JPS63271432A - Ferroelectric liquid crystal electro-optical device - Google Patents

Abstract

PURPOSE:To obtain high contrast by switching rewriting crest values in a first frame and a second frame by using a transistor. CONSTITUTION:When a HIGH voltage is inputted to a gate of a transistor TR 17, the TR 17 conducts and a power supply voltage VLC of a negative side is outputted to an output V5. When a LOW voltage is inputted to the gate, the TR 17 becomes nonconductive, and when a resistance value of a resistance 16 is denoted as R16, a voltage of N.R.VLC/(N.R+R16) is outputted to the output V5. N denotes a bias value, R denotes a resistance value of resistances 11, 12, 14 and 15, and at the time of selecting the bias value to N, it can be executed by selecting the resistance value of the resistance 13 to (N-4)R. In such a way, the high contrast can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electro-optical conversion device that drives a ferroelectric liquid crystal by mutually switching between bistable states, and particularly aims at improving the drive circuit.

[Summary of the invention]

The present invention provides a ferroelectric liquid crystal electro-optical device in which the bistable state of a ferroelectric liquid crystal is switched and driven by a pulse having a peak value higher than a dark value voltage. Focusing on the fact that the pulse height value of the pulse that switches from the second stable state to the first stable state is different, the pulse height value applied to the liquid crystal can be changed between the first frame and the second frame to increase the intensity. It was possible to increase the contrast of a dielectric liquid crystal electro-optical device.

[Conventional technology]

Conventionally, ferroelectric liquid crystal electro-optical devices have a driving method in which the bistable state of ferroelectric liquid crystal is switched and driven with a pulse having a peak value higher than the dark value voltage, and the stable state after switching is maintained with an alternating current pulse. known 0 e.g. SID'8
5 lnt' ISymposium16゜131
(1985).

First, FIG. 2 shows the structure of a conventional ferroelectric liquid crystal cell (hereinafter referred to as liquid crystal cell). 1-1 is a pair of substrates arranged opposite to each other. Reference numeral 3 denotes a ferroelectric liquid crystal, such as a chiral smectic C liquid crystal (hereinafter referred to as "SmC") thin film, sandwiched between the substrates 1-1.

2-2 is a uniaxial and random horizontally aligned film existing at the interface between the substrate 1-1 and the SmC thin film 3, which realizes a bistable state of liquid crystal molecules. ) are oriented horizontally with respect to the substrate 1 and form a layer. When observed from above, the liquid crystal molecules are divided into two domains. In the first domain, the molecular axis is 〇It is tilted. This is the first stable state 5.

Spontaneous polarization 7 of the liquid crystal molecules points upward. In the second domain, the molecular axis is tilted -0 with respect to the layer law 114. This is the second stable state 6.

At this time, spontaneous polarization 7 is pointing downward. Utilizing the fact that the directions of the spontaneous polarizations 7 are opposite to each other in both stable states, one of the bistable states is selected using positive and negative DC pulses. Reference numeral 8-8 denotes a pair of polarizing plates facing each other with their polarization axes perpendicular to each other, and optically distinguishes the first stable state and the second stable state by birefringence.
The first stable state is converted into a light blocking state (hereinafter referred to as black), and the second stable state is converted into a light passing state (hereinafter referred to as white). 9 and 10 are matrix electrodes 1 for applying a driving voltage to the SmC thin film 3. As shown in FIG. 3, 9 is a scanning electrode (hereinafter referred to as common), and 10 is a signal electrode (hereinafter referred to as segment).

+a) in FIG. 4 shows a driving waveform applied to one matrix pixel (hereinafter referred to as a dot) in line sequential driving using the AC bias averaging method. In the first frame, positive and negative (common 9 standard) pulses P and pg having peak values equal to or higher than the threshold are continuously applied during the selection period. The liquid crystal molecules are aligned in a second stable state by the positive pulse P1, and switched by the subsequent negative pulse Pt to be aligned in the first stable state. This state continues during the half selection period due to the application of AC pulses. This is because the peak value of the AC pulse is below the threshold value. Therefore, the first stable state black is written in the first frame. Subsequently, in the second frame, since the polarity of the pulse is reversed, white is written. However, in this figure, since P4 and P are less than the threshold values, white is not written, and black written in the first frame is saved. This period of P and P is called a non-selection period, and the results of measuring the change in transmitted light intensity at this time with a photomultiplier are shown in Fig. 4).

By the way, the selection period P1 and PR2 half selection period period s
The relationship between the pulse height values of the pulses in the non-selection periods P4 and P is as follows, where V is the absolute value of the pulse height values of the pulses in Pl and P2, l Ps l =V/N.

l P4 l = l Ps l =V ・(N-
2) 7N is selected. Here, N is called a bias value. Further, the peak values of the pulses P, ~P, mentioned above are V, which are generated by a bias generation circuit having resistors connected in series as shown in FIG. ,V,,V,,V,.

V4. It is created by a combination of six voltage values of Vs.

Incidentally, the bias value affects the contrast, and the situation is shown in FIG. 6. How to select this value is as per Japan Display'86 Proceed
Reported to ingPP. According to the results, in a ferroelectric liquid crystal that can obtain the pulse height value and transmitted light intensity as shown in FIG. 7, the bias value N is selected to be equal to 2T/(γ-1). Here, T is Vsat/Vth.

[Problem that the invention seeks to solve]

By the way, although FIG. 7 shows the change from black to white, in reality, the change from white to black must also be considered, and this situation is shown in FIG. 8. In the figure, a indicates a change from black to white, and b indicates a change from white to black. Depending on the liquid crystal material and alignment method, there is a difference in the peak value between the change from black to white and the change from white to black. In order to drive such a liquid crystal using the above driving method, r is Vsat, /
The value of Vthz must be used; in this case, the value of Vsat+/Vth* is naturally V sa L 1 / V
thicker than the value of th 1 (as a result, 2T/
The bias value N given by the value of (γ-1) becomes small.

As can be seen from FIG. 6, this causes a problem of poor contrast.

Means for Solving Problem c] In order to provide a ferroelectric electro-optical device with high display quality, the present invention provides a transistor in which drive voltages in the first frame and the second frame are applied to a bias generation circuit. Turn on,
This can be changed by turning it off.

Now suppose that we write from black to white in the first frame and from white to black in the second frame. Therefore, in Figure 8, the first
In the first frame, it is sufficient to pay attention to the characteristic of a, and in the second frame to the characteristic of b.In the first frame, the Vsat voltage is Vs
at++ Vth voltage is vth.

In the second frame, the Vsat voltage is Vsat4° and the vth voltage is vth. Therefore, the peak value of the rewriting from black to white in the first frame is Vsatl'';t
t voltage and the peak value of rewriting from white to black in the second frame is selected as Vsatl voltage, the threshold ratio in each frame is Vsat+/V thl , Vsat! /V
tJ, which is smaller than the aforementioned VsaLt/Vthz, so the bias value can be selected to be larger accordingly. Also, in reality, Vsat+/V th+ and V
Since the saLz/V thz ratios are not equal, in this case the larger ratio is selected and substituted into 2T/(γ-1) to determine the bias value N.

Note that a specific solution for changing the rewriting peak value between the first frame and the second frame will be explained in the embodiment section.

〔Example〕

FIG. 1 shows a specific circuit of the present invention.Resistors 11 to 16 are connected in series, and both ends thereof are connected to a power source. Resistors 11, 12, 14, and 15 have the same resistance value, and let R be that value. Now, in order to select the bias value to be N, the resistance value of the resistor 13 should be selected to be (N-4)H.

The source and drain of a transistor 17 are connected to both ends of the resistor 16, and a control signal is input to the gate. Now, when a HIGH voltage is input to the gate of the transistor 17, the transistor 17 becomes conductive because it is a transistor (nch), and the negative side power supply voltage VLC is applied to the output
is output. On the other hand, when a LOW voltage is input to the gate, the transistor 17 becomes non-conductive, and the output V is N
-R-VLC/ (NR+R16) voltage is output.

Here, R16 is the resistance value of the resistor 16. Obviously VL
The relationship c>N-R-VLC/(N-R+R16) holds true.

Therefore, VLC is set to the above-mentioned Vsat+, N-R・V
The values of VLC and R16 and R are appropriately selected so that LC/(N-R+R16) is equal to the voltage of Vsat2 mentioned above. In such a circuit, in the first frame, a HIGH voltage is applied to the gate of the transistor 17,
In the second frame, a LOW voltage is applied.

〔Effect of the invention〕

According to the present invention, by switching the rewriting peak values in the first frame and the second frame using a transistor, a large bias value can be selected, so that a ferroelectric liquid crystal electro-optical device with high contrast can be obtained. It has the effect of being able to.

[Brief explanation of the drawing]

Fig. 1 is a bias generation circuit according to the present invention, Fig. 2 is a perspective view of a conventional liquid crystal cell, Fig. 3 is an electrode arrangement diagram of a conventional liquid crystal cell, Fig. 4 is a conventional drive waveform diagram, (bl is An optical response waveform diagram at that time, FIG. 5 is a conventional bias generation circuit diagram, FIG. 6 is a diagram showing the bias value dependence of contrast ratio, and FIG. 7 is a diagram showing the relationship between pulse peak value and transmitted light intensity. Fig. 8 is a diagram showing the relationship between pulse height value and transmitted light intensity for explaining the present invention. 1-1...Substrate 2-2...Alignment film 3...Chiral smectic film a crystal 8-8...
・Polarizing plate 9...Scanning electrode 10...Signal electrodes 11 to 16...Resistor 17...Transistor and above Applicant Seiko Electronics Co., Ltd. Haze arrangement of conventional liquid crystal cell Figure 3 Conventional 4JrtJ waveform diagram Figure 4 (a) Figure 4 (a)'s 9-character eaves and eaves diagram Figure 4 (b) Bias life and death circuit diagram Figure 5 l Bias 4-direction N Fig. 6 shows the pi lower ratio of the cosidoast ratio [4 degrees and a]

Claims (4)

[Claims] (1) A ferroelectric liquid crystal thin film, an alignment film that is in contact with the thin film and realizes bistable alignment of liquid crystal molecules, a member that converts the bistable alignment state into optical brightness, and a voltage for switching the bistable state. A first stable state is written in a ferroelectric liquid crystal electro-optical device consisting of a liquid crystal panel made up of electrodes arranged in a matrix that applies voltage to the thin film, and a drive circuit that writes bistable states line-sequentially by an AC bias averaging method. A ferroelectric liquid crystal electro-optical device characterized in that a pulse height value of a pulse for writing a second stable state is different from a pulse height value of a pulse for writing a second stable state. (2) The drive circuit includes a circuit that can make the peak value of the pulse for writing the first stable state different from the peak value of the pulse for writing the second stable state. A ferroelectric liquid crystal electro-optical device according to claim 1, characterized in that: (3) The ferroelectric liquid crystal electro-optical device according to claim 2, wherein the circuit includes a transistor connected in parallel to at least one resistor of the bias generating circuit. (4) Claim 3, characterized in that a HIGH or LOW voltage is applied to the gate input of the transistor in synchronization with the first frame and the second frame.
The ferroelectric liquid crystal electro-optical device described in .