JP2766947B2 - Display device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a display device for performing gradation display using a bistable display element.

[0002]

2. Description of the Related Art Hitherto, as this type of bistable display device, there has been known a liquid crystal display device which performs a gradation display using a ferroelectric liquid crystal (FLC).

A display element used in such an apparatus is disclosed in Japanese Patent Application Laid-Open No. 61-94023 or the like, in which a transparent electrode is formed on two inner surfaces while maintaining a cell gap of about 1 to 3 microns, and alignment treatment is performed. 1 shows a liquid crystal cell in which ferroelectric liquid crystals are injected into a liquid crystal cell configured by facing glass substrates subjected to the above method.

The characteristics of the above-mentioned display device using a ferroelectric liquid crystal are that the ferroelectric liquid crystal has a spontaneous polarization, so that the coupling force between an external electric field and the spontaneous polarization can be used for switching and the length of the ferroelectric liquid crystal molecule. Since the axial direction has a one-to-one correspondence with the polarization direction of spontaneous polarization, switching can be performed depending on the polarity of an external electric field.

The ferroelectric liquid crystal generally uses a chiral smectic liquid crystal (SmC *, SmH *), and thus exhibits a twisted orientation of the liquid crystal molecule long axis in a bulk state. Can eliminate the twist of the long axis of the liquid crystal molecules (P213-P234, NA CLarke).
tal, MCLC; 1983, Vol94. ).

An actual ferroelectric liquid crystal cell uses a simple matrix substrate as shown in FIG.

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

FIG. 8 is a diagram schematically showing the relationship between the switching pulse amplitude and the transmittance of a ferroelectric liquid crystal element. One cell of a cell (element) initially in a completely light-blocking (black) state has one polarity of one polarity. 5 is a graph in which the amount of transmitted light I after application of a pulse is plotted as a coefficient of the amplitude V of a single pulse. When the pulse amplitude is equal to or smaller than the threshold value V _th (V <V _th ), the amount of transmitted light does not change, and the transmitted state after the pulse application shows a state before the application as shown in FIG. And does not change. When the pulse amplitude exceeds the threshold value (V _th <V <V _sat ), a part of the pixel transits to the other stable state, that is, the light transmission state shown in FIG. When the pulse amplitude further increases and becomes equal to or higher than the saturation value V _sat (V _sat <V), as shown in FIG. 9D, all the pixels are in a light transmitting state, and the light amount reaches a constant value.

That is, in the area modulation method, the halftone is displayed by controlling the voltage so that the pulse amplitude V satisfies _Vth <V < _Vsat .

Using this area modulation method, a driving method (hereinafter, referred to as a prior application example) described in Japanese Patent Application No. 3-73127 filed by the present applicant has been proposed. Stable analog gradation display was possible even in a display device in which variations in threshold characteristics due to temperature unevenness, cell thickness unevenness, and the like were present.

[0011]

However, in the above prior art, four write pulses and one auxiliary pulse for each pulse are required for one pixel in order to compensate for variations in threshold characteristics in the display section. Monochrome 2
There is a disadvantage that the writing time is about 10 times longer than the writing time of the value display.

The present invention has been made in view of the above-described problems in the conventional example, and compensates for a change in threshold value caused by temperature unevenness or cell thickness unevenness in a display unit, and displays gray scales quickly. It is an object of the present invention to provide a display device that can be obtained.

[0013]

In order to achieve the above-mentioned object, according to the present invention, one pixel is constituted by two sub-pixels having the same threshold characteristic and being bistable, and the first sub-pixel is composed of the first sub-pixel. After completely setting the first stable state with the writing pulse, writing the second stable state with the second writing pulse, and setting the second sub-pixel completely after setting the second stable state completely with the first writing pulse. 2
Drive means for writing the first stable state with the write pulse of (1).

[0014]

According to the present invention, with the above-described configuration, it is possible to compensate for variations in the threshold characteristics and to realize quick gradation display. A compensation method for a threshold change according to the present invention will be described with reference to FIG.

A pixel P ₁ composed of two sub-pixels A and B
And a pixel P ₂ composed of two sub-pixels A ′ and B ′ (FIG. 1
Regarding (iii)), it is assumed that the thresholds are different as shown in FIG.

[0016] In FIG. 1 (i), a threshold value characteristic for the white write pulses of the pixel P _1, b is the threshold characteristics with respect to black writing pulse for the pixel P _1, a 'is the threshold characteristics for white write pulses of the pixel P _2, b 'is the threshold characteristic for the black writing pulse for the pixel P _2. V _th is a threshold characteristic a,
The threshold voltage b and V _sat are the saturation voltages of the threshold characteristics a and b. The transmittance was 0% when the subpixel was completely black, and 100% when the subpixel was completely white.

Here, the waveform S _A of FIG. 1 (ii) is applied to the sub-pixels A and A ′, and the waveform S _B is applied to the sub-pixels B and B ′.

The waveform S _A is composed of a pulse A ₁ and a pulse A ₂ . Subpixel A becomes fully black state of the once transmittance of 0% by black writing pulse A _1, holding the transmittance alpha% Subsequent white write pulse A _2.

The waveform S _B is constituted by a pulse B ₁ and the pulse B _2. Subpixel B becomes completely white state of once 100% transmittance by white write pulses B _1, to hold the transmittance alpha% Subsequent black writing pulse B _2. Therefore, the pixel P ₁ displays a gradation of α% as shown in FIG.

Meanwhile, the sub-pixel A 'is next once transmittance of 0% by black writing pulse A _1, holding the transmittance alpha + beta%, depending subsequent white write pulse A _2.

The sub-pixel B 'is once transmittance by white write pulses B ₁ is to hold the transmittance alpha-beta% would Subsequent black writing pulse B ₂ is 100%. Therefore, the pixel P ₂ also displays a gradation of α% as shown in FIG.

Incidentally, in the triangle xyz and the triangle x'y'z 'in FIG. 1 (i), xz = x'z' and {xzy = ∠
x′z′y ′, ∠yzz = ∠y′x′z ′, and 二 xyz≡ △ x′y′z ′ because of the diagonal sides, so xy
= X'y '= β.

As described above, this compensation method is effective under the following conditions. The threshold characteristic of each pixel can be approximated by a straight line. Even if the threshold value changes, or if there is a variation in the display unit, the slope of the threshold characteristic does not change and coincides when it is translated on the coordinates. The threshold characteristics for the second stable state are the same. The maximum variation width β% of the transmittance α% of the gradation to be displayed and the variation of the threshold characteristics in the display unit is α + β.
Satisfy two formulas of ≦ 100 and α−β ≦ 0.

It has been confirmed in the aforementioned Japanese Patent Application No. 3-73127 that the ferroelectric liquid crystal element satisfies the following conditions.

If the threshold value is completely linear under the condition
a _{g V A2 + log V B2 =} log V th + logV sat.

In other words, the condition of (1) is that a display device having a transmittance variation of β% can uniformly display gradations of β to 100-β%. For example, 10%
In a display device having a variation of 10 to 90%, an analog gradation of 10 to 90% can be displayed. In addition, digital gradation every 10% is also possible. In the case of digital gradation display, the threshold characteristic does not need to be a straight line, but may be a step-like one as shown in FIG.

In FIG. 1, the gradation is displayed by changing the voltage, but the same effect can be obtained by adjusting the pulse width.

[0028]

FIG. 2 shows a liquid crystal display according to an embodiment of the present invention. This liquid crystal display device has a liquid crystal display section 101 having a matrix electrode composed of a scanning electrode 201 and an information electrode 202 shown in detail in FIG.
02, an information signal application circuit 103 applied to the liquid crystal via
A scanning signal applying circuit 102 for applying a scanning signal to liquid crystal via a scanning electrode 201; a scanning signal control circuit 104; an information signal control circuit 106; a drive control circuit 105;
And a temperature detection circuit 109 for detecting the temperature of the display unit 101 based on the output of the thermistor 108. A ferroelectric liquid crystal is disposed between the scanning electrode 201 and the information electrode 202. Reference numeral 107 denotes a graphic controller. Data sent from the graphic controller 107 is input to the scanning signal control circuit 104 and the information signal control circuit 106 through the drive control circuit 105, and is converted into address data and display data, respectively. In addition, the temperature of the liquid crystal display unit 101 is input to the temperature detection circuit 109 via the thermistor 108, and is input to the scanning signal control circuit 104 via the drive control circuit 105 as temperature data. Then, the scanning signal applying circuit 102 generates a scanning signal according to the address data and the temperature data, and applies the scanning signal to the scanning electrode 201 of the liquid crystal display unit 101. Further, the information signal application circuit 103 generates an information signal according to the display data, and applies the information signal to the information electrode 202 of the liquid crystal display unit 101.

In FIG. 3, reference numerals 203 and 204 denote sub-pixels formed by intersections between the scanning electrodes 201 and the information electrodes 202. Reference numeral 205 denotes a pixel which includes two sub-pixels 203 and 204 and is a display unit.

FIG. 4 is a partial sectional view of the liquid crystal display unit 101. In the figure, 301 is an analyzer, 306 is a polarizer, and these are arranged in crossed Nicols. 302 and 305 are glass substrates, 303 is a ferroelectric liquid crystal, 304 is a UV curing resin, and 307 is a spacer.

FIG. 5 shows the waveform of the drive signal in the apparatus of FIG. In the figure, a is a selection signal waveform output from the scanning signal application circuit 102 and applied to the first sub-pixel, and b is output in synchronization with the waveform a and applied to the second sub-pixel by the scanning signal application circuit 102. The selection signal waveform, c, is an information signal waveform having an amplitude corresponding to the gradation data output from the information signal application circuit 103. As is apparent from FIG. 3C, the time 1H required for displaying one pixel is four times the second write pulse width, that is, 4Δt.
I just need.

In this embodiment, the gray scale is displayed by a pulse having a fixed pulse width and a variable amplitude, but the same result can be obtained with a pulse having a variable pulse width and a fixed amplitude.

In this embodiment, an example is given in which a cell thickness gradient is provided in order to make the threshold characteristics in a pixel gentle. However, a capacitance gradient or a potential gradient is imparted to an electrode. Other methods may be used.

[0034]

FIG. 6 shows an embodiment in which the present invention is implemented by changing the electrode configuration. In the example shown in FIG. 3, two sub-pixels 203
And 204 are composed of different scanning electrodes 201, 201 and a common information electrode 202. However, in FIG. 6, the two sub-pixels are not common to the scanning electrode 601 and the information electrode 602. FIG. 7 shows a driving waveform at this time. In the figure, a is a scan selection waveform applied to the first sub-pixel, 6 is a scan selection waveform applied to the second sub-pixel, c is an information signal waveform applied to the first sub-pixel, and d is 7 is an information signal waveform applied to a second sub-pixel. As is clear from FIGS. 3C and 3D, the time 1H required for displaying one pixel may be twice the second write pulse width, that is, 2Δt. The time of 1H is the same as that of the conventional monochrome binary display, and is half of the example shown in FIG.

[0035]

As described above, according to the present invention,
One pixel is composed of two sub-pixels having the same threshold characteristic and is bistable, and the first sub-pixel is completely brought into the first stable state by the first writing pulse, and then becomes the second by the second writing pulse. And the second sub-pixel is provided with driving means for writing the first stable state with the second write pulse after completely setting the second sub-pixel to the second stable state with the first write pulse.
It is possible to compensate for a change in the threshold value due to temperature unevenness or cell thickness unevenness in the display unit, and to realize quick gradation display.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a driving method according to the present invention.

FIG. 2 is a diagram illustrating a configuration of a display device according to an embodiment of the present invention.

FIG. 3 is an enlarged plan view of a liquid crystal display unit in FIG. 2;

FIG. 4 is a cross-sectional view of the liquid crystal display unit.

FIG. 5 is a signal diagram of a driving waveform in the device of FIG.

FIG. 6 is an enlarged plan view of a liquid crystal display unit according to another embodiment of the present invention.

FIG. 7 is a signal diagram of a driving waveform according to the other embodiment.

FIG. 8 is a schematic diagram showing a relationship between a switching pulse amplitude and a transmittance of a ferroelectric liquid crystal.

FIG. 9 is a schematic diagram showing a light transmission state of a ferroelectric liquid crystal element according to an applied pulse.

FIG. 10 is another schematic diagram showing a light transmission state of a bistable element according to an applied pulse.

FIG. 11 is an explanatory diagram of a structure of a conventional liquid crystal element.

[Explanation of symbols]

101: liquid crystal display unit, 102: scanning signal application circuit, 10
3: information signal applying circuit, 104: scanning signal control circuit, 1
05: drive control circuit, 106: information signal control circuit, 10
7: graphic controller, 201, 601: scanning electrode, 202, 602: information electrode, 203, 204, 6
03,604: sub-pixel, 205,605: pixel, 30
1: analyzer, 302, 305: glass substrate, 30
3: ferroelectric liquid crystal, 304: protective film, 306: polarizer, 307: spacer.

Claims (13)

(57) [Claims] 1. A display section comprising a large number of bistable, first and second sub-pixels each having the same threshold characteristic and arranged in a matrix, wherein the first sub-pixel is driven when each pixel is driven. A complete first stable state is written to a pixel with a first write pulse, and then a second stable state according to the display gradation of the pixel is written with a second write pulse, and a second sub-pixel is written to a second sub-pixel. Driving means for writing a complete second stable state with a first write pulse and then writing a first stable state according to the display gradation of the pixel with a second write pulse. Display device. 2. The display device according to claim 1, wherein the first sub-pixel and the second sub-pixel have the same area. 3. The display device according to claim 1, wherein the first sub-pixel and the second sub-pixel are located at positions adjacent to each other. 4. The display according to claim 1, wherein said driving means generates a first write pulse applied to said first sub-pixel and a first write pulse applied to a second sub-pixel in synchronization with each other. apparatus. 5. The driving unit according to claim 1, wherein, in a specific pixel in the display unit, a second stable state is set in a region of x% (0 ≦ x ≦ 100) by a second writing pulse in a first sub-pixel. Write, 100-
2. The display device according to claim 1, wherein the first stable state is written in an area of x%. 6. The pixel according to claim 5, wherein the specific pixel has a threshold characteristic which is the center of the variation in the threshold characteristic in the display unit.
Display device. 7. In the voltage / transmittance characteristics for each pixel in the display unit, the voltage at which the change in transmittance ends for each characteristic is the highest voltage V _max , and the voltage at which the change in transmittance starts. And the voltage at the center where V _th is the voltage at which the change in transmittance ends is V _th .
_sat , the amplitude of the first write pulse applied to the first sub-pixel is A ₁ , the amplitude of the second write pulse is A ₂ , and the amplitude of the first write pulse applied to the second sub-pixel is B ₁ , Second
When the amplitude of the write pulse and _{B 2, A 1 ≧ V max} B 1 ≧ V max V th ≦ A 2 ≦ V sat A 2 + B 2 = V th + V sat at a display device according to claim 1. 8. In the voltage / transmittance characteristics for each pixel in the display unit, the voltage at which the change in transmittance ends for each characteristic is the highest voltage V _max , and the voltage at which the change in transmittance starts. And the voltage at the center where V _th is the voltage at which the change in transmittance ends is V _th .
_sat , the amplitude of the first write pulse applied to the first sub-pixel is A ₁ , the amplitude of the second write pulse is A ₂ , and the amplitude of the first write pulse applied to the second sub-pixel is B ₁ , Second
When the amplitude of the write pulse and _{B 2, A 1 ≧ V max} B 1 ≧ V max V th ≦ A 2 ≦ V sat log A 2 + log B 2 = log V th + log V sat at a display device according to claim 1 . 9. A pulse width for each pixel in the display unit /
In the transmittance characteristics, the longest pulse width among the pulse widths at which the change in transmittance ends for each characteristic is t _max ,
The pulse width at which the change in transmittance starts is the pulse width at the center of the pulse width t _th , and the pulse width at the end of the change in the transmittance is t _sat , and the first pulse width is applied to the first sub-pixel. The pulse width of the write pulse of A is A ₁ , the pulse width of the second write pulse is A ₂ , and the first pulse is applied to the second sub-pixel.
Assuming that the pulse width of the write pulse is B ₁ and the pulse width of the second write pulse is B ₂ , A ₁ ≧ t _max B ₁ ≧ t _max t _th ≦ A ₂ ≦ t _sat A ₂ + B ₂ = t _th + t The display device according to claim 1, wherein the display device is _sat . 10. In the pulse width / transmittance characteristic for each pixel in the display unit, the longest pulse width among the pulse widths at which the change of the transmittance ends for each characteristic is t.
_max is the pulse width at which the change in transmittance starts, and the center pulse width is t _th , and the pulse width at which the change in transmittance ends is t _sat , and the pulse width at the center is t _sat , and is applied to the first sub-pixel. The pulse width of the first write pulse is A ₁ , the pulse width of the second write pulse is A ₂ , the pulse width of the first write pulse applied to the second sub-pixel is B ₁ , and the pulse width of the second write pulse is A 1. _2. The display device according to claim 1, wherein, if a pulse width is B ₂ , A ₁ ≧ t _max B ₁ ≧ t _max t _th ≦ A ₂ ≦ t _sat log A ₂ + log B ₂ = log t _th + log t _sat . 11. The display device according to claim 1, wherein said two sub-pixels are constituted by different scanning electrodes and a common information electrode. 12. The display device according to claim 11, wherein a liquid crystal is filled between said scanning electrode and said information electrode. 13. The display device according to claim 12, wherein the liquid crystal is a ferroelectric liquid crystal.