JPH03174884A - Drive circuit for liquid crystal display device - Google Patents

Abstract

PURPOSE:To prevent luminance change due to polarity inversion of a drive signal of a liquid crystal by driving the liquid crystal symmetrically with respect to an opposite electrode level vertically even when a gate-source capacitance of a thin film transistor(TFT) exists. CONSTITUTION:A bias power supply 8 is inserted between a reference level point and an opposite electrode 4. Thus, the ground level is decreased with respect to a drain reference level (0V) being a center level of a drain signal by an opposite electrode level VB, and even when the drain signal is decreased by V0 ( V0') at the OFF state of the TFT, the waveform symmetrical in the vertical direction with respect to the opposite electrode level is formed. Even with presence of the reduction in the level of an AC drive signal due to a capacitor in existence between the gate and the source of the TFT, no luminance change is caused at the polarity inversion of the drive signal of the liquid crystal.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial Application Field The present invention relates to a method for driving a liquid crystal display device such as a liquid crystal TV.

(b) Conventional technology Conventional liquid crystal display devices, for example, "Sanyo Technical Review J VOL 16. NO, 2'84 (') pp. 23-2
The amorphous silicon thin film transistor described on page 8 (
Active matrix liquid crystal display (L
CD) The panel is T P as schematically shown in Figure 4.
A display electrode substrate (3) provided with a matrix array of T (1) and a display electrode (2), and a counter electrode (
Liquid crystal is filled between the counter electrode substrate (5) and the counter electrode substrate (5). In addition, (G), (D), and (S) are respectively TPT (
These are the gate electrode, drain electrode, and source electrode of 1).

In the above-mentioned liquid crystal display device, there is a groove bottom in which a capacitor is formed between the display electrode (2) and the counter electrode (4). For example, when used in a liquid crystal TV, the gate electrode ( When a predetermined voltage is applied to G) and it becomes conductive for one horizontal scanning period, the drain voltage applied from the drain signal source to the drain electrode during this period is accumulated in the capacitor, and this charge is retained for one vertical scanning period. It is now possible to do so.

In the above-mentioned liquid crystal display device, when driving the liquid crystal, it is usually preferable to perform AC driving in consideration of the durability of the liquid crystal. For example, in the case of a conventional liquid crystal TV, the polarity of the drain signal is inverted about the reference level (counter electrode level) every field.

Here, the applicant assumes an equivalent circuit corresponding to one pixel as shown in Fig. 4, and applies a pseudo signal for one pixel as shown in Fig. 5 to this equivalent circuit to drive the liquid crystal. The voltage was measured.

In FIG. 4, CL is a dummy capacitor equivalent to the capacitance formed between the display electrode and the counter electrode, CT is the capacitance formed between the gate and source of TPT, and (6) is a drain signal source. A gate signal and a drain signal as shown in FIG. 5 were applied to this equivalent circuit. Here, the counter electrode level and the reference level of the drain signal are the same and both are at ground potential.

The gate signal is applied for one horizontal scanning (IH) period, and the gate voltage (V c, l) T P T (1) 7! l'on,
(VGL) Becomes Teoff. In addition, the drain signal is centered around the reference level (for example, OV), and during the A field, the voltage (v
During the B field, the polarity is inverted for each field so that the voltage (Vo) is obtained.

When the above signal is applied, the dummy capacitor (CL
), as shown in Figure 6, the dummy capacitor (CL) is charged by the gate signal in the A field for a period of 1H, and the terminal voltage (VcL) is VD.
However, when the gate signal becomes vL, the terminal voltage (VcL) decreases from VD to a predetermined voltage v, and V
cL=V CLI (=V o -V -), and thereafter approximately that voltage is maintained until the next gate signal is applied.

In addition, in the B field, the terminal voltage (VCL) rises to -Vo due to the gate signal, but when the gate signal becomes ■, the terminal voltage (VCL) further decreases by V1, and becomes VCL = VCL1 (= Vo Vl). .

As mentioned above, the dummy capacitor (C
The phenomenon in which the terminal voltage decreases when the TPT is turned on even though there is sufficient charge to the capacitor (L) is shown in Figure 4.
This is thought to be due to C a).

That is, in the A field, T P T (11
is in the on state, the source electrode (S) and the drain electrode (
D) is connected, voltage VD is applied to the source electrode (S), and the dummy capacitor (CL) is instantly charged.

Furthermore, at this time, the capacity (C, T) is even higher than Vo. is applied to the gate electrode (G).

Next, T P T (1) is turned off, that is, the source electrode (S) and drain electrode (D) are separated and the gate electrode (G) is at a positive voltage V. VG which is a negative voltage from 8
When the voltage changes to L, part of the charge in the dummy capacitor (CL) moves to the capacitor (Ct) (II. Therefore, the potential of the source electrode (S) decreases by V. (Here, the capacitor (C A) is considered to have a sufficiently small capacitance value compared to the dummy capacitor (CL).) Similarly, in field B, when T P T (1) changes from on to off, the source electrode (S) The potential of is further v from −VD,
′ decreases by .

As mentioned above, due to the existence of the capacitance (Ct) between the gate and source of TPT (1), the terminal voltage (VCL) of the dummy capacitor (CL), that is, the driving voltage of the liquid crystal, is slightly reduced when the TPT is turned off. In this case, since the voltage is lowered by a predetermined amount than the applied drain signal, it becomes vertically asymmetrical with respect to the level of the counter electrode, and in the case of a liquid crystal TV, this causes a reduction in image quality such as a reduction in image contrast. In addition, the drop in liquid crystal drive voltage during A field and B field,
It was also confirmed through experiments that both of and vl were approximately equal values.

(c) Problems to be Solved by the Invention The present invention has been made in view of the above-mentioned conventional drawbacks, and provides a method for driving a liquid crystal display device in which no change in brightness occurs when the liquid crystal drive signal is reversed. It is something to do.

(d) Means for solving the problem A display electrode substrate in which a semiconductor element is bonded to each of a plurality of display electrodes arranged in a matrix, a counter electrode having a counter electrode facing each display electrode of the display electrode substrate; In a driving circuit for a liquid crystal display device including a substrate and a liquid crystal substance filled between these two substrates, a gate signal source supplies a gate signal based on a ground potential to a gate electrode of a semiconductor element of the display electrode substrate. is connected to the drain electrode of the semiconductor element, a drain signal source that supplies an alternating current drain signal with reference to ground potential is connected to the drain electrode of the semiconductor element, the display electrode is connected to the source electrode of the semiconductor element, and the opposing A bias power supply for supplying a bias voltage based on a ground potential is connected to the opposing electrode of the electrode substrate.

(E) Effect According to the present invention, even if the level of the AC drive signal is lowered due to the capacitance existing between the gate and source of the TPT, the AC drive signal becomes vertically symmetrical with respect to the level of the counter electrode.

(f) Example An embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, the equivalent circuit of the liquid crystal drive circuit in this embodiment has a bias power supply (8) inserted between the reference potential point and the counter electrode (4). Note that (7) is a gate signal source. As a result, as shown in Fig. 2, the drain reference level (OV
), the counter electrode level decreases by ■3, and as mentioned above, when the drain signal is V o
Even if the voltage decreases by (#V o '), the waveform becomes vertically symmetrical with respect to the counter electrode level. Here, ■ is V,
Set it to be equal to . The present invention can also be applied to the case where the counter electrode level and the polarity of the drain signal driven by AC are reversed for each IH.

The present invention is also effective when an auxiliary capacitor is formed in parallel with the pixel capacitor in order to obtain stable display.

(G) Effects of the Invention According to the present invention, the capacitance between the gate and the source of TPT is achieved in a TFTLCD panel in which the liquid crystal is AC driven by applying a predetermined bias voltage between the reference level of the AC drive signal and the opposing electrode. Even if there is, the liquid crystal can be driven vertically symmetrically with respect to the counter electrode level, so in the case of a liquid crystal TV, for example, there is no brightness change due to polarity reversal for one field, which causes a decline in image quality such as contrast. There is no.

[Brief explanation of drawings]

Fig. 1 is an equivalent circuit diagram of a liquid crystal drive circuit according to an embodiment of the present invention, Fig. 2 is an operational diagram of an embodiment of the present invention shown in Fig. 1, and Fig. 3 is a schematic diagram of a conventional TPT active Madnox panel. 4 is an equivalent circuit diagram of a conventional TPT active matrix panel, FIG. 5 is a drive waveform diagram of the present invention, and FIG. 6 is a terminal voltage waveform diagram of a dummy capacitor. (1)... TPT, (2) ・Display electrode, (3)...
・Display electrode substrate, (4)...Opposing pole, (5)...Counter electrode substrate, (6)...Drain signal source, (7)...
・Signal source of gate signal, (8)...Bias power supply.

Claims (1)

[Claims] (1) A display electrode substrate in which a semiconductor element is bonded to each of a plurality of display electrodes arranged in a matrix, a counter electrode substrate provided with a counter electrode facing each display electrode of the display electrode substrate, and a space between these two substrates. In a driving circuit for a liquid crystal display device including a filled liquid crystal material, a gate signal source that supplies a gate signal with reference to a ground potential is connected to the gate electrode of the semiconductor element of the display electrode substrate, and the drain of the semiconductor element is connected to the gate electrode of the semiconductor element of the display electrode substrate. A drain signal source that supplies an alternating current drain signal with reference to ground potential is connected to the electrode, the display electrode is connected to the source electrode of the semiconductor element, and the counter electrode of the counter electrode substrate is connected to the ground. A driving circuit for a liquid crystal display device, characterized in that a bias power supply for supplying a bias voltage based on a potential is connected.