KR100213971B1 - Common voltage compensation circute for liquid crystal device - Google Patents

Abstract

In general, a liquid crystal display device has a flicker phenomenon in which image quality is irregular due to a pixel voltage drop phenomenon inevitably caused by temperature and data change values.

The present invention calculates the difference by inputting the data voltage applied to the liquid crystal display device to the reference pixel to which a constant potential is applied to the gate electrode and the actual pixel driven in synchronization with the gate scan line of the liquid crystal display device in order to prevent the flicker shape. Do it.

The difference is applied in addition to the common voltage generator to maintain the potential difference between the pixel electrode and the common electrode of the liquid crystal panel. As a result, the applied to the liquid crystal becomes constant, thereby significantly reducing the flicker phenomenon of the liquid crystal panel.

Description

A common voltage compensation circuit according to a distorted pixel voltage of a liquid crystal display device.

1 is a circuit diagram showing unit pixels of a liquid crystal display.

2 is a diagram showing general waveforms of voltages applied to the gate scan line and the pixel electrode of the liquid crystal display.

3 is a schematic configuration diagram of the present invention.

4A is a circuit diagram showing Embodiment 1 of the present invention.

4B is a diagram showing waveforms applied to the electrodes of the first embodiment.

5A is a circuit diagram showing Embodiment 2 of the present invention.

5B is a diagram showing waveforms applied to each electrode of Example 2. FIG.

＊ Explanation of symbols on main parts of drawing

101: unit pixel 102: pixel electrode

103: thin film transistor 104: parasitic capacitance Cgs

105: parasitic dose Cds 106: auxiliary dose Cst

107: n-th gate scanning line 108: n-th gate scanning line

109: data scanning line 110: common electrode

111: liquid crystal capacitor C _LC

21: Gate scan waveform of the n-1th gate scan line

22: Gate scan waveform of the nth gate scan line

23: intermediate potential of signal voltage 24: potential of common voltage

25: voltage across C _LC 31: test part

32: detector 33: compensation voltage generator

34: voltage difference detector 35: sampling holder

36: second test pixel voltage 37: first test pixel voltage

38: voltage difference detector output 39: sampling holder output

40: input common voltage 41: output common voltage

42: Current Amplifier

43: first sampling period 44: second sampling period

45: common voltage cycle

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit of a liquid crystal display device, and more particularly to a circuit for preventing a flicker phenomenon caused by a pixel voltage drop ΔVp generated due to a temperature change of a liquid crystal display device and a parasitic capacitance when a gate signal falls.

The pixel 101 of a typical liquid crystal display is connected to the switching element TFT 103 and the source of the TFT formed at the intersection where the gate scan line 108 and the data scan line 109 vertically cross as shown in FIG. The auxiliary capacitor 106 and the pixel electrode 102, and the common electrode 110 provided to face the pixel electrode, and the liquid crystal capacitor 111 existing between the pixel electrode and the common electrode.

Parasitic capacitance Cgs 104 exists between the source of the TFT and the gate of the TFT, and parasitic capacitance Cds 105 exists between the source and the drain of the TFT.

One terminal split from the extension line of the source is connected to the gate scan line 107 at the front end of the gate scan line 108, and the common electrode is connected to a common voltage generator (not shown).

That is, the parasitic capacitance Cgs exists between the pixel electrode and the gate scan line (n-th gate scan line), and another parasitic capacitance Cds exists between the pixel electrode and the data scan line.

The operating principle of the liquid crystal display pixel is as follows.

When the signal 22 of the gate scan line becomes high, all the TFTs connected to the gate scan line are turned on, and data is stored in Cst and C _LC along the data scan line connected to the drain of the TFT to display an image. Display.

When the signal of the gate scan line becomes low, all the TFTs connected to the gate scan line are turned off, and at this time, the image is represented by the data stored in the Cst. However, due to the Cgs existing between the gate and the source of the TFT, the voltage applied to the pixel electrode when the signal of the gate scan line falls is reduced by ΔVp. This causes the flicker by causing the charge amount of the positive data and the negative data to be different.

Therefore, in order to reduce the flicker, the intermediate voltage 23 of the positive voltage value and the negative voltage value of the data signal cannot be applied to the common electrode, and a signal 24 lower than the intermediate potential is applied. ΔVp at this time is given as follows.

However, since C _LC depends on temperature and data values, flicker is inevitable because simply reducing the Vcom voltage cannot compensate for the ΔVp of every pixel that changes every cycle.

In the present invention, the ΔVp is detected in advance, and the value is compensated and applied to the common electrode to reduce the flicker phenomenon.

As shown in FIG. 3, the test unit 32 which detects ΔVp output from the test unit and the test unit 32 and ΔVp output from the detection unit are separately applied to the common electrode in addition to the pixels of the liquid crystal panel. Compensation voltage generator 33 is configured.

The test unit of the present invention configures a test pixel, applies a test gate pulse synchronized with the gate scan voltage of the liquid crystal panel to the test pixel, and applies an average value of data to the test pixel during one horizontal synchronization period.

The pixel voltage applied to the test pixel electrode is input to the detector to obtain a voltage difference between the pixel voltage stored in the test pixel after the test gate pulse falls and the average data voltage, and the voltage difference is input to the auxiliary voltage generator to provide a common voltage. In addition, by applying to the common electrode, the potential of the common voltage may be changed according to the change of the pixel voltage to compensate for ΔVp applied to the liquid crystal.

An embodiment of the present invention will be described with reference to FIG. 4 and FIG.

Example 1

In the liquid crystal drive circuit of the present invention as shown in FIG. 4A, a first gate voltage in synchronization with the gate scan voltage applied to the display pixel of the liquid crystal panel is applied to the thin film transistor of the first test pixel integrated separately from the display pixel of the liquid crystal panel, A second gate voltage having a constant potential is applied to the thin film transistor of the second test pixel.

At this time, the data voltages of the low frequency components passing through the Low Pass Filter (hereinafter referred to as LPF) are applied to the two test pixels.

The waveform input to each device is shown in FIG. 4B.

The first gate voltage is synchronized with the gate scan voltage applied to the display pixel configured in the liquid crystal panel, but at the same time, when the voltage falls to a low level, the first gate voltage falls first by? T time. That is, the period of the first gate voltage is the same as the period of the gate voltage applied to the TFT for driving the actual pixel electrode of the liquid crystal panel, but the application time t2 of the first gate voltage is for driving the actual pixel electrode of the liquid crystal panel. It is shorter than the application time t1 of the gate voltage applied to the TFT. In addition, the second gate voltage is maintained at a constant potential so that the TFT in the second test pixel is always on.

When the first gate voltage falls low, the pixel voltage of the first test pixel drops by ΔVp by capacitor coupling by parasitic capacitance Cgs between the gate and the source. However, since the second gate voltage is maintained at a constant value, there is no voltage variation in the second test pixel. Therefore, a voltage difference of ΔVp occurs between the pixel voltage of the first test pixel and the pixel voltage of the second test pixel.

Each pixel voltage generated at the pixel electrodes of the first test pixel and the second test pixel is applied to a voltage detector to detect a difference between the pixel voltages, that is, the pixel drop voltage ΔVp of the first test pixel. Then, the detected ΔVp is held by the sampling clock (sampling and holder, SH) by the sampling clock, and the voltage obtained by dropping the common voltage (Vcom-) output from the common voltage generator by ΔVp held by SH. (Vcom-ΔVp) is applied to the common electrode of the liquid crystal panel for Δt time.

By the above process, the voltage difference between the common electrode and the pixel electrode of the liquid crystal panel can be kept constant, thereby eliminating the flicker phenomenon.

Example 2

In the liquid crystal driving circuit of the present invention as shown in FIG. 5A, a first gate voltage in synchronization with the gate scan voltage applied to the display pixel of the liquid crystal panel is applied to the thin film transistor of the first test pixel separately integrated with the display pixel in the liquid crystal panel, A second gate voltage having a constant potential is applied to the thin film transistor of the second test pixel, and a signal voltage applied to each test pixel is delayed by one period by passing through the first sampling holder.

The first gate voltage is applied to the thin film transistor of the first test pixel in synchronization with the period of the first sampling holder. In addition, the pixel voltage is output from the thin film transistor of the first test pixel by the first gate voltage applied above.

In this embodiment, the delayed signal voltage is applied to the thin film transistors of the first test pixel and the second test pixel to output respective pixel voltages, and each of the output pixel voltages is amplified by passing through a current amplifier. After that, it is applied to the voltage difference detector and passed through the second sampling holder to detect the drop of the pixel voltage.

Since the period of the second sampling holder is synchronized with the period of the first gate voltage falling low, the drop of the pixel voltage can be accurately detected. The detailed waveform is shown in FIG. 5B.

The drop of the pixel voltage may be applied to the common voltage generator to maintain the difference between the pixel electrode and the common electrode by applying a voltage compensated for the drop of the common voltage and the pixel voltage to the common electrode of the liquid crystal panel.

That is, the present invention detects ΔVp that changes according to temperature and data in advance, and changes the common voltage in units of one horizontal cycle, thereby eliminating flicker by compensating Vcom in consideration of the size of data. In other words, the pixel voltage to be applied to the actual pixel electrode of the liquid crystal panel is applied to the test pixel electrode one horizontal period in advance, and the pixel drop voltage output from the test pixel electrode is applied to the common voltage generator to be applied to the actual common electrode of the liquid crystal panel. By providing a circuit for compensating the common voltage, there is an effect of eliminating the flicker generated in the liquid crystal panel by maintaining the potential difference between the pixel electrode and the common electrode which changes according to the temperature or the data when it is positive and negative.

Claims (5)

A detection means for detecting a voltage difference by comparing the pixel voltage of the liquid crystal panel with a reference voltage, and a voltage applying means for applying the voltage difference to a common electrode of the liquid crystal panel by adding the voltage difference to a common voltage output from the common voltage generator. A driving circuit for a thin film transistor liquid crystal panel. The detector according to claim 1, wherein the detection means is maintained at a constant potential with the first test pixel that outputs the actual pixel voltage in accordance with the first test gate voltage having a short application time in synchronization with the scanning time of the gate voltage applied to the TFT of the liquid crystal panel. A second test pixel for outputting a reference pixel voltage with little voltage drop due to the second test gate voltage; A voltage detector detecting the difference between the actual pixel voltage and the reference pixel voltage and a sampling holder (SH) sampling and storing the pixel voltage drop output from the voltage detector at a half period of a common voltage; A driving circuit for a thin film transistor liquid crystal panel. The driving circuit of claim 2, wherein the first test pixel and the second test pixel comprise a switching element TFT, a storage capacitance, and a liquid crystal capacitance. 4. The driving circuit of claim 3, wherein each of the storage capacitances connected to the first test pixel and the second test pixel is connected to the same gate scan line. The liquid crystal panel according to any one of claims 1 to 4, wherein the voltage applying means includes a voltage detector for adding the pixel voltage drop and the common voltage output from the common voltage generator in synchronization with the period of the common voltage. Driving circuit.