A. Field of the Invention
The present invention relates to a compensation circuit for a liquid crystal display, especially to a compensation circuit which can remove the flicker occurring on the panel of a liquid crystal display by providing a compensation signal for each pixel of the liquid crystal display.
B. Description of the Prior Art
Referring to FIG. 5(A), it shows the structure of a conventional active matrix liquid crystal display (LCD). The liquid crystal display mainly includes a pixel array 51, a data signal line driving circuit 52, and a scan signal line driving circuit 53. The pixel array 51 includes a plurality of pixels. A thin film transistor (TR) drives each pixel 54. The source of each thin film transistor (TR) is connected to a common voltage Vcom via a liquid crystal capacitor Clc, and connected to a saturation voltage Vst via a storage capacitor Cst. Parasitic capacitance Cgs exists between the source and the gate. Moreover, distributed resistance-capacitance (R-C) indispensably exists on each scan signal line SL, which can be represented by a plurality of Rd and Cd distributed in each pixel as illustrated in FIG. 5(A). If the storage capacitor Cst is connected to the common voltage Vcom, then the circuit can be simplified as illustrated in FIG. 5(B).
As described above, due to the existence of the distributed RC on each scan signal line SL, the scan pulse generated by the scan signal line driving circuit 53 is subject to distortion more and more severely from left side to right side when it reaches a pixel on the same scan signal line SL (with reference to FIG. 5(A), the left side represents the side of the scan signal line driving circuit 53). The distortion of the scan pulse can be illustrated more clearly by FIGS. 6(A) and 6(B). When a scan pulse is generated by the scan signal line driving circuit 53 and transmitted to a scan signal line SL, the waveform as illustrated in FIG. 6A represents the signal applied to the gate of the leftmost pixel transistor, the waveform as illustrated in FIG. 6B represents the signal applied to the gate of the rightmost pixel transistor. For instance, take a 14.1 inch LCD with resolution of 1024xc3x97768 for an example. It takes only a few nanoseconds for the waveform of FIG. 6A to rise from low to high. However, it takes a few microseconds for the waveform of FIG. 6B to rise from low to high.
Referring to FIG. 5(B), due to the existence of the parasitic capacitance Cgs, the potential of the source will be pulled down by the signal applied to the gate of the thin film transistor (TR) via the parasitic capacitance Cgs. Such effect will become less and less apparent for the pixel transistors from left side to right side on the same scan signal line SL. In other words, when the drain of each pixel transistor on the same scan signal line is provided with the same data signal, the potential on the source of each pixel transistor is getting higher and higher from left side to right side. Since the brightness of each pixel is determined by the voltage applied to the equivalent capacitance Clc+Cst, the resulting brightness of each pixel will therefore be different. This result is undesirable because the brightnesses of two pixels are supposed to be the same if they are provided with the same potential. The inconsistency of the pixel brightness will cause flicker on the LCD.
Accordingly, it is an object of the present invention to provide a compensation circuit for a liquid crystal display, thereby removes the flicker by adjusting the potential applied to each pixel. Eventually, when the active element of each pixel on the same scan signal line is provided with the same data signals, the potential on each pixel and the brightness of each pixel will be the same.
It is another object of the present invention to provide a compensation circuit for a liquid crystal display which has the merits of simple structure and low cost. Moreover, the compensation circuit in accordance with the present invention can be selectively enabled or disabled according to practical applications.
In accordance with the present invention, the compensation circuit mainly includes: a memory device, a buffer, a digital/analog converter and a data signal line driving circuit. The memory device stores a plurality of digital data set each of which corresponds to the pixel array on a scan line. Each digital data set has length n which is equal to the length of a pixel array. Each of the n digital data is used as the compensation signal of a pixel on the pixel array. The buffer is connected to the memory device for temporarily store a digital data set coming from the memory device in response to an external clock CK1. The digital/analog converter is connected to the buffer for converting the digital data set coming from the buffer into an analog data set in response to an external clock CK2. The data signal line driving circuit, consisting of n units each of which is composed of a sample/hold circuit unit and an output circuit unit, provides adequate data signals for the n pixels of each pixel array. Each of the sample/hold circuit units receives an analog data of the analog data set coming from the digital/analog converter or a ground potential, samples the received data and holds the sampled result. Each of the output circuit units receives the output data coming from the corresponding sample/hold circuit unit and an external signal, outputs the summed result of the two received data to a corresponding pixel.
When flicker is to be compensated, each sample/hold circuit unit receives an analog data of the analog data set coming from the digital/analog converter. On the other hand, when flicker is not to be compensated, each sample/hold circuit unit receives a ground potential.