JP2593657B2 - Ferroelectric liquid crystal electro-optical device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electro-optical conversion device that drives a ferroelectric liquid crystal by switching between bistable states, and particularly to an improvement in a driving circuit.

[Summary of the Invention]

The present invention relates to a ferroelectric liquid crystal electro-optical device that switches and drives a bistable state of a ferroelectric liquid crystal with a pulse having a peak value equal to or higher than a threshold voltage, wherein a pulse for switching from a first stable state to a second stable state is provided. Focusing on the fact that the peak value differs from the peak value of the pulse that switches from the second stable state to the first stable state, the peak value applied to the liquid crystal is switched between the first frame and the second frame. The contrast of the transparent liquid crystal electro-optical device could be increased.

[Conventional technology]

Conventionally, there is known a driving type ferroelectric liquid crystal electro-optical device in which a bistable state of a ferroelectric liquid crystal is switched by a pulse having a peak value equal to or higher than a threshold voltage, and a stable state after the switching is maintained by an AC pulse. Had been. For example, SID'85 | n
t '| Symposium 16, 131 (1985).

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

Reference numeral 2-2 denotes a uniaxial and random horizontal alignment film existing at the interface between the substrate 1-1 and the SmC ^* thin film 3, and realizes a bistable state of liquid crystal molecules. The major axis of the liquid crystal molecules (hereinafter referred to as molecular axis) is oriented horizontally with respect to the substrate 1 and forms a layer. When this is observed from above, the liquid crystal molecules are distinguished into two domains. In the first domain, the molecular axis is inclined by + θ with respect to the normal 4 of the layer. This is the first stable state 5.
The spontaneous polarization 7 of the liquid crystal molecules points upward. In the second domain, the molecular axis is inclined by -θ with respect to the normal 4 of the layer. This is the second stable state 6.

At this time, the spontaneous polarization 7 faces downward. By utilizing the fact that the directions of the spontaneous polarization 7 are opposite to each other in the both stable states, one of the bistable states is selected by positive and negative DC pulses. Reference numeral 8-8 denotes a pair of polarizing plates arranged so as to face each other with their polarization axes orthogonal to each other, and optically distinguishes between the first stable state and the second stable state by birefringence. For example, the first stable state is changed to a light blocking state (hereinafter, referred to as black),
The second stable state is converted to a light passing state (hereinafter, referred to as white). Reference numerals 9 and 10 denote matrix electrodes for applying a drive voltage to the SmC ^* thin film 3, as shown in FIG.

FIG. 4A shows a drive 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, a positive or negative signal having a peak value equal to or larger than the threshold value during the selection period (common 9)
Pulses P ₁ and P ₂ of the reference) are added sequentially. Liquid crystal molecules by the positive pulse P ₁ is aligned to the first stable state and switched by the negative pulse P ₂ to Tsuzuku aligned to the second stable state. This state is maintained by the application of the AC pulse during the half-selection period.
This is because the peak value of the AC pulse is equal to or less than the threshold. Therefore,
In the first frame, black in the first stable state is written.
Subsequently, in the second frame, white is written because the polarity of the pulse is reversed. However, in this view, since P ₄ and P ₅ is a threshold below white is not written, black written in the first frame is stored. The duration of this P ₄ and P ₅ is referred to as a non-selection period. FIG. 4 (b) shows the result of measuring the change in transmitted light intensity at this time using a photomultiplier.

By the way, the relationship between the pulse heights of the pulses in the selection periods P ₁ and P ₂ , the half-selection period P ₃ , and the non-selection periods P ₄ and P ₅ is such that the absolute value of the pulse heights of the pulses P ₁ and P ₂ is V. If | P ₃ | = V / N, | P ₄ | =
| P ₅ | = V · (N−2) / N. Here, N is called a bias value. The pulse wave height of the P ₁ to P ₅ are of _{V DD, V 1, V 2} , V 3, V 4, V 5 produced by the bias generation circuit connected to the resistance shown in FIG. 5 in series 6
It is made by a combination of two voltage values.

Incidentally, the bias value affects the contrast, and the state is shown in FIG. How to choose this value is reported in Japan Display '86 Proceeding PP. According to the result, in the ferroelectric liquid crystal having the pulse peak value and the transmitted light intensity as shown in FIG. 7, the bias value N is selected to be equal to 2γ / (γ−1). Where γ is Vsat / V
th.

[Problems to be solved by the invention]

FIG. 7 shows the change from black to white, but in practice, the change from white to black must also be considered. This is shown in FIG. In the figure, a is black to white, b
Indicates a change from white to black. Depending on the liquid crystal material and the alignment method, there is a difference between the peak values of the change a from black to white and the change b from white to black. Such liquid crystals to be driven by the drive method shall use the value of Vsat ₁ / Vth ₂ as gamma. In this case, naturally, Vsat ₁ / Vth ₂
Is greater than the value of Vsat ₁ / Vth ₁ , resulting in 2
The pierce value N given by the value of γ / (γ-1) becomes small. This causes a problem that the contrast deteriorates as can be seen from FIG.

[Means for solving the problem]

According to the present invention, in order to provide a ferroelectric electro-optical device having a high display quality, a transistor in which a driving voltage in a first frame and a second frame is added to a bias generation circuit is turned on and off.
It is made to change by making it FF.

Now, assume that writing is performed from black to white in the first frame and from white to black in the second frame. Therefore, in FIG. 8, the characteristics of a in the first frame and the characteristics of b in the second frame may be noted. In the first frame, the Vsat voltage is Vsat ₁ ,
Vth voltage is Vth _1, Vsat voltage in the second frame Vsa
The t ₂ and Vth voltages are Vth ₂ . Therefore, if the peak value of rewriting from black to white in the first frame is selected as Vsat ₁ voltage and the peak value of rewriting from white to black in the second frame is selected as Vsat ₂ voltage, the threshold ratio in each frame is Vsat 1 ₁ / Vth ₁ , Vsat ₂ / V
th _2, and the correspondingly to become smaller than Vsat ₁ / Vth ₂ described above, it is possible to choose a large bias value. Also,
Actually, the ratio between Vsat ₁ / Vth ₁ and Vsat ₂ / Vth ₂ is not equal, so in this case, the larger ratio of both is selected and 2γ / (γ
-1) to determine the bias value N.

A specific answer for changing the rewrite peak value between the first frame and the second frame will be described in the section of the embodiment.

〔Example〕

FIG. 1 shows a specific circuit of the present invention. The resistors 11 to 16 are connected in series, and both ends are connected to a power supply. Resistance 1
1, 12, 14, and 15 have the same resistance value, and the value is R. 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) R.

The source and the drain of the transistor 17 are connected to both ends of the resistor 16, and a control signal is input to the gate. Now, when the HIGH voltage to the gate of the transistor 17 is input, the transistor 17 is conductive for nch transistor, the output V _5, the power supply voltage V _LC of negative side is output. On the other hand, when LOW voltage is input to the gate, transistor 17
Becomes non-conductive, the output _{V 5 N · R · V LC} / (N · R +
R16) is output.

Here, R16 is the resistance value of the resistor 16. Clearly V _LC > N
・ The relationship of R · V _LC (N · R + R16) is established.

Accordingly, the V _LC to Vsat ₁ described _{above, N · R · V LC /} (N
・ R + R16) should be equal to the above-mentioned voltage of Vsat2.
_VLC and the values of R16 and R are chosen appropriately. In such a circuit, a HIGH voltage is applied to the gate of the transistor 17 in the first frame, and a LOW voltage is applied in the second frame.

〔The invention's effect〕

According to the present invention, it is possible to obtain a ferroelectric liquid crystal electro-optical device having a high contrast because a bias value can be largely selected by switching a rewrite peak value between a first frame and a second frame using a transistor. There is an effect that can be.

[Brief description of the drawings]

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 the conventional liquid crystal cell, FIG. b) 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 the contrast ratio, and FIG. 7 is a diagram showing the relationship between the pulse peak value and the transmitted light intensity. FIG. 8 is a diagram showing a relationship between a pulse peak value and transmitted light intensity for explaining the present invention. 1-1: substrate 2-2: alignment film 3: chiral smectic C liquid crystal 8-8: polarizing plate 9: scanning electrode 10: signal electrode 11-16: resistance 17: transistor

Claims (3)

(57) [Claims] 1. A ferroelectric liquid crystal thin film, an alignment film which is in contact with the thin film and realizes bistable alignment of liquid crystal molecules, a member for converting a bistable alignment state into optically bright and dark, and a switch for switching the bistable state. In a ferroelectric liquid crystal electro-optical device comprising a liquid crystal panel comprising electrodes arranged in a matrix for applying a voltage to the thin film and a driving circuit for writing a bistable state line-sequentially by an AC bias averaging method, Drive means capable of making 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 saturation voltage Vsat ₁ and a threshold voltage Vth for writing the first stable state ₁ ratio gamma ₁ and two values gamma ₁ ratio gamma ₂ of the saturation voltage Vsat ₂ and the threshold voltage Vth ₂ obtained when the first write second stable state _between, with the larger gamma _2, the bias of the drive waveform Drive with determined value N Ferroelectric and having a stepped liquid crystal electro-optical device. 2. A ferroelectric liquid crystal thin film, an alignment film in contact with the thin film to realize bistable alignment of liquid crystal molecules, a member for converting a bistable alignment state to optically bright and dark, and a switch for switching the bistable state. In a ferroelectric liquid crystal electro-optical device comprising a liquid crystal panel comprising electrodes arranged in a matrix for applying a voltage to the thin film and a driving circuit for writing a bistable state line-sequentially by an AC bias averaging method, The peak value of the pulse for writing the first stable state is different from the peak value of the pulse for writing the second stable state, and the drive circuit has the peak value of the pulse for writing the first stable state,
A circuit capable of making the peak value of a pulse for writing the second stable state different from that of the pulse,
A ferroelectric liquid crystal electro-optical device, wherein a transistor is connected in parallel to at least one resistor of the bias generation circuit. 3. A high voltage or a low voltage synchronized with the first frame and the second frame is applied to a gate input of the transistor.
Item 8. The ferroelectric liquid crystal electro-optical device according to Item 1.