JPH0634944A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To increase the number of display gradations of the liquid crystal display device by controlling the pulse amplitude and then improving the precision of a gradational display. CONSTITUTION:This liquid crystal display device is provided with a matrix electrode consisting of a scanning electrode group 201 and an information electrode group 202 crossing the group opposite, liquid crystal which is arranged between the electrode groups and driven with an electric field applied through the electrode groups, and driving means 102 and 103 which drive those electrode parts with a signal waveform including a period wherein the applied potential is continuously varied and adjusts the length of the period to make the gradational display.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device which performs gradation display using a liquid crystal element.

[0002]

2. Description of the Related Art Conventionally, a liquid crystal display element for displaying image information by filling a liquid crystal compound between a scanning electrode group and an information electrode group of a matrix electrode to form a large number of pixels is well known. As a driving method of this display element, a time-division driving method in which an address signal is sequentially and selectively applied to the scanning electrode group and a predetermined information signal is selectively applied to the information electrode group in parallel in synchronization with the address signal. Has been adopted.

Further, a liquid crystal compound using a ferroelectric liquid crystal (FLC) as a liquid crystal compound to be filled has attracted attention because it has bistability and has a quick response to an electric field.
Regarding a display element using this ferroelectric liquid crystal, as shown in Japanese Patent Laid-Open No. 94023/1986, two glass substrates each having a transparent electrode formed on the opposite surface and subjected to an alignment treatment are used. It is known that a ferroelectric liquid crystal is injected into a liquid crystal cell formed by facing each other while maintaining a cell gap of about 3 μm.

The characteristic of this display element is that the ferroelectric liquid crystal has spontaneous polarization, so that the coupling force between the external electric field and the spontaneous polarization can be used for switching, and the long-axis direction of the ferroelectric liquid crystal molecules exhibits spontaneous polarization. Since there is a one-to-one correspondence with the polarization direction, it is possible to switch depending on the polarity of the external electric field.

Since a chiral smectic liquid crystal (SmC *, SmH *) is generally used as the ferroelectric liquid crystal,
In the bulk state, the long axes of the liquid crystal molecules show a twisted orientation,
Twisting of the long axis of the liquid crystal molecule can be eliminated by putting the cell in the cell gap of about 1 to 3 μm (P213-P234, NA CLarket).
al, MCLC 1983, Vol. 94).

An actual ferroelectric liquid crystal cell is composed of a simple matrix substrate as shown in FIG. Figure 1
2A is a sectional view of the cell, and FIG. 12B is a plan view of the electrode substrate. As shown in the figure, this cell is constructed by sandwiching a ferroelectric liquid crystal 6 between two electrode substrates 81 and 82. The electrode substrates 81 and 82 are, respectively, a glass substrate 61, an ITO stripe electrode 62 for driving a liquid crystal formed thereon, a SiO ₂ insulating film 63 formed thereon,
And a polyimide alignment film 64 laminated thereon. Reference numeral 65 is a sealing member.

The ferroelectric liquid crystal is mainly used as a binary (white / black) display element in which two stable states are used as a light transmitting state and a light blocking state, but a multi-value display, that is, a halftone display is also possible. One of the halftone display methods is to create an intermediate light transmission state by controlling the area ratio of bistable states in a pixel. Hereinafter, this method (area modulation method) will be described in detail.

FIG. 13 is a diagram schematically showing the relationship between the switching pulse amplitude and the transmittance of the ferroelectric liquid crystal element,
8 is a graph in which the amount of transmitted light I after applying a single-shot pulse of one polarity to a cell (element) that was initially in a completely light-blocking (black) state was plotted with the amplitude V of the single-shot pulse as a variable. When the pulse amplitude V is less than or equal to the threshold value V _th (V <V _th ), the amount of transmitted light does not change, and the transmitted state after pulse application is shown in FIG.
As shown in (b), there is no difference from that in (a) of FIG. When the pulse amplitude V exceeds the threshold value V _th (V _th <
(V <V _sat ) A part of the pixel makes a transition to the other stable state, that is, the light transmission state shown in FIG. 7C, and shows an intermediate amount of transmitted light as a whole. When the pulse amplitude V further increases and reaches the saturation value V _sat or more (V _sat <V), all the pixels are in the light transmitting state as shown in FIG.

That is, in the area modulation method, the voltage is controlled so that the pulse amplitude V becomes V _th <V <V _sat and halftone is displayed. Therefore, when driving the ferroelectric liquid crystal on the matrix electrodes, as shown in FIG. 15, a transistor is formed in each output circuit that applies a voltage waveform to each electrode, and a given floor is provided. The pulse amplitude is controlled by amplifying the key information.

[0010]

However, in the above-mentioned conventional example, when the gradation information is amplified by the transistors, the driving ability of each electrode reflects the ability of each transistor. The applied potential varies. In a large-screen display, since the load current of the transistor is large, the variation of the driving ability for each electrode is particularly remarkable. For example, 12 with output capability of 0-5V
If there are 80 output circuits, the current should be 50-200.
Since the signal is amplified about twice, even if the same gradation information is given, a variation of about 50 to 100 mV occurs between the output terminals. As a result, even when the graphic controller sends highly accurate information of, for example, 512 gradations, the display gradation is limited to 50 to 100 colors due to the variation between the output circuits.

An object of the present invention is to improve the accuracy of gradation display and increase the number of display gradations in a liquid crystal display device in view of the above-mentioned conventional technique.

[0012]

In order to achieve the above object, a liquid crystal display device of the present invention comprises a matrix electrode composed of a group of scanning electrodes and a group of information electrodes facing and intersecting the scanning electrodes, and the electrodes. A liquid crystal that is arranged between the groups and driven by an electric field applied through the electrode group, and these electrode groups,
And a driving unit that performs gradation display by driving with a signal waveform including a period in which the applied potential is continuously changed, and adjusting the length of the period.

In a preferred embodiment of the present invention, the drive signal waveform includes a period in which the applied potential is monotonically increased or monotonically decreased, and gradation display is performed by adjusting the length of the period. Further, the information signal waveform includes a period in which the applied potential is continuously changed. Further, the liquid crystal filled between the scan electrode group and the information electrode group is a ferroelectric liquid crystal.

[0014]

According to the present invention, since the control of the pulse amplitude is performed by controlling the pulse width, the variation of the pulse amplitude between the output circuits is suppressed, the accuracy of gradation display is improved, and the number of display gradations is increased. be able to.

A specific example of the waveform output method of the present invention will be described with reference to the output circuit diagram of FIG. First, a sawtooth wave and a constant potential from external signal sources SS1 and SS2 are supplied. Then, transistors TR1 and TR2 are provided in the output circuit 401 to switch between the signal sources SS1 and SS2 and the output terminal Out to form an output waveform. Here, the gradation signal defines the timing of switching through the timing circuit 402.

FIG. 7 shows a timing chart of each signal in the output circuit 401. Here, the timing signals T1, T2
Is a signal sent from the timing circuit 402 to the transistor. The transistor TR1 is turned on when the timing signal T1 is "H", and turned off when the timing signal T1 is "L". Similarly, when the timing signal T2 is "H", the transistor TR2
Turns on, and turns off when "L".

Returning to FIG. 6, for example, the amplitude V of a sawtooth wave
_When a potential of 50% is applied to _X , the transistor TR1 is turned off at a timing of ΔT / 2 corresponding to a half period of the sawtooth cycle width ΔT, and the transistor TR2 is turned off.
Turn on. Then, the output waveform applied from the output terminal Out partially has a sawtooth wave, and the maximum potential in the ΔT period is 1/2 V _X. Similarly, when a 70% potential is applied, the sawtooth wave SS1 is switched to a constant voltage SS2 at a timing of 7/10, and the maximum potential within the ΔT period becomes 7V _X / 10. That is, by adjusting the timing of switching from the sawtooth wave signal source SS1 to the constant voltage signal source SS2, the maximum potential within the ΔT period can be controlled with each output waveform.

With this circuit configuration, the switching timing varies in the order of several nSec in each output circuit, and since the amplification factor of the current is about 10 times, the pulse amplitude can be suppressed to several mV. Further, even when the output timing of the pulse between the output circuits has a variation of 100 to 200 nSec, the precision of gradation display can be improved by adjusting the ratio of the pulse width to the voltage fluctuation (for example, by reducing). When the frequency width ΔT of the sawtooth wave corresponding to the writing period is set to 20 μSec, gradation display of about 1,000 colors is possible.

In the above description, a sawtooth wave is used as a signal source that periodically fluctuates the potential, but this may be, for example, another ramp waveform (ramp: a waveform that simply increases or decreases), a sine wave, or a plurality of continuous waves. For example, the peak value of the rectangular pulse may be continuously changed.

MOSFET as a switching element
However, another switching element such as a bipolar transistor may be used.

As described above, since the gradation display method of the present invention can control the electric potential with high accuracy, for example, an ITO film is sputtered to a thickness of about 150 nm on a glass substrate having a thickness of 11 mm, and then Hitachi Chemical Co., Ltd. Made polyimide alignment film LQ-
1802 of which about 20 nm was applied and baked, was rubbed with a nylon cloth, a beads spacer having a diameter of 1.1 μm was dispersed, and the periphery of the substrate was sealed with an epoxy-based sealant and cured, Like a gray scale display device that mainly controls the number of inversion domains generated in a pixel when a liquid crystal cell in which a pyrimidine-based liquid crystal material having the properties shown in Table 1 is driven in a low voltage region of about 2 V, It is particularly effective for the liquid crystal element in which the pulse amplitude is dominantly affected, but it is almost the same as the pulse amplitude like the transmittance in the pixel when the liquid crystal cell is driven in the high voltage region of about 20V. It can also be used for liquid crystal devices affected by the pulse width.

[0022]

[Table 1]

[0023]

1 shows the structure of a liquid crystal display device according to an embodiment of the present invention. This liquid crystal display device has a scanning electrode 201.
And a liquid crystal display unit 101 having a matrix electrode composed of information electrodes 202, a scanning signal output circuit group 102 for applying a scanning signal to the liquid crystal via the scanning electrode 201, and an information signal for applying the liquid crystal to the liquid crystal via the information electrode 202. An information signal output circuit group 103, a scanning signal control circuit 104, an information signal control circuit 106, and a drive control circuit 105 are provided. Ferroelectric liquid crystal is arranged between the scanning electrode 201 and the information electrode 202. Reference numeral 107 is a graphic controller, and the data transmitted from this is the drive control circuit 10.
5 is input to the scanning signal control circuit 104 and the information signal control circuit 106, and is converted into address data and gradation display data, respectively. Then, the scanning signal output circuit group 102 generates a scanning signal according to this address data and applies it to the scanning electrode 201 of the liquid crystal display unit 101. Further, the information signal output circuit group 103 generates an information signal according to the gradation display data and applies it to the information electrode 202 of the liquid crystal display unit 101.

FIG. 2 is a partial sectional view of the liquid crystal display section 101. In the figure, 203 is a polarizer serving as an analyzer, and 204 is a polarizer serving as a polarizer, which are arranged in crossed Nicols. 205 and 206
Is a glass substrate, 207 and 208 are insulating films, and 209 and 21.
Reference numeral 0 is an alignment film, 211 is a ferroelectric liquid crystal, and 212 is a seal member.

FIG. 3 shows drive waveforms in the liquid crystal display device of FIG. In the figure, (a) shows the scanning signal output circuit group 10
2 shows a scanning selection signal waveform output by one scanning signal output circuit, and (b) shows an information signal waveform output by one information signal output circuit of the information signal output circuit group 103 according to a gradation signal. This information signal waveform is a partial ramp waveform (ram
p). FIG. 3C is a composite signal waveform of the scan selection signal and the waveform information signal waveform. This combined signal waveform is applied between one scan electrode and one information electrode, that is, one pixel formed at the intersection of the scan electrode and the information electrode in the liquid crystal display unit 101. Figure 3
(D), (e), and (f) are gradation levels of 0%,
It is an information signal waveform corresponding to 50% and 100%.

A method of forming the information signal waveform will be described with reference to FIG. 4 which is a partial schematic enlarged view of the information signal output circuit group 103 and FIG. 5 which is a timing chart of each signal in the output circuit.

As shown in FIG. 4, one output circuit 401
Are connected to two signal sources SS1 and SS2, and the transistors TR1 and TR2 are controlled by the timing signal generated by the timing circuit 402 according to the gradation signal, and the output terminal Ou
The information signal waveform is output from t. The output terminal Out of each output circuit 401 corresponds to the corresponding information electrode 202 (FIG. 1).
The information signal waveform output from each output circuit 401 is applied to each information electrode.

As shown in FIG. 5, the output of the signal source SS1 is a ramp waveform having a period width of 1H and an amplitude of 2Va, and the output of the signal source SS2 is a constant potential of 0V. Timing signals T1 and T2 are signals that control switching of the transistors TR1 and TR2, and are output from the timing circuit 402 in FIG. The transistor TR1 has a timing signal T
When 1 is "H", it is turned on, and when it is "L", it is turned off. Similarly, the transistor TR2 is turned on when the timing signal 2 is "H", and turned off when the timing signal 2 is "L".

1H shown in FIGS. 3 and 5 is a signal source SS1.
Is a period width that is a unit of the information signal, and ΔT is a period for applying a pulse for writing gradation in a half period of 1H.

FIGS. 8, 9 and 10 show examples of different drive signal waveforms in the device of FIG.

In FIG. 8, switching is performed between a sine wave, a cosine wave, and a constant voltage, and since the high potential portion is gentle, the potential can be accurately controlled in this portion.

FIG. 9 shows a waveform to which the auxiliary pulse ± Vb is added, which has the effect of preventing crosstalk and flicker and improving the image quality.

In FIG. 10, writing is performed in two of ΔT1 and ΔT2.
This is a waveform when divided into two periods, and has an effect of facilitating control of the number of inversion domains generated in a pixel.

FIG. 11 shows an example of an output circuit that forms the drive waveforms of FIGS. 9 and 10.

The ferroelectric liquid crystal used in the above-mentioned examples contains a pyrimidine component and has the characteristics shown in the following table.

[0036]

[Table 2] In the drive waveform used in this example, 1H is 40 to 2
00 μSec, ΔT, ΔT1, ΔT2 is 20 to 50 μS
ec, V1 and V2 are 5 to 20V, and Va, Vb and V
3, V4 was set in the range of 2 to 7V.

[0037]

As described above, in the present invention, the maximum potential in the applied waveform is controlled by providing the signal waveform with a period for continuously changing the applied potential and adjusting the length of the period. As a result, it was possible to perform higher control than the conventional potential control method.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of a liquid crystal display device according to an embodiment of the present invention.

2 is a cross-sectional view of the liquid crystal display unit in FIG.

3 is a drive waveform diagram in the apparatus of FIG.

FIG. 4 is a partially enlarged view of the output circuit group in FIG.

FIG. 5 is a timing chart in the output circuit.

FIG. 6 is a circuit diagram of an output circuit for explaining the present invention.

FIG. 7 is a timing chart for explaining the present invention.

8 to 10 are waveform charts showing other examples of drive waveforms in the apparatus of FIG. 1, respectively.

11 is a circuit diagram showing an example of an output circuit for forming the drive waveform of FIG. 9 or FIG.

FIG. 12 is a cross-sectional view and a schematic plan view of a conventional liquid crystal cell.

13 and 14 are explanatory diagrams of conventional gradation display.

FIG. 15 is a circuit diagram of a conventional output circuit.

[Explanation of symbols]

101: liquid crystal display unit, 102: scanning signal output circuit group, 1
03: Information signal output circuit group, 104: Scan signal control circuit, 105: Drive control circuit, 106: Information signal control circuit, 107: Graphic controller, 201: Scan electrode, 202: Information electrode, 203: Analyzer, 20
4: Polarizer, 205, 206: Glass substrate, 20
7, 208: insulating film, 209, 210: alignment film, 21
1: Ferroelectric liquid crystal, 212: Seal member, 401: Output circuit, 402: Timing circuit, Out: Output terminal, SS
1, SS2: signal source, TR1, TR2: transistor.

Claims (5)

[Claims] 1. A matrix electrode composed of a scanning electrode group and an information electrode group facing and intersecting with the scanning electrode group, and a liquid crystal which is arranged between the electrode groups and driven by an electric field applied through the electrode groups. And a driving means for driving the liquid crystal by a signal waveform including a period in which the potential applied to the electrode group continuously changes and the length of the period being adjusted according to gradation information. Liquid crystal display device. 2. The applied potential monotonically increases or monotonically decreases during the period in which the applied potential continuously changes.
The described liquid crystal display device. 3. The liquid crystal display device according to claim 1, wherein the applied potential having the continuously changing period is included in an information signal waveform applied to the information electrode. 4. The liquid crystal is a ferroelectric liquid crystal.
The described liquid crystal display device. 5. The driving means selects a plurality of signal sources including a signal source that periodically changes the potential in units of horizontal scanning periods and a constant voltage source that supplies a constant voltage, and each of these signal sources. 2. The liquid crystal display device according to claim 1, further comprising a switching unit that is electrically connected to the output terminal, and a unit that adjusts the timing of signal selection in the switching unit according to the gradation signal.