JPH0572997A - Driving method of liquid crystal display device - Google Patents

Abstract

PURPOSE:To prevent the baking defect of a liquid crystal display panel by setting the potential difference between the reference level of an AC driving signal and the level of a counter electrode with the least influence upon a DC component for the correction of the DC component having variance of impression of liquid crystal about a drain signal voltage. CONSTITUTION:In the active matrix liquid crystal display device which uses thin film transistors, a source potential, i.e., a voltage impressed to liquid crystal shifts by the potential drop DELTAVSC of a drain signal in a gate-voltage OFF state from a counter electrode potential in a driving waveform wherein the reference level 8 of the AC driving signal and the counter electrode level 4 are entered when the liquid crystal is oriented in parallel to a display electrode. This shift is corrected with a bias voltage VT. In this case, the bias voltage VT is set as an optimum setting method to the DELTAVSG value when the liquid crystal is arranged in parallel to a picture element electrode and the amplitude of a liquid crystal driving voltage becomes small.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving a liquid crystal display device such as a liquid crystal TV.

[0002]

2. Description of the Related Art As shown schematically in FIG. 4, an active matrix L using a conventional liquid crystal display device, for example, an a-Si-TFT (amorphous silicon thin film transistor).
In a CD (liquid crystal) device, a counter electrode 4 faces a substrate 3 provided with a matrix array of TFTs 1 and display electrodes 2, and liquid crystal is filled between them. It should be noted that G, D and S in the figure are a gate electrode, a drain electrode and a source electrode of the TFT 1, respectively.

In this liquid crystal display device, a capacitor having a liquid crystal as a dielectric is formed between the display electrode 2 and the counter electrode 4. For example, when used in a liquid crystal TV, a gate electrode G is used. When a predetermined voltage is applied and it is in a conductive state for one horizontal scanning period (TFT is on), the drain voltage is charged in the capacitor during this period, the voltage of the pixel electrode becomes equal to the drain voltage, and this voltage is 1 vertical. It is designed to be held during the scanning period. The liquid crystal drive circuit in this one pixel can be represented by the equivalent circuit in FIG.

When driving the liquid crystal, the above-mentioned liquid crystal display device performs AC driving in consideration of durability of the liquid crystal and the like. For example, in the case of the conventional liquid crystal TV, the polarity of the signal is inverted for each field around the reference level (counter electrode level).

As shown in FIG. 6, when the TFT is turned on, the drain signal voltage applied to the liquid crystal is shifted in the negative (-) potential direction of the gate voltage when the TFT is turned off. A certain amount of potential drop (ΔV _SG ) occurs. That is, it becomes vertically asymmetric with respect to the reference level and the counter electrode level, and as a result, ΔV _SG is added as a DC component to the liquid crystal drive voltage. When a DC component is applied to the liquid crystal drive voltage, electric charge is accumulated in the panel, and when the same image is displayed for a long time, the previous image remains even if it changes to another image. Become.

A means for solving the problem that the DC component is added to the liquid crystal driving voltage is disclosed in Japanese Patent Laid-Open No. 61-1163.
No. 92 publication.

According to this, as shown in FIG. 7, it is possible to correct the DC voltage applied to the liquid crystal by providing a predetermined potential difference (V _T ) between the reference level of the AC drive signal and the counter electrode level. Proposed.

However, the capacitance C _L of the liquid crystal changes depending on the alignment state of the liquid crystal (the tilt angle of the liquid crystal molecule with respect to the pixel electrode), and usually differs from pixel to pixel.

Here, the C _L and the above-mentioned potential decrease ΔV _{S G} of the liquid crystal charging voltage when the gate voltage is turned off will be described.

The following relationship is known between this C _L and the above-described potential drop ΔV _SG of the liquid crystal charging voltage when the gate voltage is off (T.Yanagisawa et al .: Japan Display '86, p.
192).

[0011]

[Equation 1]

C _L : liquid crystal capacitance, C _GS : TFT gate source / source
As is clear from the inter-cell capacitance <Equation 1>, when C _L increases, ΔV
_{When SG} decreases and C _L decreases, ΔV _SG increases.
That is, it can be seen that the value of ΔV _SG varies among all pixels of the panel. According to the proposal of the above-mentioned publication, since the set value of V _T is a constant value for all pixels of the panel, ΔV _SG cannot be completely corrected by this V _T.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional drawbacks, and a DC having a variation applied to the liquid crystal with respect to this drain signal voltage.
In order to correct the component, the potential difference between the reference level of the AC drive signal and the counter electrode level is set so that the influence of the DC component is minimized, thereby driving the liquid crystal display device without causing the burn-in defect in the liquid crystal display panel. It provides a method.

[0014]

According to a method of driving a liquid crystal display device of the present invention, an active matrix liquid crystal display device using a TFT is provided with a value of a potential difference provided between a reference level of a drive signal and a counter electrode level. , Which is induced by the gate voltage and applied to the pixel liquid crystal when the liquid crystal is oriented parallel or nearly parallel to the pixel electrode.
It is set to the voltage value.

[0015]

According to the driving method of the liquid crystal display device of the present invention, the voltage level between the reference level and the counter electrode level of the AC drive signal corresponding to the variation in the amount of charge charged in the liquid crystal cell due to the DC component of the liquid crystal drive voltage is between. The potential difference is optimally set.

[0016]

FIG. 1 shows an equivalent circuit of an embodiment of the liquid crystal driving method of the present invention. As shown in the figure, the bias power source 7 is inserted between the reference potential point 8 and the counter electrode 4.
Reference numeral 6 is a gate signal source.

FIG. 2 shows drive waveforms indicating the reference level and the counter electrode level of the AC drive signal when the liquid crystal is aligned parallel to the display electrodes. As shown in the figure, the source potential with respect to the counter electrode potential, that is, the voltage applied to the liquid crystal is shifted by the potential decrease amount ΔV _SG of the drain signal when the gate voltage is off, and the shift amount is bias voltage V.
Correct with _T.

ΔV_SGIs expressed by <Equation 1>, the liquid crystal
Capacity C_LMaximum value of (the liquid crystal drive voltage amplitude is large, that is, the liquid crystal
(When arranged vertically to the pixel electrode) is A, the volume of the liquid crystal
Quantity C _LMinimum value (Liquid crystal drive voltage amplitude is small, that is, the liquid crystal is a pixel
If B is set when the electrodes are arranged parallel to the electrodes, then ΔV
_SGTakes the values shown in Table 1.

[0019]

[Table 1]

As described above, the optimum setting value of V _T is the small liquid crystal drive voltage amplitude, that is, the ΔV _SG value when the liquid crystal is arranged in parallel with the pixel electrode.

[0021]

[Equation 2]

It becomes

The capacitance in this <Formula 2> tends to change depending on the voltage. Moreover, it was not clear what kind of increase / decrease the capacity of <Equation 2> would show depending on the voltage.

Therefore, the present applicant has proposed that the liquid crystal drive voltage DC
It was found that the amount of charge accumulated in the liquid crystal cell by the component tends to increase as the amplitude of the driving voltage decreases. That is, the smaller the amplitude, the easier the charge is to accumulate.

Based on this matter, a method for setting the optimum V _T value will be described below.

The liquid crystal capacitance C _L is minimum when the liquid crystal molecules are parallel to the pixel electrodes, increases as the angle of the liquid crystal molecules increases, and maximum when the liquid crystal molecules are perpendicular to the pixel electrodes. I take the. Therefore, from <Formula 1>, the potential decrease ΔV _SG of the liquid crystal charging voltage when the gate voltage is off is the maximum (ΔV _SG) when the liquid crystal molecules are aligned parallel to the pixel electrodes.
_SGB ), which is the minimum (ΔV _SGA ) when the liquid crystal molecules are aligned perpendicularly to the pixel electrodes. When the optimum setting of V _T is performed, the amount of charges accumulated in the panel when ΔV _{SG of} an arbitrary pixel on the panel is ΔV _{S GB} and when ΔV _SGA is as follows. (1) In a pixel with ΔV _SG = ΔV _SGB , charge is easily accumulated due to a small amplitude, but since V _T = ΔV _{SG, the} driving voltage D
There is no C component, and the amount of accumulated charge is 0 (the timing chart of each signal is shown in FIG. 2). (2) In a pixel in which ΔV _SG = ΔV _SGA , the DC component of the drive voltage is ΔV _SGB −ΔV _SGA , but the amount of accumulated charge is small because the amount of charge is less likely to accumulate at large amplitude (timing charge of each signal -Fig. 3).

Therefore, from the above (1) and (2), ΔV _SG is Δ
In the voltage range of V _{SGA to} ΔV _SGB, the amount of accumulated charge is small, and particularly when ΔV _SG = ΔV _SGB , the amount of image sticking is the smallest. That is the optimum value of the V _T value.

[0028] That is, as the method of setting the V _T, taking a V _T value of the accumulation state of easily the most charge, ie optimum setting of V _T, the liquid crystal is oriented parallel to the pixel electrode It is optimal to set the ΔV _SG value when the liquid crystal drive voltage amplitude becomes small.

When the optimum value is set, a good image free from image sticking can be obtained regardless of the variation in the distribution of charges accumulated on the liquid crystal display surface.

By the way, the driving method of the liquid crystal display device of the present invention is not limited to the amorphous silicon TFT, but can be adopted for polysilicon or the like.

Further, an auxiliary capacitor for driving the liquid crystal in a stable manner is generally provided in parallel with a capacitor formed of the liquid crystal, but this is of course also effective.

Furthermore, the display mode can be adopted in either case of normally white or normally black.

[0033]

As described above, according to the present invention, in the TFT active matrix liquid crystal display device, an arbitrary screen is displayed and a certain amount of potential drop distribution is generated with respect to the drain signal on the panel, and Even when the screen is continued for a long time, the difference between the minimum value and the maximum value of the charge accumulation distribution due to the potential drop distribution can be suppressed to the minimum.
Since the occurrence of image sticking defects can be prevented, the image quality of the liquid crystal display device can be improved.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram showing an embodiment of a driving method of a liquid crystal display device of the present invention.

FIG. 2 is a drive waveform diagram when a liquid crystal is aligned in parallel according to a driving method of the present invention.

FIG. 3 is a driving waveform diagram when a liquid crystal is vertically aligned according to a driving method of the present invention.

FIG. 4 is a schematic view of a general amorphous silicon thin film transistor.

FIG. 5 is an equivalent circuit diagram of a liquid crystal drive circuit diagram in one pixel showing a driving method of a conventional liquid crystal display device.

FIG. 6 is a drive timing chart of a conventional liquid crystal display device.

FIG. 7 is a drive timing chart of a conventional liquid crystal display device.

[Explanation of symbols]

 1 TFT 2 display pixel electrode 3 glass substrate 4 counter electrode 5 drain signal power source 6 gate signal source 7 bias signal source 8 reference level of each signal

Claims (1)

[Claims] 1. In an active matrix liquid crystal display device using a TFT, a value of a potential difference provided between a reference level of a drive signal and a counter electrode level is induced by a gate voltage, and liquid crystal is applied to a pixel electrode. A method of driving a liquid crystal display device, wherein a value of a DC voltage applied to a pixel liquid crystal when set in a parallel or nearly parallel state is set.