Patent Publication Number: US-2013249882-A1

Title: Liquid Crystal Display Device and Driving Method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of displaying techniques, and in particular to a liquid crystal display device and a driving method thereof. 
     2. The Related Arts 
     A liquid crystal display device often comprises a first substrate, a second substrate, and a liquid crystal layer arranged between the first substrate and the second substrate. The liquid crystal display device comprises a plurality of pixel units, each of which comprises a pixel electrode made of indium tin oxide and formed on the first substrate and a common electrode formed on the second substrate. 
     As shown in  FIG. 1 , a single pixel unit is taken as an example for illustration purposes. A known driving circuit for liquid crystal display device comprises: a scan line  110 , a data line  120 , a first TFT (Thin Film Transistor)  130 , a liquid crystal capacitor  141 , and a storage capacitor  142 . The liquid crystal capacitor  141  is constituted by a pixel electrode  1411  formed on the first substrate and a common electrode  1413  formed on the second substrate. The storage capacitor  142  is constituted by the pixel electrode  1411  and a common electrode  1423  formed on the first substrate. The first TFT  130  has a gate terminal g electrically connected to the scan line  110 , a source terminal electrically connected to the data line  120 , and a drain terminal d electrically connected to the pixel electrode  1411  of the liquid crystal capacitor  141  and the storage capacitor  142 . 
     In operation, a scan signal is applied through the scan line  110  to the gate terminal g of the first TFT  130  to conduct the first TFT  130  on. A data signal is applied through the data line  120  to the source terminal s of the first TFT  130 . When the scan signal sets the first TFT  130  in a conduction condition, the data signal is applied through the drain terminal d of the first TFT  130  to the pixel electrode  1411  of the liquid crystal capacitor  141 . When the voltage applied across the liquid crystal capacitor  141  varies, the orientation of liquid crystal molecules of the liquid crystal layer is changed to thereby change the transmission rate of light transmitting through the pixel unit and thus controlling displayed brightness of the pixel unit.  FIG. 2  is a plot showing the scan signal and the waveform of voltage detected on the pixel electrode of the circuit shown in  FIG. 1 . Also referring to  FIG. 2 , due to the existence of parasitic capacitor  150 , at the very moment when the first TFT  130  is turned off (namely the time when the scan signal  210  is at the descending edge), the parasitic capacitor  150  conducts the scan signal  210  to the pixel electrode  1411 , thereby lowering the level of voltage  220  applied to the pixel electrode  1411 . Such an amount of voltage reduced in this way is often referred to as “feed-through voltage”. 
     Since the parasitic capacitors  150  of a specific scan line  110  are gradually increased from two opposite sides of a display panel toward a center, this leads a gradually reduction of the feed-through voltage applied through the parasitic capacitor  150 , whereby voltage difference between the pixel electrode  1411  and the common electrode  1413  that is formed on the second substrate is gradually increased. Consequently, the level of the feed-through voltage at different locations is different, wherein the feed-through voltage is relatively large at locations close to edges of the display panel and the feed-through voltage at the central area of the liquid crystal display panel is relatively small. As a consequence, for a low grey scale image, the left and right side edges of the liquid crystal display panel show relatively great brightness, leading to a defect of non-uniformity of brightness and affecting the quality of displaying. 
     SUMMARY OF THE INVENTION 
     The primary technical issue to be addressed by the present invention is to provide a liquid crystal display device and a driving method thereof, which may correct difference of feed-through voltages of the same scan line that is caused by parasitic resistor and parasitic capacitor in order to improve uniformity of brightness of the liquid crystal display device. 
     To address the above technical issue, the present invention adopts a technical solution by providing a liquid crystal display device. The liquid crystal display device comprises a plurality of pixel units arranged in an array and the pixel unit comprises a first substrate and a second substrate that are arranged opposite to each other and a liquid crystal layer interposed between the first and second substrates; wherein the first substrate comprises a data line and a scan line intersecting the data line, a pixel electrode formed in an area delimited by two adjacent scan lines and two adjacent data lines, and a first thin film transistor arranged at the intersection of the data line and the scan line, the first thin film transistor having a gate terminal connected to the scan line, a source terminal connected to the data line, and a drain terminal connected to the pixel electrode; wherein the liquid crystal display device further comprises: a first voltage source, which functions to provide a first voltage; a second voltage source, which functions to provide a second voltage; and a switching unit, which is arranged at the connection between the gate terminal of the first thin film transistor and the scan line, the switching unit having a control terminal electrically connected to the scan line, an input terminal electrically connected to the first voltage source, and an output terminal electrically connected to the second voltage source and a common electrode of a storage capacitor of the pixel unit; wherein the switching unit comprises at least one thin film transistor and the switching unit has a gate terminal electrically connected to the scan line, a source terminal electrically connected to the first voltage source, and a drain terminal electrically connected to a common terminal of the second voltage source and the common electrode of the storage capacitor of the pixel unit, the storage capacitor being formed of the pixel electrode and the common electrode of the storage capacitor, wherein the pixel electrode and the common electrode of the storage capacitor are formed on the first substrate; whereby when the switching unit receives a scan signal, the first voltage source supplies the first voltage to the common electrode of the storage capacitor of the pixel unit; and when the switching unit receives no scan signal, the second voltage source supplies the second voltage to the common electrode of the storage capacitor of the pixel unit so as to reduce difference of feed-through voltage between a plurality of pixel units of the scan line; and wherein the first voltage is less than the second voltage. 
     Wherein, the liquid crystal capacitor is constituted by the pixel electrode, the common electrode formed on the second substrate, and the liquid crystal layer, the common electrode formed on the second substrate being electrically connected to the second voltage source. 
     Wherein, the first voltage has a voltage value of 6.8V and the second voltage has a voltage value of 7.5V. 
     To address the above technical issue, the present invention adopts another technical solution by providing a liquid crystal display device. The liquid crystal display device comprises a plurality of pixel units arranged in an array and the pixel unit comprises a first substrate and a second substrate that are arranged opposite to each other and a liquid crystal layer interposed between the first and second substrates; wherein the first substrate comprises a data line and a scan line intersecting the data line, a pixel electrode formed in an area delimited by two adjacent scan lines and two adjacent data lines, and a first thin film transistor arranged at the intersection of the data line and the scan line, the first thin film transistor having a gate terminal connected to the scan line, a source terminal connected to the data line, and a drain terminal connected to the pixel electrode; wherein the liquid crystal display device further comprises: a first voltage source, which functions to provide a first voltage; a second voltage source, which functions to provide a second voltage; and a switching unit, which is arranged at the connection between the gate terminal of the first thin film transistor and the scan line, the switching unit having a control terminal electrically connected to the scan line, an input terminal electrically connected to the first voltage source, and an output terminal electrically connected to the second voltage source and a common electrode of a storage capacitor of the pixel unit; whereby when the switching unit receives a scan signal, the first voltage source supplies the first voltage to the common electrode of the storage capacitor of the pixel unit; and when the switching unit receives no scan signal, the second voltage source supplies the second voltage to the common electrode of the storage capacitor of the pixel unit so as to reduce difference of feed-through voltage between a plurality of pixel units of the scan line; and wherein the first voltage is less than the second voltage. 
     Wherein, the switching unit comprises at least one thin film transistor and the switching unit has a gate terminal electrically connected to the scan line, a source terminal electrically connected to the first voltage source, and a drain terminal electrically connected to a common terminal of the second voltage source and the common electrode of the storage capacitor of the pixel unit. 
     Wherein, the switching unit comprises at least one bipolar transistor and the switching unit has a base terminal electrically connected to the scan line, a collector terminal electrically connected to the first voltage source, and an emitter terminal electrically connected to a common terminal of the second voltage source and the common electrode of the storage capacitor of the pixel unit. 
     Wherein, the switching unit comprises a composite bipolar transistor comprising a plurality of thin film transistors and bipolar transistors, the control terminal of the switching unit being electrically connected to the scan line, the input terminal being electrically connected to the first voltage source, the output terminal being electrically connected to the common terminal of the second voltage source and the common electrode of the storage capacitor of the pixel unit. 
     Wherein, the storage capacitor is constituted by the pixel electrode and the common electrode of the storage capacitor, wherein the pixel electrode and the common electrode of the storage capacitor are both formed on the first substrate. 
     Wherein, the liquid crystal capacitor is constituted by the pixel electrode, the common electrode formed on the second substrate, and the liquid crystal layer, the common electrode formed on the second substrate being electrically connected to the second voltage source. 
     Wherein, the first voltage has a voltage value of 6.8V and the second voltage has a voltage value of 7.5V. 
     To address the above technical issue, the present invention adopts a further technical solution by providing a method for driving liquid crystal display device. The liquid crystal display device comprises a plurality of pixel units arranged in an array. The driving method comprises the following steps: providing a first voltage source, which functions to provide a first voltage; providing a second voltage source, which functions to provide a second voltage; providing a first switching unit, which functions to control the first voltage source and the second voltage source to supply the first voltage or the second voltage to the common electrode of the storage capacitor of the pixel unit; wherein when the switching unit receives a scan signal, the first voltage source supplies the first voltage to a common electrode of a storage capacitor of the pixel unit; and when the switching unit receives no scan signal, the second voltage source supplies the second voltage to the common electrode of the storage capacitor of the pixel unit so as to reduce difference of feed-through voltage between a plurality of pixel units of a scan line; wherein the first voltage is less than the second voltage. 
     Wherein, the method further comprises: providing a second switching unit, which functions to control the data line of the liquid crystal display device to supply data voltage to the pixel unit; wherein an identical scan signal is supplied to the first switching unit and the second switching unit so as to have both the first switching unit and the second switching unit to turn on or off simultaneously. 
     Wherein, a common voltage is applied to the common electrodes of the liquid crystal capacitors of a plurality of pixel units of a scan line, the common voltage having a voltage value that is equal to the first voltage. 
     Wherein, the first switching unit is a thin film transistor or a bipolar transistor and the second switching unit is a thin film transistor. 
     Wherein, the first voltage has a voltage value of 6.8V and the second voltage has a voltage value of 7.5V. 
     The efficacy of the present invention is that to be distinguished from the known techniques, according to the present invention, when a scan signal is received through the switching unit, the first voltage source supplies a first voltage to the pixel unit, and when the switching unit receives no scan signal, the second voltage source supplies a second voltage to the pixel unit, and the first voltage is less than the second voltage, whereby correction can be effected on the difference of feed-through voltage that is caused by different parasitic resistors and parasitic capacitors of the same scan line of the liquid crystal display device, thus the brightness uniformity of the liquid crystal display device can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a driving circuit of a conventional liquid crystal display device; 
         FIG. 2  is a plot showing a scan signal and waveform of voltage detected on a pixel electrode of the circuit shown in  FIG. 1 ; 
         FIG. 3  is a schematic view showing the structure of a liquid crystal display device according to the present invention; 
         FIG. 4  is a diagram showing a driving circuit of the liquid crystal display device according to the present invention; 
         FIG. 5  is a circuit diagram of an embodiment of the driving circuit shown in  FIG. 4 ; 
         FIG. 6  is a flow chart showing a driving method of liquid crystal display device according to the present invention; and 
         FIG. 7  is a plot showing comparison of signal waveforms for two pixel units of the same scan line and respectively located at an edge and a center of the liquid crystal display device according to the p″resent invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A detailed description will be given hereinafter with reference to the accompanying drawings and embodiments. 
     The present invention aims to provide a liquid crystal display device, which comprises a plurality of pixel units arranged in an array. As shown in  FIG. 3 , each pixel unit  30  comprises a first substrate  301  and a second substrate  302  that are arranged opposite to each other and a liquid crystal layer (not shown) interposed between the first substrate  301  and the second substrate  302 . In the embodiment, the first substrate  301  is a TFT (Thin Film Transistor) substrate and the second substrate  302  is a CF (Color Filter) substrate. 
       FIG. 4  is diagram showing a driving circuit of the liquid crystal display device according to the present invention. Referring to both  FIGS. 3 and 4 , taking a single pixel unit as an example, in the instant embodiment, the driving circuit of the liquid crystal display device comprises: a scan line  410 , a data line  420 , a first TFT  306 , a liquid crystal capacitor  441 , a storage capacitor  442 , a parasitic capacitor  450 , a first voltage source  460 , a second voltage source  470 , and a switching unit  480 . 
     The scan line  410 , the data line  420 , and the first TFT  306  are formed on the first substrate  301  in an insulated intersection manner. The scan line  410  is connected to a gate drive  412  for transmitting a scan signal provided by the gate drive  412 . The data line  420  is connected to the source drive  422  for transmitting a data signal provided by the source drive  422 . 
     The pixel electrode  303  is formed on the first substrate  301  in an area delimited by two adjacent scan lines  410  and two adjacent data lines  420 . 
     The liquid crystal capacitor  441  is constituted by the pixel electrode  303  and a common electrode  304  formed on the second substrate  302  and the liquid crystal layer. The storage capacitor  442  is constituted by the pixel electrode  303  and a common electrode  305  that is also formed on the first substrate  301 . 
     The first TFT  306  is arranged at the intersection between the scan line  410  and the data line  420 . The first TFT  306  has a gate terminal g 1  that is electrically connected to the scan line  410 , a source terminal s 1  that is electrically connected to the data line  420 , and a drain terminal d 1  that is electrically connected to the pixel electrode  303 . 
     The first voltage source  460  functions to provide a first voltage. In the instant embodiment, the first voltage has a voltage value of 6.8V. 
     The second voltage source  470  functions to provide a second voltage. In the instant embodiment, the second voltage has a voltage value of 7.5V. It is noted that in the present invention, the values of the first voltage and the second voltage are not limited to above example values and it is only required that the relationship that the voltage value of the first voltage is smaller than that of the second voltage is satisfied. 
     The common electrode  304  of the liquid crystal capacitor  441  and the common electrode  305  of the storage capacitor  442  are both electrically connected to the second voltage source  470 . 
     The parasitic capacitor  450  has two terminals that are respectively and electrically connected to the gate terminal g 1  and the drain terminal d 1 . 
     The switching unit  480  is arranged at the connection between the gate terminal g 1  of the first TFT  306  and the scan line  410  to effect selective connection with the first voltage source  460  or the second voltage source  470 . The switching unit  480  has a control terminal c that is electrically connected to the scan line  410 , an input terminal i that is electrically connected to the first voltage source  460 , and an output terminal o that is electrically connected to the second voltage source  470 , the common electrode  304  of the liquid crystal capacitor  441 , and the common electrode  305  of the storage capacitor  442 . 
     Referring to  FIG. 5 ,  FIG. 5  is a circuit diagram of an embodiment of the driving circuit of the liquid crystal display device according to the present invention. 
     In the instant embodiment, a TFT  580  is provided to serve as the switching unit, comprising a gate terminal g 2  that is electrically connected to the scan line  410 , a source terminal s 2  that is electrically connected to the first voltage source  460 , and a drain terminal d 2  that is electrically connected to the second voltage source  470 , the common electrode  304  of the liquid crystal capacitor  441 , and the common electrode  305  of the storage capacitor  442 . 
     It is understood that in the above discussed embodiment, the TFT  580  can be replaced by a bipolar transistor. In this regard, the bipolar transistor has a base terminal that is electrically connected to the scan line  410 , a collector terminal that is electrically connected to the first voltage source  460 , and an emitter terminal that is electrically connected to a common terminal of the second voltage source  470  and the common electrode  305  of the storage capacitor  442  of the pixel unit. 
     Similarly, in the above-discussed embodiment, the TFT  580  can be constituted by a composite bipolar transistor that comprises multiple TFTs or multiple bipolar transistors, or a composite bipolar transistor that comprises multiple TFTs and bipolar transistor, in order to form other embodiments. The present invention applies no specific limitation thereto. 
     It is noted that in the present invention, the common electrode  304  of the second substrate  302  and the common electrode  305  of the first substrate  301  may be arranged to be directly connected to each other and are instead provided, respectively with electrical voltages from two different voltage sources with the voltages of the two voltage sources being severely set equal to each other. 
     According to another aspect of the present invention, the present invention also provides a method for driving a liquid crystal display device. As shown in  FIG. 6 , the driving method according to the present invention comprises the following steps: 
     Step  601 : providing a first voltage source. 
     The first voltage source functions to provide a first voltage. In the instant embodiment, the first voltage has a voltage value of 6.8V. 
     Step  602 : providing a second voltage source. 
     The second voltage source functions to provide a second voltage. In the instant embodiment, the second voltage has a voltage value of 7.5V. The voltage value of the second voltage is greater than that of the first voltage. 
     Step  603 : providing a first switching unit. 
     The first switching unit can be a TFT or a bipolar transistor, which functions to control the first voltage source and the second voltage source to supply the first voltage or the second voltage to the common electrode of the storage capacitor of the pixel unit. 
     Step  604 : determining if the first switching unit receive a scan signal. If the answer is positive, then executing Step  605 ; otherwise if the answer is negative, then executing Step  606 . 
     Step  605 : the first voltage source supplying a first voltage to the common electrode of the storage capacitor of the pixel unit. 
     Step  606 : the second voltage source supplying a second voltage to the common electrode of the storage capacitor of the pixel unit. 
     In the following, a driving circuit of the display device according to the present invention that realizes the above described driving method and an actual operation of the driving method will be described in details. 
     The actual operation of the driving circuit and the driving method according to the present invention are as follows: 
     Referring again to  FIG. 4 , since the gate terminal of the first TFT  306  and the control terminal c of the switching unit  480  are connected to the same node of the scan line  410 , they can receive the same scan signal to be simultaneously conducted on or off. 
     Specifically, when the scan line  410  is supplied with the scan signal, the first TFT  306  and the switching unit  480  are conducted on and the data signal applies a load through the data line  420  and the first TFT  306  to the pixel electrode  303 . Also, since the switching unit  480  is conducted on, the first voltage source  460  supplies the first voltage through the switching unit  480  to the common electrode  305  of the storage capacitor  442  of the pixel unit to induce a voltage difference between the pixel electrode  303  and the common electrode  304  that is formed on the second substrate, causing re-orientation of the liquid crystal layer. 
     It is noted that in the instant embodiment, the liquid crystal display device adopts column scanning, whereby each time a scan is made, a common voltage is applied to the common electrodes  304  of the liquid crystal capacitors  441  of a number of pixel units in connection with the same scan line  410  and the common voltage is set at a value that is equal to the first voltage provided by the first voltage source  460 . 
     When no scan signal is received, the first TFT  306  is set off and the switching unit  480  is also switched off. Under this condition, the second voltage source  470  directly supplies the second voltage to the common electrode  305  of the storage capacitor  442  of the pixel unit. Since the first voltage is smaller than the second voltage, the second voltage that is larger and is supplied at the time when the first TFT  306  is off raises the voltage level of the pixel electrode  303 , effecting a correction of feed-through voltage. 
     Referring to  FIG. 7 ,  FIG. 7  is a plot showing comparison of signal waveforms for two pixel units of the same scan line and respectively located at an edge and a center of the liquid crystal display device according to the present invention. 
     Curve  711  is the voltage signal of the control terminal of the switching unit for the edge-located pixel unit and curve  721  is the voltage signal of the control terminal of the switching unit for the center-located pixel unit. In the instant embodiment, when curves  711 ,  712  are of high levels, the first TFT is conducted on. 
     Curves  712 ,  722  respectively indicate voltage signals of the common electrodes of the storage capacitors for the pixel units. 
     Curves  713 ,  723  respectively indicate voltage signals of the pixel electrodes of the pixel units, which are the voltage signals of the pixel electrodes after the correction of the feed-through voltages according to the present invention. 
     Curves  714 ,  724  respectively indicate voltage signals of the pixel electrodes of the pixel units under a condition where the second voltage source and the switching unit are not provided, namely the voltage signals of the pixel electrodes with no correction of feed-through voltage being made. 
     It can be seen from  FIG. 7  that when curves  711 ,  712  are at low levels and the first TFT is turned off, the voltage differences between the pixel electrodes and the common electrodes of the two pixel units that are respectively located at an edge and a center according to the present invention show a difference therebetween that is smaller than the case where the second voltage source and the switching unit are not provided. Further, referring to  FIG. 4 , according to the present invention, when the parasitic capacitor  450  is relatively small (such as for the edge-located pixel unit), the correction made on the feed-through voltage is great; on the other hand, when the parasitic capacitor  450  is relatively great (such as for the center-located pixel unit), the correction made on the feed-through voltage is small. In this way, the difference of feed-through voltage caused by different parasitic resistors and parasitic capacitors of pixel units of the same scan line but at different locations can be corrected, making the difference in feed-through voltages of different pixel units significantly reduced thereby effectively alleviating the problem of relatively great brightness found at the left and right sides of a display screen for low grey scale condition. 
     Experiments show that before the correction, the voltage of the pixel electrode of an edge-located pixel unit is −1.44108V and that of the pixel electrode of a center-located pixel unit is −0.99628V; and after the correction, the voltage of the pixel electrode of the edge-located pixel unit is −1.01600V and that of the pixel electrode of a center-located pixel unit is −0.99628V, the voltage difference of the two being less than 0.02V, making it possible to achieve excellent result. 
     To be distinguished from the known techniques, according to the present invention, when a scan signal is received through the switching unit, the first voltage source supplies a first voltage to the pixel unit, and when the switching unit receives no scan signal, the second voltage source supplies a second voltage to the pixel unit, and the first voltage is less than the second voltage, whereby correction can be effected on the difference of feed-through voltage that is caused by different parasitic resistors and parasitic capacitors of the same scan line of the liquid crystal display device, thus the brightness uniformity of the liquid crystal display device can be improved. 
     Embodiments of the present invention have been described, but not intending to impose any unduly constraint to the appended claims. Any modification of equivalent structure or equivalent process made according to the disclosure and drawings of the present invention, or any application thereof, directly or indirectly, to other related fields of technique, is considered encompassed in the scope of protection defined by the clams of the present invention.