JPH0766257B2 - Driving method for liquid crystal display device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention forms a switch array using thin film transistors (hereinafter abbreviated as TFT).
The present invention relates to a driving method for mounting a liquid crystal display .

[0002]

2. Description of the Related Art FIG. 1A shows the structure of a conventional matrix type liquid crystal display device, in which 101 to 103 are gate lines, 104 to 107 are data lines, and 108 to 110 are pixels. The pixel is the gate line 11 as shown in FIG.
The switching TFT 113, the floating capacitance 114, and the liquid crystal cell 115 formed at the intersection of 1 and the data line 112. Reference numerals 116 and 117 are a drive electrode and a common electrode of the liquid crystal cell, respectively.

The data line 112 in FIG. 1B is shown in FIG.
A voltage corresponding to the data signal such as V _D of
A gate line driving voltage such as V _G in FIG. 1C is applied to the gate line 111 in FIG. When the TFT 113 is an N-type TFT, the data line 1 is supplied during the period when V _G is high.
Twelve signals are written to the liquid crystal cell 115, and the signal written to 115 is held during the period when V _G is low.

In FIG. 1C, A represents a negative frame in which V _D <0, and B represents a positive frame in which V _D > 0.

Conventionally, a MOS transistor formed in a single crystal silicon substrate has been used as a switch of a matrix type liquid crystal display device. Voltage-current characteristics of single crystal silicon MOS transistor (gate-source voltage V _GS-
The drain / source current I _DS characteristic) has a large ON / OFF ratio as shown by 201 in FIG. 2, the current change is abrupt in the weak inversion region, and the leakage current in the cutoff region is small. As shown in FIG. 3, the bias relationship between the gate line drive signal 302 and the data line drive signal 301 was set so that the TFT at that time operates in the region 202 of FIG.

[0006]

However, when an attempt is made to drive a matrix type liquid crystal display device using a TFT as a switch by a method similar to that of a matrix type liquid crystal display device using a single crystal silicon MOS transistor as a switch, The problems described arise. TFT with silicon thin film
FIG. 4 shows an example of the voltage-current characteristics of the above.

I _DS is the drain-source current, and V _GS is the gate-source voltage. Reference numerals 401 and 402 denote voltage-current characteristics in which the drain-source voltage V _DS is V _DS = V ₁ and V _DS = V ₂ , respectively. However, V ₂ > V ₁ . TF
The characteristics of the voltage-current characteristic of T are that the current change in the weak inversion region 403 is slower than that in the single crystal silicon MOS transistor, the leak current level in the cutoff region is large, and the PN junction leak in the cutoff region 404. That is, I _DS is increased by the current.

When the liquid crystal cell is driven in the bias relationship as shown in FIG. 3, the gate-source voltage V _{GS of the} TFT changes within the range of 405 in FIGS. At this time, T when not conducting
The PN junction current flowing through the FT becomes considerably large, which appears as display unevenness on the screen and greatly impairs display performance.

[0009]

In the liquid crystal display device of the present invention,
The driving method includes a plurality of gate lines and a plurality of data lines intersecting the gate lines, and a substrate having a thin film transistor formed in the vicinity of each intersection and a transparent electrode serving as a common electrode. Liquid crystal display device in which liquid crystal is sandwiched between
, The center voltage of the data signal is V _B = C _S · V _G / (C _LC + C _S ), where C _S is the parasitic capacitance of the thin film transistor, and V _G is the gate line drive voltage during selection and non-selection. The difference, C _LC is the pixel capacitance, and V _B is expressed by
Characterized by being assaulted .

[0010]

[0011]

[0012]

[Function] Between the central voltage of the data line drive signal and the common electrode
By applying a constant bias voltage V _B to the liquid crystal,
By removing the DC component of the voltage _{V LC} applied _to, the _{V LC}
It is possible to compensate the asymmetry of the waveform and prevent the deterioration of the liquid crystal due to the DC drive.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, by providing an appropriate bias voltage V _B according to the voltage-current characteristics of a TFT between the zero level of the gate line drive signal V _{G and} the zero level of the data line drive signal V _D. The present invention provides a method for driving a matrix type liquid crystal display device which solves the above-mentioned drawbacks and has good display performance, which will be described in detail below.

In the first embodiment of the driving method of the present invention, as shown in FIG. 5, a constant bias according to the voltage-current characteristics of the TFT is provided between the zero level of the gate line driving signal and the zero level of the data line driving signal. By providing the voltage V _B , the TFT at the time of non-conduction has a minimum leak current within its operating range.

In FIG. 5, 501 is a gate line and 502
Is a data line, 503 is a switching TFT, 504 is a liquid crystal cell, 505 is a data line drive signal source, and 506 is a gate line drive signal source.

6 and 7 show an embodiment of the present invention. Reference numerals 601 and 602 are shown in FIGS.
2 is a curve of the same voltage-current characteristics as No. 2. 601, 60
When the switch of the matrix type liquid crystal display device is formed by the TFT having the voltage-current characteristic of 2, the TFT in the non-conduction state
The operating range that minimizes the leakage current of
The range is shown in 3. Thus, in order to set the operating range of the TFT in the non-conducting state, that is, the changing range of the gate-source voltage V _GS of the TFT in the non-conducting state in the region where the leak current is minimized, in the present invention, As shown in FIG. 7, a bias voltage of V _B is provided between the zero level V _G = 0 of the gate line driving signal 702 and the zero level V _D = 0 of the data line driving signal 701.

According to the above driving method, the average value of the leak current of the TFT at the time of non-conduction is reduced by 50% to 90% as compared with the driving method as shown in FIG. In the circuit configuration for realizing the driving method of the present invention, it is desirable that the value of V _B can be externally adjusted as shown in the example of FIG. By making V _B correctable from the outside, it is possible to easily deal with variations in TFT characteristics between manufacturing lots. Another effect produced by externally setting the value of the bias voltage V _B is to reduce the charging / discharging current to the parasitic capacitance added to the gate and the gate line of the switching TFT and achieve low power consumption. That is. Most of the TFT parasitic capacitance and the gate line parasitic capacitance are attached between the gate lines and between the gate lines or between the gate line and the liquid crystal drive current. Therefore, by setting V _B to a negative value, the current for charging and discharging the parasitic capacitance is reduced and low power consumption is achieved.

FIG. 8 and FIG. 9 show a second embodiment of the present invention.

In FIG. 8, reference numeral 801 denotes a gate line and 802.
Is a data line, 803 is a TFT, 804 is a liquid crystal cell, 80
Reference numeral 5 is a data line drive signal source, 806 is a gate line drive signal source, 807 is a first bias power supply, 808 is a second bias power supply, and C _S is a parasitic capacitance of the TFT. In FIG. 9, 901 is a data line drive signal waveform and 902 is a gate line drive signal waveform.

In FIG. 10, the liquid crystal cell has a gate line drive signal V.
_The state of being driven by _G and the data line drive signal V _D is shown, and V _DC shows the state of change of the voltage applied to the liquid crystal cell. Due to the influence of the parasitic capacitance C _S of the TFT shown in FIG.
At the same time when the gate line drive signal V _G falls, V _LC

[0021]

[Equation 1]

Only changes to the negative side. Therefore, in the negative frame A, the effective value of V _LC increases immediately after the data is written, and in the positive frame B, the effective value of V _LC changes immediately after the data is written. To do. Therefore, the waveform of V _LC contains a constant DC component as shown in FIG. 10, which causes a problem of significantly shortening the life of the liquid crystal.

In the second embodiment, a constant bias voltage V _B2 is provided between the zero level of the data line driving vibration and the common electrode of the liquid crystal cell so that the DC voltage contained in the above V _LC can be controlled. _Is removed to compensate the vertical asymmetry of the V _LC waveform and prevent deterioration of the liquid crystal due to DC driving. As shown in FIG. 8, the bias voltage V _B1 of the gate line drive signal source and the bias potential V _B2 of the data line drive signal source can be independently set externally to reduce the leak current of the TFT at the time of non-conduction. It is possible to simultaneously realize the high reliability of the display device by removing the display unevenness and the DC voltage applied to the liquid crystal.

[0024]

As described above, the liquid crystal display device of the present invention is
The potential difference between the central potential of the data signal and the common potential signal is different.
_VLC is substantially positive / negative symmetrical in the vertical scanning period.
Since it is biased in the direction of
It is possible to remove the direct current component contained in the above _VLC ,
Liquid crystal display device while preventing deterioration of liquid crystal due to current drive
It will be possible to greatly improve the display performance of
effective. Further, the driving method of the liquid crystal display device of the present invention
Is the potential difference between the central potential of the data signal and the common potential signal,
_VLC is substantially positive / negative symmetrical in different vertical scanning periods
Is it characterized by being biased in the direction
, It is possible to remove the DC component contained in the above V _LC.
Prevent the deterioration of the liquid crystal due to DC drive, and
It is possible to greatly improve the display performance of the display device.
There is an effect.

[Brief description of drawings]

FIG. 1A is a diagram showing a configuration of a matrix type liquid crystal display device. FIG. 6B is a diagram for explaining the configuration of the pixel.
(C) is a figure which shows a drive signal.

FIG. 2 is a diagram showing voltage-current characteristics of a single crystal silicon MOS transistor used in a conventional matrix type liquid crystal display device.

FIG. 3 is a diagram showing a conventional example of a method for driving a gate line and a data line.

FIG. 4 is a diagram for explaining driving of a switching TFT by a conventional driving method.

FIG. 5 is a diagram for explaining the first embodiment of the present invention.

FIG. 6 is a diagram for explaining the first embodiment of the present invention.

FIG. 7 is a diagram for explaining the first embodiment of the present invention.

FIG. 8 is a diagram for explaining a second embodiment of the present invention.

FIG. 9 is a diagram for explaining the second embodiment of the present invention.

FIG. 10 is a diagram for explaining the second embodiment of the present invention.

[Explanation of symbols]

101, 102, 103, 111, 404, 501, 8
01 Gate line 104, 105, 106, 107, 112, 502, 8
02 data lines 108, 109 and 110 pixel 113,503,803 TFT 114 stray capacitance 115,504,804 liquid crystal cell 116 driving electrode 117 common electrode 401 drain-source voltage V _DS of V _DS = V ₁ and the voltage of the liquid crystal cell -Current characteristic weak inversion region 402 Voltage where drain-source voltage V _DS is V _DS = V ₂ -Current characteristic weak inversion region 403 Blocking region 505,805 Data line drive signal source 506,806 Gate line drive signal source 807 1st Bias power supply 808 Second bias power supply C _S TFT parasitic capacitance 901 Data line drive signal waveform 902 Gate line drive signal waveform

Claims (1)

[Claims] 1. A liquid crystal is enclosed in a pair of substrates,
It has a common electrode on one substrate and a common electrode on the other substrate.
Multiple gate lines and multiple data formed in the shape of a lix
Line, near each intersection of the plurality of gate lines and the plurality of data lines
A plurality of formed pixel electrodes, and the plurality of pixel electrodes
It has a plurality of thin film transistors formed correspondingly, and a scanning signal is supplied to the thin film transistors from the gate line.
The phase of the pixel electrode is inverted every vertical scanning period.
Data signal is transmitted through the thin film transistor
In a liquid crystal display device supplied from a line, the center voltage of the data signal is V _B = C _S · V _G / (C _LC + C _S ), where C _S is the parasitic capacitance of the thin film transistor and V _G is the time of selection. The difference in gate line drive voltage when not selected, C _LC is the pixel capacitance, and V _B represented by
Driving method for liquid crystal display device characterized by being removed
Law.