panel without the help of the gate drive circuit  100 . 
     In this way, V com  of the common voltage output terminal  12  is used as the reference voltage of V on  of the turn-on voltage output terminal  11 , so that a linkage between V com  and V on  is established. Thus, V com  is under the influence of the ripple of V com , so that V on  has a ripple similar to that of V com . At this time, since V com  and V off  have similar ripples, V on  and the V off  also have similar ripples. Therefore, the difference ΔV g  between V on  and V off  can be kept relatively uniform among the drive circuits on the array substrate of the liquid crystal display, which reduces the Y-Block phenomenon. 
       FIG. 4  is a diagram showing the structure of a device for controlling the gate drive voltage in the liquid crystal display according to a second embodiment of the invention. As shown in  FIG. 4 , the device for controlling the gate drive voltage in the liquid crystal according to this embodiment includes a turn-on voltage output terminal  11 , a turn-off voltage output terminal  14 , and a capacitor  13 . The turn-on voltage output terminal  11  is coupled with the turn-off voltage output terminal  14  through the capacitor  13 . Specifically, one end of the capacitor  13  is connected with both the turn-on voltage output terminal  11  and a gate drive circuit  100 , and the other end of the capacitor  13  is connected with both the turn-off voltage output terminal  14  and the gate drive circuit  100 . 
     In this way, V off  of the turn-off voltage output terminal  14  is used as the reference voltage of V on  of the turn-on voltage output terminal  11 , so that a linkage between the V off  and the V on  is established. Thus, V on  is under the influence of the ripple of V off , so that V on  has a ripple similar to that of the V off . Therefore, the difference ΔV g  between the V on  and the V off  can be kept relatively uniform among the gate drive circuits, which reduces the Y-Block phenomenon. 
       FIG. 5  is a waveform diagram showing the voltages when the device for controlling the gate drive voltage according to the first or second embodiments is employed in the liquid crystal display. In  FIG. 5 , line a represents V off  of the first gate drive circuit, line b represents V off  of the second gate drive circuit, line d 1  represents V on  of the first gate drive circuit, line d 2  represents V on  of the second gate drive circuit, ΔV g-1 ′ is ΔV g  of the first gate drive circuit, and ΔV g-2 ′ is ΔV g  of the second gate drive circuit. As shown in  FIG. 5 , because of the control circuit such as the capacitor, V on  is under the influence of the ripple of V off  or V com  so that ΔV g-1 ′ and V g-2 ′ are similar to each other. Therefore, a significant variation of ΔV g  among the gate drive circuits will not be caused and the Y-Block phenomenon can be reduced. 
     It should be understood by those skilled in the art that although a simple circuit element such as the capacitor is used to realize the embodiments of the invention, the embodiments of the invention can employ various circuits such as a RC circuit, by which the turn-on voltage can be influenced by the ripple of the common voltage or the turn-off voltage so that the turn-on voltage has a ripple similar to that of the common voltage or the turn-off voltage and ΔV g  variation among the gate drive circuits can be eliminated. 
     A liquid crystal display is provided in another embodiment of the invention. The liquid crystal display includes the device for controlling the gate drive voltage, for example, shown in  FIG. 3  or  FIG. 4 . The liquid crystal display is a TFT-LCD. 
     It should be appreciated that the embodiments described above are intended to illustrate but not limit the present invention. Although the present invention has been described in detail herein with reference to the preferred embodiments, it should be understood by those skilled in the art that the present invention can be modified and some of the technical features can be equivalently substituted without departing from the spirit and scope of the present invention.