Patent Publication Number: US-7221350-B2

Title: Method of reducing flickering and inhomogeneous brightness in LCD

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to a TFT scan line control circuit for LCDs and, in particular, to a circuit that solve the problems of flicker and inhomogeneous brightness in LCDs. 
   2. Related Art 
   The LCD (Liquid Crystal Display) is a flat display with low power consumption. In comparison with the CRT (Cathode Ray Tube) of the sane screen size, the LCD is much smaller in its space occupation and weight. Unlike the curved screen in conventional CRTs, it has a planar display screen. With these advantages, LCDs have been widely used in various products, including palm calculators, electronic dictionaries, watches, mobile phones, notebook computers, communication terminals, display panels or even personal desktop computers. In particular, there is tendency that the TFT-LCD (Thin Film Transistor Liquid Crystal Display) is gradually replacing the low-level STN-LCD due to its superior properties in visible angles, contrast, and response time. 
   As shown in  FIG. 1 , there are liquid crystal capacitors  100  and transistors  110  disposed in an array. Scan lines  120  connect the gates  111  of the transistors  110 . Data lines  130  connect the sources  112  of the transistors  110 . Each liquid crystal capacitor  100  connects between a transistor  110  and a reference potential  115 . Each scan line  120  imposes in order a rectangular voltage on the gate  111  of the transistor  110  at an interval of roughly a scanning time, which is a positive frame time divided by the number of scan lines. At the moment, the voltages D 1 , D 2  and D 3  are existent on the data lines  130 . The corresponding charges are then stored in the crystal capacitors  100  at the intersection of the data lines  130  and each scan line  120  in order at times t 1 , t 2 , and t 3 . The shaded squares  140  in the drawing schematically explain the data storage of the rectangular waves on the data lines and the scan lines. 
   With further reference to  FIG. 1 , aside from the transistors  110  and the crystal capacitors  100  connected by the scan lines  120 , there are also stray capacitors  116  and resistors  121 . For currently available LCDs with a resolution of 1024×768, 1024×3 data lines are required, where the factor  3  accounts for the red, green and blue color signals for a point. The resistance  121  is generated by the generic resistance in thin and long wires (10 μm×12–14 in.). The resistance is about 0.35Ω/sq. The above-mentioned resistors  121  and the stray capacitors  116  definitely cause RC time delays. Therefore, even each scan line  120  is input with a rectangular wave that is steep at its edges, as shown in  FIG. 2   a , the voltage imposed on the gate of the first pixel transistor (composed of a transistor  111  and a liquid crystal capacitor  100 ) is almost invariant in its shape ( FIG. 2   b ). However, on the n&#39;th pixel, the voltage imposed on the gate has some shape deformation. 
   The voltages V GH  and V GL  in  FIG. 3   a  are the maximum and minimum voltages at the gate of the first pixel.  FIG. 3   b  shows that the starting (the transistor turned on) time and the decreasing (the transistor turned off) time of the scan line rectangular wave at the gate of the last pixel. Therefore, to respond such a change in the waveform, the usual scan line and data line produce a time difference Δt on purposes, as shown in  FIG. 3   c . That is, the data line has to wait until the previous scan line is turned off before it writes the data signals while the next scan line is turned on. 
   Since there is an unavoidable parasitic capacitor C GS  between the TFT source/drain and gate and C GS  is pretty large, although C GS  does not generate any influence when the transistor is turned on, it does generate the charge coupling effect when the transistor is turned off after writing data into the liquid crystal capacitor C LC  and a storage capacitor C S .  FIG. 4  shows that the voltage at the drain of the transistor drops from V D  by ΔV D  to (V D −ΔV D )  142 . This voltage is maintained till the end of the positive frame time, which is about 16.7 ms. The ΔV D  is C GS (V GH −V GL )/(C GS +C S +C LC ). To prevent decomposition of the liquid crystal from, a negative frame time (when the voltage V D  is negative) has to be imposed after a frame time (when the voltage V D  is positive). At this moment, the charge coupling effect due to the capacitor C GS  still produces a voltage drop of ΔV D  to the voltage −V D −ΔV D    144 .  FIG. 5  illustrates such a situation. 
   In the n&#39;th pixel of the scan lines, the RC time delay deforms the square waveform of the scan line and makes the capacitor C GS  generate the charge coupling effect. Therefore, the gate voltages of the n&#39;th pixel and the first pixel are different, resulting in the flicker problem of a large TFT-LCD. To conquer the above problem, a common method is to change the IC design of the scan line driver. Nevertheless, this will increase the cost and thus is not economical at all. It is thus an object of the invention to provide an effective method that solves the above problem. 
   SUMMARY OF THE INVENTION 
   An object of the invention is to provide a method to solve the flickering problem in a large TFT-LCD. 
   The invention discloses a scan line circuit that solves the problems of screen flickering and inhomogeneous brightness in the LCD. Each scan line circuit contains a scan line connecting the gates of the TFTs of a plurality of pixels in a row and a resistor connecting in series. The resistor is placed between the first pixel on the scan line and the voltage input terminal of the scan line, so that the gate voltage entering the TFT in the first pixel deforms. The voltage of the TFT decreases when it is turned off, solving screen flickering due to the capacitor charge coupling effect between the first pixel and the last pixel on a scan line and, at the same time, the problem of inhomogeneous brightness due to imperfect exposure junctions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a schematic layout of a conventional TFT-LCD; 
       FIGS. 2   a  to  2   c  illustrate the rectangular waveforms when imposing a rectangular waveform voltage on the first pixel and the n&#39;th pixel; 
       FIGS. 3   a  and  3   b  illustrate the maximum and minimum voltages on the gates of the first pixel and the last pixel, respectively, and  FIG. 3   c  shows that the data line can start to write the data signals from the next scan line only after the previous pixel is turned off because there is a time difference Δt between the scan line and the data line; 
       FIG. 4  illustrates the voltage drop ΔV D  on the drain voltage due to the C GS  capacitor coupling effect; 
       FIG. 5   a  shows a typical rectangular wave voltage input from a scan line, and  FIG. 5   b  shows a difference between the drain voltages on the first and the last pixels due to the C GS  capacitor coupling effect; 
       FIG. 6  shows an equivalent circuit of the scan line with a resistor made of ITO added between the scan line voltage input terminal and the first pixel gate in a TFT-LCD according to a first preferred embodiment of the invention; 
       FIG. 7   a  shows a square voltage at the scan line input terminal, and  FIG. 7   b  shows the scan line voltage of transistor gate of the first pixel and the scan line voltage of transistor gate of the last pixel according to the equivalent circuit in  FIG. 6 ; and 
       FIG. 8  shows an equivalent circuit of the scan line wherein a thin film transistor with source/gate connection is connected between the scan line voltage input terminal and the first pixel gate in a TFT-LCD according to a second preferred embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In view of the foregoing description, due to the RC time delay on the n&#39;th pixel of each scan line, the deformed square waveform voltage input on the scan line and the charge coupling effect produced by the capacitor C GS , there is flickering in a large TFT-LCD. 
   The specification further describes flickering occurred in a TFT-LCD hereinafter and then discloses a method to solve the problem. 
   With reference to  FIG. 5   a , a typical rectangular waveform voltage entering a scan line has a high voltage V GH  of about 15V and a low voltage V GL  of about −7V. At this moment, no time delay occurs in the transistor of the first pixel when going from V GH  to V GL  such that the voltage of the first pixel is the same as that at the input terminal of the scan line. However, due to the charge coupling effect produced by the capacitor C GS , the drain voltage V D  of the transistor experiences a voltage drop ΔV D1  when the signal input moves from one scan line to the next scan line during a positive frame time, as shown by the curve  170  in  FIG. 5   b . Thus, the voltage V D  drops from 5V down to 4V. In a negative frame time, the voltage V D  also drops from −5V to −6V due to the charge coupling effect of the capacitor C GS . For the liquid crystal, accordingly, the biases of the positive frame time and the negative frame time are different. This affects the brightness of the display so that it is brighter in the positive frame time than in the negative frame time. Therefore, the reference voltage has to be adjusted. In the current embodiment, for example, if the reference voltage is adjusted to −1V, the DC bias of the liquid crystal in the positive and negative frame times become very close to each other. As shown by the curve  175  in  FIG. 5   b , when the scan line transmits the signal to the n&#39;th pixel, the RC time delay for the scan line square wave voltage to change from V GH  to V GL  is very significant for a large size LCD (e.g. a 10 μm×14 in. metal scan line). The scan line square wave seriously deforms. Therefore, in the positive frame time, the voltage is V T  when the transistor of the n&#39;th pixel is turned off, where V T  is the threshold voltage when the TFT is turned off. Due to the charge coupling effect, the voltage is dropped by ΔV Dn  to become C GS (V T −V GL )/(C GS +C S +C LC ). Since V T &lt;V GH , ΔV Dn  is smaller, e.g. 0.5V. In the negative frame time, it is also decreased by 0.5V. Therefore, such a 0.5V difference results in the difference of the biases of the positive and negative frame times. The bias is larger in the positive frame time (low brightness) and smaller in the negative frame time (high brightness). Flicker thus takes place on the liquid crystal display. 
   Using the conventional method described in prior art to solve the problem of flickering is very difficult. It is because one needs to modify the IC design of the scan line driver. Not only are the effects bad, the main reason is that the cost of the scan line driver manufacturers increases because of different capacitors required by different LCD manufacturers. 
     FIG. 6  shows an equivalent circuit of the scan line a resistor  200  made from ITO installed between the scan line voltage input terminal  202  and the first pixel gate  204  in a TFT-LCD according to a first preferred embodiment of the invention. The voltage drop ΔV D1  and ΔV Dn  at the first and the n&#39;th pixels, respectively, due to the charge coupling effect then become closer. 
   With reference to  FIGS. 7   a  and  7   b , since a resistor  200  with a resistance of about 10–100Ω/sq is provided to each scan line before connecting to the first pixel transistor, there is a time delay in the scan line voltage drop even at the transistor gate of the first pixel. Therefore, the turn-off time of the first pixel transistor is not the time when the scan line signal is removed, but at a later time when the voltage reaches V T1 . Therefore, the difference between V T1  and V Tn  becomes smaller so that the voltage drop ΔV D1  of the first pixel transistor and ΔV Dn  of the n&#39;th pixel transistor become closer. 
   Please refer again to  FIG. 7   b . For example, when no resistor is installed, V GH −V GL =15V−(−7V)=22V. After inserting ITO resistor  200 , V GH  becomes V T1 . At the moment, if V T1  is 7V, then V T1 −V GL =7V−(−7V)=14V. Thus, the voltage drop ΔV D1  of the first pixel transistor and ΔV Dn  of the n&#39;th pixel transistor become closer. This decreases screen flickering. 
     FIG. 8  shows an equivalent circuit of the scan line wherein a TFT  300  with source/gate connection is connected between the scan line voltage input terminal  302  and the first pixel gate  304  in a TFT-LCD according to a second preferred embodiment of the invention. The source  300   a  and the gate  300   b  of the TFT  300  are connected so that they have the same electric potential. When the voltage input terminal  302  imposes a positive voltage at the source  300   a , the gate  300   b  also opens so that the current can flow through the TFT  300 . Inserting the TFT  300  with connection of source and gate before the first pixel gate  304 , the decrease and waveform deformation of the voltage at the first pixel gate can achieve the one shown in  FIG. 7   b , shortening the difference between V T1  and V Tn , improving the screen flickering phenomena. 
   Moreover, since the LCD is a large area display, the exposure in the photolithography procedure for making source/drain areas can not be done in one step. The exposure is done by one image field after another. Since the LCD manufacture procedure does not allow alignment marks between the image fields, errors of the gate and source/drain in one transistor between different image fields is unavoidable. Therefore, the capacitor C GS  varies, resulting in changing ΔV D . The variation of ΔV D  causes the so-called shut mura, meaning imperfect exposure junctions and inhomogeneous brightness. 
   The invention can use the thin film resistor made by ITO or the TFT with source/gate connection to bring V T1  and V Tn  closer, solving the shut mura problem. Thus, the disclosed method can significantly decrease the cost and improve the problems of screen flickering and inhomogeneous brightness. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.