JPH0667146A - Method for driving liquid crystal display device - Google Patents

Abstract

PURPOSE:To improve display image quality, visual angle dependency in particular, by imparting a first modulation signal different from each other at every first wiring and uniformizing transmission light intensity in an entire picture at a certain viewpoint. CONSTITUTION:Respective display elements are scanning signal-wired, and a TFT(thin film transistor) 3 is provided at a crossing of an image signal wiring 2. A parasitic capacity exists in the TFT 3, and further, a liquid crystal capacity 7, a storage capacity 8 exist as the capacity formed intentionally. Drive voltage is impressed to these respective element electrodes from the outside, and a scanning signal Vg is impressed to a scanning signal wiring 1, and an image signal voltage Vsig is impressed to the image signal wiring 2, and first modulation signals Ve(+), Ve(-) corresponding to the polarity of an image advance signal inverting at every 1 field are impressed to one side electrode of the storage capacity 8, and a constant voltage is impressed to the counter electrode of the liquid crystal capacity 7 at every field. At this time, potential fluctuation appearing on a pixel electrode is set to optional magnitude when Ve(+) and Ve (-) are controlled, and by changing the potential fluctuation, a visual angle characteristic is changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention uses an active matrix having a switching element such as a thin film transistor (hereinafter referred to as a TFT) and a pixel electrode in a matrix to drive a display material such as liquid crystal with an alternating current to display an image. It is an object of the present invention to improve the display image quality of a display device to be performed by a driving method.

[0002]

2. Description of the Related Art The display image quality of an active matrix liquid crystal display device has been remarkably improved in recent years, and has reached a level comparable to that of a CRT. However,
In terms of image quality, flicker, brightness change in the vertical direction of the screen,
That is, it cannot be said that the image memory phenomenon, the gradation display performance, the viewing angle dependency, etc., which remain as if the image of the fixed image was burned in immediately after displaying the brightness gradient / fixed image, is comparable to that of the CRT.

In particular, the improvement of the viewing angle dependency is one of the important problems for a liquid crystal display device whose screen has been enlarged in recent years. In general, a liquid crystal display device has a large viewing angle dependency, and an image varies greatly depending on the viewing angle of the display screen. Therefore, in a large-screen liquid crystal display device, when the display screen is viewed from a certain point of view, the display image is not uniform in the upper part and the lower part of the display screen, and it looks as if there is a brightness gradient in the vertical direction of the screen.

The following techniques are known as measures for improving flicker. That is, Japanese Patent Application Laid-Open No. 60-1516 discloses that the polarity of the signal voltage is inverted for each field on the display screen.
No. 15, JP 61-256325, and JP 61-2.
There is, for example, Japanese Patent No. 75823. Japanese Patent Laid-Open No. Sho 60-1985 discloses that the characteristics of the signal voltage are inverted every scanning line on the display screen.
No. 3698, No. 60-156095, No. 6
There is a publication such as 1-275522. Further, Japanese Patent Application Laid-Open No. 61-275824 discloses an apparatus which inverts every scanning line while performing field inversion. However, these methods do not compensate for the DC voltage that is inevitably generated due to the dielectric anisotropy of the display material such as liquid crystal described below and the parasitic capacitance inside the display device. (2) Instead of reducing the flicker, it is a reduction in the apparent flicker as a whole.

In a special active matrix configuration example, K. Oki and others: Euro Display (Euro D
isplay) '87 P55 (1987) is known. W. E. Howard et al .: I. D. R. C (InternationalDi
splay Research Conference) '88 P230 (19
88) is known. This method compensates for a crosstalk voltage component after supplying an image signal. There is no particular consideration for compensating a DC voltage due to the dielectric anisotropy of the liquid crystal described later.

Next, as a publicly known document intended to compensate the DC voltage inevitably generated in the display device due to the dielectric anisotropy of the liquid crystal, basically reduce the flicker and improve the driving reliability. , There are the following two cases. The first is T. Yanagisawa et al .: Japan Display '8
6 P192 (1986). This is to compensate for this DC voltage by changing the positive and negative amplitudes with respect to the amplitude center voltage (Vc) of the image signal voltage (Vsig). Secondly, K. Suzuki: Euro Display '
87 P107 (1987). Here, a negative additional signal (Ve) is applied after the scanning signal to try to compensate.

Further, in the liquid crystal display device, the scanning signal affects the display electrode potential through the parasitic capacitance Cgd between the gate and drain of the TFT, and the potential between the average potential of the image signal wiring and the average potential of the display electrode is increased. Although a direct current potential difference is generated, when the liquid crystal is driven by an alternating current, if the potential of each part of the display device is set so that the average DC potential difference between the display electrode and the counter electrode becomes zero, the direct current potential difference faces the image signal wiring. Inevitably appears between the electrodes. This DC potential difference causes a serious display defect such as an image memory. However, a method for compensating the DC potential difference to be essentially zero has not been realized yet.

[0008]

SUMMARY OF THE INVENTION The present invention is intended to improve the display image quality, especially the viewing angle dependency among the above problems.

[0009]

In order to solve the above-mentioned problems, the present invention has a matrix of pixel electrodes connected to a first wiring via a capacitor, and the pixel electrode is provided with an image signal wiring. A switching element electrically connected to the scanning signal wiring is connected to drive the liquid crystal material held between the pixel electrode and the counter electrode by alternating current, and a signal voltage is applied to each field of the display screen during the ON period of the switching element. The image signal voltage whose polarity is inverted is transmitted to the pixel electrode, and the first modulation signal whose polarity is inverted for each field is applied to the first wiring during the OFF period of the switching element, whereby the potential of the pixel electrode is changed. In a liquid crystal display device in which a potential is changed and an image signal voltage is superimposed and / or canceled to apply a voltage to a display material, a different first modulation signal is applied to each first wiring. By Rukoto, the first modulated signal as a transmitted light intensity of the entire screen in a certain viewpoint uniform and optimum value.

[0010]

By the above means, the switching element is a TFT
In the case of (thin film transistor), the potential change Vg of the scanning signal and the potential change CgdVg / ΣC with the image signal induced via the gate-drain capacitance Cgd occur in the negative direction. Therefore, by providing asymmetrical positive and negative first modulation signal widths Ve (+) and Ve (-) with the polarity reversed for each field via the storage capacitor Cs, CsVe in the negative direction is given.
A potential change of (+) / ΣC and CsVe (−) / ΣC in the positive direction is generated in the pixel electrode and superposed on the above-described potential change CgdVg / ΣC. Here, if the relationship of these potential changes is set to satisfy the following equation, the dielectric anisotropy of the liquid crystal and at least a part of the DC component induced by the scanning signal via the gate-drain capacitance are compensated, High quality display is possible by removing factors such as flicker and image memory.
The driving reliability of the display device is also high.

CsVe (+) / ΣC + CgdVg / ΣC = CsVe (−) / ΣC−CgdVg / ΣC = ΔV * ΣC: Total capacitance of one pixel Here, Cs, Cgd, ΣC, and Vg are fixed values. Then Ve
(+) And Ve (-) are not uniquely determined, but satisfy the following relationships.

(Ve (−) − Ve (+)) / 2 = CgdVg / Cs = Veo Veo: Constant Ve (+) and Ve (−) are changed while satisfying the above formula, that is, ΔV * is changed. Thus, the input signal voltage-transmittance characteristic of the display device can be changed.

Generally, in a liquid crystal display device, the input signal voltage-transmittance characteristic changes greatly with respect to changes in the viewing angle, particularly in the vertical direction. The change in the characteristics due to this viewing angle,
By canceling the change in the input signal voltage-transmittance characteristic due to ΔV *, the transmitted light intensity on the entire screen can be made uniform from a certain viewpoint.

[0014]

EXAMPLES Examples of the present invention will be described below.

FIG. 1 shows an electrical equivalent circuit of a display element of a TFT active matrix drive LCD. Each display element has a TFT 3 at the intersection of the scanning signal wiring 1 and the image signal wiring 2.
Have. The TFT 3 has a gate-drain capacitance 4 (Cgd) and a source-drain capacitance 5 (C
sd) and a gate-source capacitance of 6 (Cgs). Further, as a capacitance intentionally formed, a liquid crystal capacitor 7 (Clc
*), Storage capacity 8 (Cs).

External drive voltages are applied to the respective element electrodes, the scanning signal Vg is applied to the scanning signal wiring 1, the image signal voltage Vsig is applied to the image signal wiring 2, and one electrode of the storage capacitor 8 (Cs) is applied. Is a first modulation signal Ve (+), Ve (-) corresponding to the polarity of the image signal inverted for each field, and a constant voltage is applied to the counter electrode of the liquid crystal capacitor 7 (Clc *) for each field. Is applied.

The influence of the drive voltage is affected by the driving voltage through the above parasitic or various capacitors intentionally installed (point A in the figure).
Appear in.

Vg and Ve shown in FIGS. 2 (a) to 2 (d) defined as the change component of the voltage related to the nth scanning line.
When (+), Ve (-), Vt and Vsig are applied to the respective points in FIG. 1, the potential change of the pixel electrode due to capacitive coupling is ΔV *,
It is expressed by equations (1) and (2) in even and odd fields (however, the potential change component at point A due to conduction from the image signal wiring caused by turning on the TFT is excluded).

ΔV * + = (CsVe (+) + CgdVg−CsdVsig) / Ct ... (1) ΔV * − = (CsVe (−)-CgdVg−CsdVsig) / Ct …… (2) Ct = Cs + Cgd + Csd + Clc * = Cp + Csd + * The first term in the above equations (1) and (2) is the potential change due to the first modulation signal. The second term is a potential change induced in the pixel electrode by the scanning signal Vg through the parasitic capacitance Cgd of the TFT. The third term shows a potential change that the image signal voltage induces in the pixel electrode through the parasitic capacitance. Clc * is the capacitance of the liquid crystal that changes under the influence of its dielectric anisotropy as the alignment state of the liquid crystal changes depending on the magnitude of the signal voltage (Vsig). Therefore, Clc * and ΔV * are large (Clc (h)) and small (Cl
c (l)) respectively. Here, Cgs is the capacitance between the gate and the signal electrode, but the scanning signal line and the image signal line are both driven by a low impedance power supply, and this coupling does not directly affect the display electrode potential, so it is ignored.
If the potential changes ΔV * + and ΔV * − in the even and odd fields are equal to each other, symmetrical AC drive can be performed without applying a DC voltage to the liquid crystal. That is, the following formula should be satisfied.

(CsVe (+) + CgdVg−CsdVsig) = (CsVe (−) − C
gdVg-CsdVsig) (3) Since Vsig gives a signal that is inverted every field, the effect of the third term CsdVsig is canceled in each field.
Therefore, the formula (3) is simplified as (CsVe (+) + CgdVg) = (CsVe (-)-CgdVg) (4).

Here, rewriting the equation (4) gives (Ve (-)-Ve (+)) = 2CgdVg / Cs (4a).

The first point to note is that the electric potential ΔV * induced in the pixel electrode can be made equal to positive and negative in the even and odd fields with respect to the counter electrode regardless of the liquid crystal capacitance.
Therefore, positive and negative polarities are equally applied to the liquid crystal, and flicker is essentially reduced.

The second point to note is that Clc * does not appear in equations (3) and (4). That is, equations (3) and (4)
If driven under a condition that satisfies the condition, the influence of the dielectric anisotropy of the liquid crystal disappears, and the DC voltage due to Clc * does not occur inside the display device.

A third point is that, under the driving condition satisfying the expressions (3) and (4), the scanning signal Vg also cancels out the DC potential induced between the image signal wiring and the display electrode through the parasitic capacitance Cgd, so that it is zero. Can be Further, in the driving method of the present embodiment, a signal of positive / negative reverse polarity with respect to the potential of the counter electrode is applied for each field, so that when two fields are seen, a DC electric field is generated between the potentials of the pixel electrode, the signal electrode, and the counter electrode. Does not occur. Since the driving method does not apply a DC voltage to the liquid crystal, it is advantageous in terms of reliability.

The fourth point is that the conditional expressions (3) and (4) have two voltage parameters Ve (+) and Ve (-) that can be arbitrarily set on the display device side. Therefore, Ve (+) and Ve (-)
By controlling in accordance with equations (3) and (4), the potential fluctuation ΔV * appearing in the pixel electrode can be set to an arbitrary magnitude. By changing the ΔV *, the viewing angle characteristics of the display device can be changed, and the improvement of the viewing angle dependency, which is the object of the present invention, can be realized. Further, if ΔV * is equal to or higher than the threshold voltage of the liquid crystal, Vsig can be reduced, which is effective for power saving. On the other hand, Vg is a semi-fixed constant determined by the driving condition, but its influence can be corrected by Ve (+) and Ve (-). On the other hand, Vsig is the display data itself and changes arbitrarily between the maximum value and the minimum value.

2 (e) and 2 (f) are potential changes of the pixel electrode (point A in FIG. 1) when the drive signals Vg, Vsig and the modulation signal Ve are input to each electrode of the display element of FIG. Indicates. For example, when Vsig is Vs (h) in the odd field as shown by the solid line in FIG. 2D, when the scanning signal Vg is input at T = T1, the TFT becomes conductive and the potential Va at the point A becomes Vs (h). Charge until it becomes equal to. Before the TFT of T = T2 is turned off (preferably between T1 and T2 where the TFT is in a conductive state), Ve gives a signal of Ve (-) in the negative direction. Next, when the scanning signal disappears, this change in Vg appears as a potential change of ΔVg at point A through Cgd. T after the delay time τd
When Ve changes in the positive direction by Ve (-) at = T4, this effect appears as a positive displacement of the potential Va as shown in the figure. After that, when Vsig changes from Vs (h) to Vs (l) at T = T5, the potential fluctuation at the point A also appears. This capacitive coupling component is also shown as ΔV * in the figure.

After that, when a scanning signal is input in the even field, the TFT changes the point A to a low level Vs (l) of Vsig.
Charge up to. When the TFT is turned off, the capacitive coupling potential ΔV * appears as in the above. When Vsig is at a high level and Ve is at a low level when the TFT is turned off as described above, or conversely, when Vsig is at a low level and Ve is at a high level and Ve changes after the TFT is turned off. Is the amplitude of the effective voltage applied to the liquid crystal, Ve, relative to the image signal amplitude Vsigpp.
ff is almost (2ΔV * + Vsigpp) as shown in the figure, and both overlap each other. In other words, the image signal output IC
Output amplitude can be reduced by 2ΔV * (hereinafter, the case where Ve and Vsig have the above-mentioned phase relationship is referred to as antiphase).

On the other hand, for the modulation signal Ve, Vsig is shown in FIG.
When there is a phase relationship like the dotted line in (d) (hereinafter referred to as in-phase), the effective applied voltage amplitude at point A is almost (2ΔV * −).
Vsigpp), and ΔV * and Vsig cancel each other out.

FIG. 3 shows the relationship between the applied voltage of the liquid crystal and the transmitted light intensity, and also shows an example of the voltage range for controlling the transmitted light by ΔV * and Vsig. The voltage range in which the transmitted light of the liquid crystal changes is from the threshold voltage Vth of the liquid crystal to the saturation voltage Vmax. If ΔV * is set to Vth or more, the required maximum signal voltage becomes (Vmax-Vth) when the phase control is not performed. By setting the applied voltage by ΔV * to VCT and controlling the amplitude and phase of the signal voltage, the required maximum signal amplitude voltage can be reduced to about (Vmax-Vth) / 2.

FIG. 4 shows the relationship between the applied voltage and the transmitted light intensity when the viewing angle is changed. Same value ΔV for all scanning lines
When * is given, the applied voltage-transmitted light intensity characteristic changes greatly depending on the viewing angle. Therefore, when ΔV *, which is the closest to the applied voltage vs. transmitted light intensity characteristic at the front, is given for each scanning line at the viewing angle corresponding to each scanning line, the applied voltage vs. transmitted light intensity at each viewing angle is obtained as shown in FIG. The characteristics are considerably close to each other, and the transmitted light intensity on the entire surface of the display device becomes uniform at a certain viewpoint on the front surface of the display device, and the improvement of the viewing angle dependency, which is the object of the present invention, can be achieved.

FIG. 6 shows a circuit diagram of the apparatus of the second embodiment of the present invention. Reference numeral 9 is a scan drive circuit, 10 is a video signal drive circuit, 11 is a first modulation circuit, and 12 is a second modulation circuit. 13a, 13b, ... 13z are scanning signal wirings, 14
a, 14b, ... 14z are image signal wirings, 15a, 15
b, ... 15z is a common electrode of the storage capacitor Cs, 16a, 1
Reference numerals 6b, ..., 16z are liquid crystal counter electrodes. In the present embodiment, as described above, the storage capacitor and the counter electrode are formed separately for each scanning signal wiring, and the first modulation signal is also applied corresponding to each scanning signal wiring. FIG. 7 shows a time chart of the scanning signal / modulation signal. In the figure, the scanning signal / modulation signal for the n-th scanning signal wiring and the (n + 1) -th scanning signal wiring are respectively shown. The mutual relationship between the modulation signal / image signal and ΔV *, Vsig is essentially as shown in FIG.
Is equivalent to That is, the polarity of the video signal / modulation signal is 1
Invert every field. Here, ΔV * for the nth scanning signal wiring and the n + 1th scanning signal wiring have different values.

In this embodiment, it is assumed that a liquid crystal display device having a height of about 25 cm is viewed from the front side at a distance of about 40 cm. At this time, when the upper end and the lower end of the display device are viewed, they have an angle of about 16 ° in the vertical direction. Here, if ΔV * corresponding to each scanning signal wiring is all set to the same value, the applied voltage-transmitted light intensity characteristics at each viewing angle are greatly different, as shown in FIG. 4, and a luminance gradient occurs in the vertical direction of the screen. It looks like you are. In addition, when ΔV * for each scanning signal wiring is adjusted so as to be as close as possible to the applied voltage-transmitted light intensity characteristic from the front direction, it becomes as shown in FIG. At this time, the difference in ΔV * between the upper part and the lower part of the screen was about 1.5V. As a result, the improvement of the viewing angle dependency, which is the object of the present invention, can be realized, and the transmitted light intensity can be made substantially uniform from the upper part to the lower part of the screen when viewed from the front of the display device. Further, there was little flicker, the output amplitude of the signal voltage was 3 Vpp, and the entire area from white to black could be driven, and display with good contrast was possible. Further, there was almost no DC component between the electrodes, and the long-term reliability of the liquid crystal was good.

[0033]

As described above, according to the present invention, in the active matrix liquid crystal display device having the viewing angle dependency, each scanning line is supplied with Ve (+) and Ve (-) as independent voltage values. By changing the viewing angle dependency for each scanning line, it is possible to obtain a display screen in which the transmitted light intensity is uniform from the upper part to the lower part of the screen at a certain viewpoint.

[Brief description of drawings]

FIG. 1 is a configuration diagram for explaining a first embodiment of the present invention.

FIG. 2 is a voltage waveform diagram applied to the same configuration.

FIG. 3 is a characteristic diagram showing the relationship between transmitted light intensity of liquid crystal and applied voltage.

FIG. 4 is a diagram showing a relationship between a signal voltage of a liquid crystal display device and transmitted light intensity.

FIG. 5 is a characteristic diagram showing the relationship between applied voltage and transmitted light intensity when optimal Ve (+) and Ve (-) are set at each viewing angle.

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

FIG. 7 is a voltage waveform diagram applied to the same configuration.

[Explanation of symbols]

 1 scanning signal wiring 2 image signal wiring 3 TFT 4 gate-drain capacitance 5 source-drain capacitance 6 gate-drain capacitance 7 liquid crystal capacitance 8 storage capacitance 9 scanning drive circuit 10 video signal drive circuit 11 first modulation signal drive Circuit 12 Second modulation signal drive circuit 13a, 13b, ... 13z Scan signal wiring 14a, 14b, ... 14z Image signal wiring 15a, 15b, ... 15z Storage capacitor common wiring 16a, 16b, ... 16z Counter electrode Common wiring

Claims (9)

[Claims] 1. A switching element electrically connected to an image signal wiring and a scanning signal wiring is connected to the pixel electrode, the pixel electrode being connected to the first wiring through a capacitor in a matrix form. The liquid crystal material held between the pixel electrode and the counter electrode is AC-driven, and an image signal voltage whose signal voltage polarity is inverted for each field of the display screen is transmitted to the pixel electrode during the ON period of the switching element. , The potential of the pixel electrode is changed by applying a first modulation signal in a direction opposite to the image signal voltage to the first wiring for each field during the OFF period of the switching element. In a liquid crystal display device that applies a voltage to the liquid crystal material by superposing and / or canceling a change and the image signal voltage with each other, the first modulation signal different for each of the first wirings is applied. A method for driving a liquid crystal display device, which gives a uniform transmitted light intensity over the entire screen from a certain viewpoint by giving a given point. 2. The image signal voltage transmitted during the on period of the switching element inverts the polarity of the signal voltage for each scanning line of the display screen, and the first during the off period of the switching element.
2. The method for driving a liquid crystal display device according to claim 1, wherein the polarity of the first modulated signal applied to the wiring is reversed every scanning line. 3. A first modulation signal Ve (+), V having an inverted polarity, which is applied to the first wiring during the off period of the switching element.
3. The method for driving a liquid crystal display device according to claim 1, wherein the absolute values of e (−) are different. 4. The method for driving a liquid crystal display device according to claim 3, wherein a part of the potential of the first modulation signal is changed before the end of the ON period of the switching element. 5. The switching element is a thin film transistor, and the positive and negative first modulation signals with inverted polarities are respectively supplied to Ve.
(+), Ve (-), the potential change of the scanning signal is Vg, the storage capacitance, the gate-drain capacitance, and the source-drain capacitance are Cs, Cgd, Csd, and the total capacitance per pixel. When ΣC, the potential change CsVe (+) / ΣC, CsVe (-) / ΣC of the pixel electrode due to the first modulation signal and the potential change CgdVg / Σ of the pixel electrode due to the change of the scanning signal voltage
The relationship with C is CsVe (+) / ΣC + CgdVg / ΣC = CsVe (-) / ΣC-
4. The method of driving a liquid crystal display device according to claim 3, wherein CgdVg / ΣC = ΔV * is satisfied, and the value ΔV * is not less than a threshold voltage of the liquid crystal. 6. An image signal voltage is transmitted to a pixel electrode during an ON period of a switching element, and an average DC voltage generated between a counter electrode, a signal wire and a display electrode is smaller than CgdVg / ΣC. Item 6. A method for driving a liquid crystal display device according to item 5. 7. The method of driving a liquid crystal display device according to claim 3, wherein the potential of the counter electrode is held in an electrically floating state. 8. The electric wiring according to claim 3, wherein the first wiring is shared with the scanning signal wiring, and the first modulation signal is applied to the scanning signal wiring by superimposing it on the scanning signal. A method for driving the described liquid crystal display device. 9. The method of driving a liquid crystal display device according to claim 1, wherein the potential of the counter electrode of the liquid crystal display device is constant at least in each field period.