JP2519420B2 - Ferroelectric liquid crystal electro-optical device - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an electro-optical conversion device that drives bistable states of ferroelectric liquid crystals by switching them from each other, and particularly good electro-optical conversion by temperature compensation. The purpose is to obtain the device.

[Outline of Invention]

The present invention relates to a driving type 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, and holds the bistable state with an AC pulse. By switching the bias value, the amount of light transmitted in the bistable state was adjusted, and a better display contrast could always be obtained.

[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.

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 2 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. 3-3 is a uniaxial and random horizontal alignment film existing at the interface between the substrate 1-1 and the SmC ^* thin film 2, 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. Observing this from above, the liquid crystal molecules are divided into two domains. In the first domain, the molecular axis is tilted by + θ with respect to the layer normal 4. 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 -θ with respect to the layer normal.
Leaning. 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. 8
Reference numeral -8 is a pair of polarizing plates arranged so as to face each other with their polarization axes orthogonal to each other, and optically distinguish between the first stable state and the second stable state by birefringence. For example, 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 Sm
Matrix electrodes for applying a driving voltage to the C ^* thin film 2 are scan electrodes (hereinafter referred to as common) 9 and signal electrodes (hereinafter referred to as segments) 10 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 AC pulse application during the non-selection period.
This is because the peak value of the AC pulse is equal to or less than the threshold. Therefore, black in the first stable state is written in the first frame. Subsequently, in the second frame, white is written because the polarity of the pulse is reversed. However, in this figure, since P ₄ and P ₅ are less than the threshold value, white is not written and black written in the first frame is retained.

[Problems to be solved by the invention]

However, in the conventional driving method, the light intensity transmitted through the liquid crystal cell fluctuates as shown in FIG. 4B in synchronization with the AC pulse applied during the non-selection period. The result of measuring the relationship between this fluctuation ΔI and the peak value of the AC pulse is
Shown in the figure. It can be seen that the fluctuation ΔI increases as the peak value increases. This fluctuation ΔI leads to a decrease in contrast, and the larger the value, the lower the contrast. Therefore, the peak value of the AC pulse must be made as small as possible. On the other hand, the peak value of this non-selective AC pulse, the above-mentioned write pulses P ₁ and P ₂ , and the half-select pulse
Relationship between the peak value of P ₄ and P _5, the half-selection pulse P _4, P ₅ If based on the peak value of P ₁ and P ₂ (i.e. N / N) (N-
2) / N, non-selective AC pulse is 1 / N. This is generally called the so-called AC bias averaging method.
Here, N is called a bias value.

By the way, looking at the relationship between the above-mentioned fluctuation ΔI and the bias value, it can be seen that the peak value of the non-selective AC pulse can be made small so that the bias value N can be made large in order to prevent the deterioration of the contrast.

However, the following problems occur here. That is, when the bias value N is increased, the peak values of the half-selection pulses P ₄ and P ₅ are (N−2) / N, so that the peak values are increased and may exceed the threshold value. This phenomenon becomes more remarkable as the temperature becomes higher, and in practice, driving cannot be performed over the entire temperature range unless the bias value is reduced. This causes a problem that the contrast is lowered.

[Means for solving problems]

The present invention has an object to solve the above-mentioned problems of the prior art, and therefore, the contrast value is prevented from being lowered by switching the bias value depending on the temperature. Actually, the bias value may be decreased on the high temperature side because it may exceed the threshold value, and the bias value may be increased on the low temperature side because the possibility decreases.

〔Example〕

FIG. 1 shows a circuit for switching the bias value depending on the temperature. Temperature detector For example, thermistor 12 is a constant current source
11 is connected and works. The voltage generated by the thermistor 12 is input to the negative side of the operational amplifier 13. The positive input of the operational amplifier 13 is connected to the DC power source E and the output via the resistors R ₁ and R ₂ , respectively, thereby forming a hysteresis comparator. On the other hand, the output of the operational amplifier 13 is
A resistor group 17 and a transistor 1 for generating a bias voltage input to the transistors 14, 15 and 16 and applied to the liquid crystal.
4,15,16 are connected in parallel. Output V _DD of resistor group 17,
_{V 1, V 2, V 3} , V 4, V LC in order to produce a waveform of 4 (a), is input to the drive circuit of the common and segment. Since the circuit of this part is known, the description thereof is omitted.

Next, the operation will be described. For example, if the thermistor 12 has a positive temperature characteristic, its output voltage V ₁
Rises. When the voltage value V ₁ does not exceed the DC voltage E (the voltage component due to hysteresis is omitted here for clarity of explanation), the output voltage of the operational amplifier 13 is the V _DD voltage. This V _DD voltage turns off transistors 14 and 16 and turns on only transistor 15. Therefore,
Both ends of the resistor R3 are shorted and the bias voltage is V _DD , V ₁ , V
There are 5 types, ₂ = V ₃ and V ₄ V _LC , and the bias value is N = 4. When the temperature rises and the voltage V ₁ across the thermistor 12 rises and exceeds the DC voltage E, the output of the operational amplifier 13 is inverted.
It becomes the V _SS voltage. Therefore, the transistor 15 is turned off and the transistors 14 and 16 are turned on instead. As a result, the bias voltage output is V _DD , V ₁ = V ₂ , V ₃ = V ₄ , V _LC
There are four types, and the bias value is N = 3.

Here, the DC voltage E may be determined as follows. FIG. 6 is a diagram showing the temperature range in which the bias value can be selected, which was tested by using the ferroelectric liquid crystal composition manufactured by Chisso.
It can be seen that, with a boundary of C, N = 4 bias must be applied on the lower temperature side and N = 3 bias on the higher temperature side. Therefore, the DC voltage E should be adjusted to the voltage value V ₁ output by the thermistor 12 at 34 ° C.

It should be noted that the present invention is not limited to the switching between the bias values N = 3 and 4 shown in FIG. 1 since the temperature and the bias value to be switched differ depending on the liquid crystal material.

[Advantages of the Invention] According to the present invention, by changing the bias value of the bias voltage applied to the ferroelectric liquid crystal depending on the temperature, it is possible to obtain a display with high contrast even at low temperatures.

[Brief description of drawings]

FIG. 1 is a circuit diagram for changing the bias value according to temperature, 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, and FIGS. 4 (a) and 4 (b). Is a diagram showing a driving waveform and a transmitted light characteristic of a conventional liquid crystal cell, FIG. 5 is a diagram showing a relationship between a peak value of an AC pulse and a fluctuation intensity of transmitted light induced thereby, and FIG. 6 is a selection of a bias value. It is a figure which shows the temperature range which should be performed. 1-1 ... Substrate, 2-2 ... Alignment film 3 ... Chiral smectic C liquid crystal 8-8 ... Polarizing plate, 9 ... Operation electrode 10 ... Signal electrode, 11 ... Constant current source 12 ... Thermistor, 13 ... Opamp 14,15, 16 ... Transistor 17 ... Resistor group

Claims (1)

(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 which converts a bistable aligned state into optical brightness and darkness, and a bistable state switching device. A liquid crystal cell composed of a matrix electrode for applying a voltage to the thin film and a voltage for writing one of bistable states to a selected pixel are applied, and an AC pulse for maintaining a bistable state is applied to a non-selected pixel. In a ferroelectric liquid crystal electro-optical device including a driving circuit for applying, a bistable state for the non-selected pixel with respect to a peak value of a voltage applied when writing one of the bistable states for the selected pixel. The bias value, which is the ratio of the peak value (hereinafter referred to as the bias voltage) of the AC pulse applied when the voltage is held, is switched depending on the temperature. Ferroelectric liquid crystal electro-optical device, characterized by being switched by connecting the transistors in parallel to the resistance group that generates.