JPS62215241A - Ferroelectric liquid crystal electrooptic device - Google Patents

Abstract

PURPOSE:To set inverted pulse width to a proper value all the time and to stably drive liquid crystal over a wide operating temperature range by adjusting a reference clock within a specific range for the compensation of the switching temperature dependency of a ferroelectric liquid crystal cell. CONSTITUTION:Temperature compensation is so performed that the inverted pulse width tau is set in an area determined by an upper-limit line and a lower- limit line; and the oscillator of a variable frequency oscillator 11 is a multivibrator composed of two inverters, one capacitor C, and one thermistor R17 and its oscillation frequency (f) is 1/2.2CR. When the thermistor 17 has negative temperature characteristics, (f) becomes smaller and smaller as the temperature is lower and lower. When (f) decreases, the frequency of CL2 also decreases and inverted pulses which are in inverse proportion to it increase in tau. The thermistor varies in R according to an exponential function, so (f), therefore, taualso varies according to an exponential function. The temperature dependency of a switching speed varies according to an exponential function, so the temperature compensation which uses the thermmistor sets tau within a preferable area.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a ferroelectric liquid crystal electro-optical device.

In particular, it relates to improvements in drive circuits that compensate for the temperature dependence of ferroelectric liquid crystals.

[Summary of the invention]

The present invention provides a ferroelectric liquid crystal electro-optical device in which a drive circuit is configured as a temperature-responsive frequency variable oscillation circuit, and a timing for dividing the frequency of an output reference clock signal from the oscillation circuit at a constant rate, shaping the waveform, and outputting a control signal. Since the generator circuit is configured to include a driver that supplies drive pulses with a predetermined pulse width in response to the control signal, it is possible to control temperature-dependent ferroelectric liquid crystals over the entire operating temperature range. A driving pulse can be applied that has a pulse width to which the molecules are electrically responsive.

[Conventional technology]

Ferroelectric liquid crystals have traditionally been used, such as chiral liquid crystals (
A ferroelectric liquid crystal cell (hereinafter simply referred to as a liquid crystal cell) and its driving circuit, which is driven by electrically switching (hereinafter referred to as switching) the bistable state of molecules (hereinafter referred to as "SmC"), has been known.

Furthermore, it has been known to adjust the pulse width of the applied pulse (hereinafter referred to as inversion pulse) required for switching in order to compensate for the temperature-dependent operation of liquid crystal cells (
For example, see Japanese Patent Application Laid-open No. 18931/1983).

FIG. 2 shows a perspective view of a conventional liquid crystal cell. (2) 1 is a pair of substrates placed opposite each other. 2.2 is a uniaxial or random horizontal alignment film provided on the inner plane of the substrate. 3
is a SmC'' thin film sandwiched between alignment films 2.2.

SmC" originally has a helical layer structure, but when it is made into a thin film sandwiched between alignment films, the liquid crystal molecules form layers and are horizontally aligned as shown in the figure. However, when looking at the SmC thin film 3 from above, the molecular axis is in the direction of the layer. It is tilted by θ with respect to line 4. There are two positions for this, a first stable state 5 and a second stable state 6. By the way, the head of the SmC'' molecule has a spontaneous polarization 7 in a direction perpendicular to the molecular axis. The direction of the spontaneous polarization 7 is perpendicular to the substrate 1 and has opposite polarity between bistable states.

It is therefore possible to switch between stable states by applying pulses of a predetermined polarity. 8.8 is a pair of polarizing plates having polarization axes orthogonal to each other, and is an optical conversion member that identifies the first stable state domain and the second stable state domain of liquid crystal molecules as light passing and light blocking by birefringence. It is. Electrodes 9 and 10 are placed opposite each other and apply an inversion pulse to SmC''.

Figure 3 shows the electrode configuration. 9 is a scanning electrode, and 10 is a signal electrode. A well-known time-division drive is performed by forming a matrix with both electrodes.

FIG. 4 is an example of a driving waveform applied to one of the matrix pixels shown in FIG. 3. First, the wave height value above the threshold during the first selection period■. An alternating current pulse with P and pulse width τ is −
Add cycle. The polarity of the first half pulse now makes the liquid crystal molecules first.
6, the polarity of the second half pulse performs switching in the opposite direction (second stable state - first stable state).

As a result, the switching of the first half pulse is not preserved, and the switching of the second half pulse is always effective. Therefore, the second half pulse is an inverted pulse.

Next, during the non-selection period, an AC pulse having a peak value below the threshold value is applied, and the previously obtained first stable state is preserved. Further, during the second selection period, a pulse having a polarity opposite to that applied during the first selection period is applied. Therefore, switching from the first stable state to the second stable state is performed by the second half pulse. Mutual switching of the bistable states is performed with the above as one frame scan.

By the way, the switching speed in a bistable state has a temperature dependence, and as the temperature is low (I see, the response speed becomes slow. Therefore, as shown in Fig. 4, the technology for increasing the temperature compensation by increasing the inversion pulse width τ as the temperature decreases). is disclosed in the prior literature mentioned above.

[Problem that the invention seeks to solve]

However, the prior literature does not disclose any specific drive circuit for temperature-correcting τ. Furthermore, it has become clear that not only a lower limit but also an upper limit exists as an appropriate value for the inversion pulse width τ at each temperature. Therefore, in order to achieve complete temperature compensation, it is necessary to always maintain the τ value between the upper and lower limits. However, there has been a problem in that temperature compensation that takes both of these factors into consideration has not been done in the past.

[Means for solving problems]

The present invention aims to solve the problems of the prior art, and provides an inversion pulse width τ over the entire operating temperature range of a liquid crystal cell.
is set between the two limit values and the lower limit value for each temperature. In particular, it utilizes reference clock pulse frequency control of the drive circuit.

That is, in a ferroelectric liquid crystal electro-optical device consisting of a liquid crystal cell and its driving circuit that operate by switching the bistable state of SmC5 whose response characteristics are temperature dependent,
The drive circuit includes a variable oscillation circuit that generates a reference clock that exists in a predetermined frequency range depending on the external temperature, a timing generation circuit that divides the frequency of the reference clock at a constant rate and generates a control signal, and a timing generator that generates a control signal. The device is constructed of a driver that applies an inverted pulse having a pulse width that allows the liquid crystal to respond to the electrodes.

[Effect]

Figure 1 shows a liquid crystal cell (the cell shown in Figure 2 has the following SmC"
Figure 2 shows the dependence of the inversion pulse width on temperature for a encapsulated mixture. Driving conditions ■. , = 5V 173 bias method.

? 113 CJ,, -C"-Czl140Q-wa0R82 part expansion・CJ5-C"-C2I+ 400-1>ORz
Part 3? 113 CJs-C"-Coo◎-7y 0R82 Part I As is clear from the figure, in the region below the straight line indicating the lower limit value, τ is too small and the liquid crystal molecules do not respond. As the temperature decreases, the response becomes slower. In the region above the straight line that indicates the upper limit value, the inversion pulse width τ is too large and excessive torque is applied to the liquid crystal molecules, making switching unstable and causing the liquid crystal molecules, which have been in a stable state, to return to their original stable state. As a result, switching is not performed.Therefore, in the present invention, the inverted puff
- Temperature compensation is performed so that the pulse width τ is always set in the region between the upper limit line and the lower limit line.

[Embodiment 1] A preferred embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 5 shows a linear temperature compensated drive circuit according to the first embodiment. This will be explained in conjunction with the time chart shown in FIG. 11 is a variable frequency oscillation circuit having positive temperature responsiveness. Details will be explained later.

The output of the oscillation circuit 11 is frequency-divided by a frequency divider 12 as desired to provide a reference clock pulse CL2. Reference numeral 13 denotes a timing generation circuit which inputs CL2 and shapes the divided waveform of CL according to a predetermined frequency division ratio, thereby creating control signals FLM, CL, and DF necessary for driving the scan electrodes. FLM is a scanning electrode (hereinafter referred to as common) selection signal, and one common is selected during this period. CL is a common shift signal that allocates FLM to each common line-wise.

DF is an alternating current signal and converts the signal applied to the common into alternating current for one cycle. As can be seen from FIG. 6, the inversion pulse .tau. matches the DF pulse width .tau.' by -8=. τ' is 1/2 of the period of DF. DF is CL
Since it is obtained by dividing □, the period 1υ of DF] is determined by the frequency of CLz. Therefore, by adjusting the frequencies of C1 and C2, the pulse width τ of the inversion pulse (see FIG. 4) can be adjusted. That is, the frequency of CL2 and τ of the inversion pulse are inversely proportional. 14 is a common driver,
It inputs FLMXCL and DF, synthesizes a common drive voltage, and outputs it to the common of cells I and C. 15 is a ROM that stores display data; It is read by CL2.

16 is a segment driver for driving signal electrodes (hereinafter referred to as segments), and is a segment driver for driving signal electrodes (hereinafter referred to as segments).
TA is input, and in response to this, segment drive voltages are output in parallel to each segment in synchronization with CL. Note that the LC cells shown in FIGS. 2 and 3 are used. The SmC" material shown in the section on operation is used. The drive waveform shown in FIG. 4 is used.

Now, the variable frequency oscillator 11 will be described in detail. This oscillator is a multi-by-break composed of two inverters, one capacitor C, and one thermistor R17, and has an oscillation frequency f=1/2.2CR. If a thermistor 17 having negative temperature characteristics is used here, R will increase as the temperature decreases, and therefore f will decrease. As f becomes smaller, the frequency of Cl3 also becomes smaller, and τ of the inversion pulse, which is inversely proportional to this, becomes larger. Since the change in R of the thermistor is exponential, f and therefore τ also change exponentially. As can be seen from FIG. 1, since the temperature dependence of the switching speed is also exponential, temperature compensation using a thermistor can set τ in the preferable range shown in FIG.

[Embodiment 2] FIG. 7 shows a second embodiment of the linear temperature compensation drive circuit. Embodiment 1 shown in FIG. 5 except for the variable frequency oscillator 11
is the same as

The variable frequency oscillator 11 causes a constant current source 18 to supply a constant current to the thermistor 17 having positive temperature characteristics.
The voltage generated across the voltage controlled oscillator 19 (V CO
). The VCO used here has an output frequency that increases in proportion to the input voltage (
(See Figure 9). Therefore, when the temperature decreases, the thermistor 1
Since R of 7 has positive temperature characteristics, it becomes small. Therefore, the voltage across the thermistor 17 becomes small. Therefore vC
The frequency of the output f of O becomes smaller, and the inverse pulse τ, which is inversely proportional to this, becomes larger. Since this tendency matches the tendency of the response characteristics shown in FIG. 1, it is possible to perform temperature compensation in which τ is always set in the preferable region shown in FIG.

[Embodiment 3] FIG. 8 is a diagram showing a third embodiment of a drive circuit with discrete temperature compensation. The difference from the second embodiment shown in FIG. 7 is that the output voltage (2) across the thermistor 17 is digitally processed and then input to the VCO 19, and the rest is the same.

This digital processing sets τ to a constant value of 40 m5ec in the range of 0°C to 10°C, 18 m5ec in the range of 10°C to 20°C, and 9 m5ec in the range of 21°C to 30°C in Fig. 1.
m5ec, τ at 31℃~40'C 4m5ec
It is also carried out to set τ to 2m5ec at 41°C to 50°C. That is, τ is set stepwise within the preferable region shown in FIG.

It is a 20Lt A/D converter and converts the analog output voltage (2) of the thermistor 17 into five-stage digital data according to the above-mentioned temperature divisions. Therefore, the output only needs to be at least 3-bit data. 21 is a ROM with VCOI9
The input voltage value of is stored as digital information. A
It receives the 3-bit output of the /D converter 20 and uses this as an address to output predetermined voltage value digital data.

This voltage value digital data is input to the D/A converter 22, and a predetermined voltage is applied to the VCO 19. Now, the digital processing during this time will be summarized in the table below.

First, 3-bit data (0, O, O) to (1, O, O) is output from the A/D converter 20 in accordance with the five temperature ranges. Using this as an address, a voltage value (0.26 to 0.70) is read from the conversion ROM table 21 and applied to the VCO 19 via the D/A converter 22. According to the characteristic curve of input voltage vs. oscillator angular frequency of VCO 19 shown in FIG. 9, the VCO output (assuming this is used as it is for Cl3) fCL2300 Hz ~ 60
Get 00Hz.

Now, the relationship between fcL2 and τ is fCL2 = (1/τX2)X24, assuming that the number of segment electrodes is 24.

In other words, since the common scan frequency is 1/τ x 2 (doubling is because τ corresponds to a half period), 24 serial segment data are processed for each scan, so 1/τ A reference clock CL that is 24 times faster than ×2,
is necessary.

Now, the results of calculating τ from the value of f CL2 using this formula are shown on the right side of the table. From the table, τ
It is understood that discrete temperature compensation is performed.

〔Effect of the invention〕

As described above, according to the present invention, in order to compensate for the switching temperature dependence of a ferroelectric liquid crystal cell, the inversion pulse width can always be set to an appropriate value by adjusting the reference clock within a predetermined range. This has the effect of allowing stable driving over a wide operating temperature range.

Furthermore, since the reference clock is adjusted, the operation of the entire drive circuit is controlled all at once in accordance with each temperature condition, resulting in good efficiency.

[Brief explanation of drawings]

Figure 1 is a diagram showing the temperature dependence of the inversion pulse width, Figure 2 is a perspective view of a conventional liquid crystal cell, Figure 3 is a diagram of the electrode arrangement of a conventional liquid crystal cell, and Figure 4 is a diagram of the drive of a conventional liquid crystal cell. Waveform diagram, 5th
Figure 7 and Figure 7 are linear temperature compensation drive circuit diagrams, Figure 6 is a time chart 1 for explaining circuit operation, Figure 8 is a discrete temperature compensation drive circuit diagram, and Figure 9 is VCO oscillation frequency. It is a characteristic circle. 1.1... Substrate 2.2... Alignment film 3... Chiral smectic liquid crystal thin film 8.8...
Polarizing plate 9.10... Electrode 11... Frequency variable oscillation circuit 13... Timing generation circuit 14... Common driver 16... Segment driver or more anti-axis. , 5□%,) 2 degrees dependent 4. j, 1 10゛Fig. 1@Isaki Mi's liquid crystal cell Rishinzan 1st Night Izushima Takumi Gravity Circuit Diagram Figure 8 VCO Inspection Temporary Shushiro My Signature Diagram Figure 9

Claims (4)

[Claims] (1) A ferroelectric liquid crystal thin film, a pair of substrates that sandwich the thin film, an alignment film that is in contact with the plane of the thin film and provides a bistable state for liquid crystal molecules, and a predetermined film that sandwiches the thin film and switches the bistable state. a liquid crystal cell comprising an electrode for applying a voltage with a pulse width of and a conversion member for optically identifying a bistable state;
In a ferroelectric liquid crystal electro-optical device comprising a drive circuit that applies a drive voltage to the electrode, the drive circuit includes a variable oscillation circuit that generates a reference clock within a predetermined frequency range depending on external temperature; A ferroelectric device comprising a timing generation circuit that divides a clock at a constant rate to generate a control signal, and a driver that applies a driving pulse having a pulse width that allows the liquid crystal to respond to the control signal to an electrode in response to the control signal. liquid crystal electro-optical device. (2) The ferroelectric liquid crystal electro-optical device according to claim 1, wherein the variable oscillation circuit is a multivibrator including a thermistor. (3) The ferroelectric liquid crystal electro-optical device according to claim 1, wherein the oscillation circuit is a voltage-to-frequency converter controlled by inputting a voltage generated across a thermistor. (4) The ferroelectric liquid crystal electro-optical device according to claim 3, wherein the oscillation circuit digitally processes the voltage generated across the thermistor and then inputs it to the voltage-to-frequency converter.