JPS6215850B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that displays gradations using two-frequency dynamic driving and has a function of temperature compensation.

Liquid crystal display elements are currently widely used for digital displays such as calculators and wristwatches. In order to apply this liquid crystal display element to devices with a large number of pixels, such as XX matrix displays or display elements for televisions, dynamic It is essential to adopt a driving method. Furthermore, in order to extend the life of the liquid crystal display element, it is necessary to perform alternating current driving, so the currently used dynamic driving method for the liquid crystal display element is a method called a voltage averaging method.

_In order to perform dynamic drive and obtain good contrast, _the ratio (V _O
N /V _OFF ) is required to be large. The following equation is V _ON /V _OF in the optimal state in the voltage averaging method.
This is the formula that gives _F.

Here, n is the number of digits and is the reciprocal of the duty ratio. As is clear from this equation, as the number of digits increases, the value of V _ON /V _OFF decreases rapidly. To display a television image, 200 digits (n) are required. In this case, from equation 1,
V _ON /V _OFF = 1.073. When driving two digits simultaneously using a double matrix, that is, when n=100, the value is 1.106. Figure 1 shows the voltage-contrast characteristics of a liquid crystal with relatively excellent characteristics. In the case of this liquid crystal, V _S /V _T =1.45. When driving as above, V _OFF = V _T and V
_ON is shown in Figure 1, n = 200.100, respectively.
It is represented by B. When n=200, the contrast is limited to 10%, and even when n=100, the contrast is limited to 15%. Furthermore, when performing gradation display, only the contrast obtained by further dividing this contrast by the number of gradations per gradation can be obtained. As described above, due to problems with the characteristics of liquid crystal, good contrast cannot be expected in gradation display using the conventional voltage averaging method. The two-frequency dynamic drive solves this drawback, that is, the inability to obtain a large voltage ratio as in Equation 1, and is a gradation display using the two-frequency dynamic drive. Below, we will first describe a gradation display method using two-frequency dynamic drive.
Next, temperature compensation according to the present invention will be described.

Figure 2 shows the frequency characteristics of the relative dielectric constant (ε ₁₁ ) parallel to the liquid crystal molecules and the relative dielectric constant (ε⊥) perpendicular to the liquid crystal molecules of a nematic liquid crystal used in two-frequency dynamic drive, where dispersion occurs at relatively low frequencies. It is something that
c is the frequency at which the dielectric anisotropy, that is, the sign of ε ₁₁ −ε⊥ is reversed, _L is a frequency lower than c, and Δε _L is the frequency at which ε ₁₁ −ε
It is the value of ⊥ and is positive. Further, _H is a frequency higher than c, and Δε _H is the dielectric anisotropy at that time, which is negative.

The following describes the driving of a liquid crystal display element which is subjected to a horizontal alignment process such as a twisted nematic system. This is a liquid crystal display element in which the liquid crystal molecules are not lit when they are parallel to the liquid crystal cell, and are lit when they are perpendicular to the liquid crystal cell. Therefore, in the case of a liquid crystal with the characteristics shown in Figure 1, if a voltage is applied at frequency _L ,
It lights up at a certain voltage, but when the frequency is _H , it does not light up and a force is applied to the liquid crystal molecules, which are already oriented horizontally, to turn them even more horizontally. This phenomenon is utilized in two-frequency dynamic driving, and gradation display using two-frequency dynamic driving.

Next, gradation display using two-frequency dynamic drive will be described.

FIG. 3 shows an example of a drive signal used for dynamic gradation display using the two-frequency method. R is a digit selection signal, and the voltage is ±V as shown in the figure. C ₀ , C ₁ ,
C ₂ ... is a data signal corresponding to each gradation. This voltage is ±mV. m is a proportionality constant. As a result, voltages G ₀ , G ₁ , G _{2 .} . . are applied to the liquid crystal pixels. The number of high frequency voltages varies depending on the gradation.

Here, the number of gradations is p+1 and the gradation is q. Third
In the case of the figure, p=10, and when _G8 , q=8. Further, let the effective voltage of Gq to the liquid crystal be Vq, and let the force applied to the liquid crystal at this time be ε ₀ Fq. ε ₀
is the dielectric constant of vacuum.

The following equation calculates Fq by Gq.

Fq=n-1/nΔε _L V ₂ ―n-1/n｜Δε _H ｜m ² V ² ―｜Δε _H ｜/nV ² {q/p(1-m) ² +p-q/p(1+m
) ² }... n is the number of digits. (Reciprocal of duty ratio) q
As becomes larger, Fq also becomes larger. In other words, the force that makes the liquid crystal molecules stand up increases. Next, the equation shows the effective voltage applied to the liquid crystal at this time.

As mentioned above, it can be seen that the value of Vq changes depending on the value of q. Therefore, when q=0, the value of Vq is
By setting m so that the values of Vq become V _T and V _S in FIG. 1, respectively, when q=p, gradation display with extremely good contrast can be achieved. FIG. 4 is an example showing the relationship between gradation and contrast. This is the case when n=100. Frequencies _L and _H are 200Hz and 30KHz, respectively. The frame frequency was set to 30Hz and 21 gradations were obtained. In this way, we were able to obtain good contrast. This is an extremely high-quality display compared to a gray scale display using the conventional voltage averaging method.

As described above, the gradation display using the two-frequency dynamic drive is extremely effective in terms of contrast. However, there is a drawback that the characteristics of the liquid crystal composition change greatly with respect to temperature changes, and as a result, the entrance becomes poor. The present invention solves this problem.

The relationship between gradation and contrast in Figure 4 above is
Obtained at 20°C. In other words, the voltages V and mV shown in Figure 3 were set so that the contrast shown in Figure 4 could be obtained at 20°C. Fifth
The relationship between gradation and contrast in Figures 6 and 6 is for the cases of 18°C and 22°C, respectively. In FIG. 5, the light is turned off too much, and in FIG. 6, it is turned on too much. The main reason why the contrast fluctuates due to temperature changes is that Δε _L and Δε _H change greatly with respect to temperature changes. Figure 7 shows this situation. 7-1 is _L
= Δε _L value when 200Hz, and 7-2 is _H
This is the absolute value of Δε _H when = 30KHz. As shown in this figure, each value changes with temperature. This causes a change in contrast. One way to solve this problem is to make sure that the values of Δε _L and Δε _H do not change due to changes in temperature.
It is conceivable to change _L and _H , which give Δε _L and Δε _H. However, changing the frequency means changing the number of gradations as well. Therefore, it is not possible to fix Δε _L and Δε _H to certain constant values by changing the frequency. So _L ,
The present invention is a method of changing the drive voltage in order to perform temperature compensation while keeping _H fixed.

In order to eliminate contrast fluctuations in each gradation even with changes in temperature, the maximum and minimum contrasts may be fixed. Therefore, 0 gradation (q=
0) is set to V _T in Fig. 1, and the effective voltage Vp that gives a middle gradation (q = p) is set as V T in Fig _. 1.
is set to V _S in the same figure, and the drive voltage V and mV shown in FIG. 3 are changed so that this condition is always satisfied even with temperature changes.

FIG. 8 shows the values of V and mV that satisfy the above-mentioned conditions as a function of temperature. 8-1 is a curve of V, that is, the digit selection signal voltage, and 8-2 is a curve of mV, that is, the data signal voltage.

By changing the drive voltage in response to temperature changes in this way, it was possible to always obtain good contrast.

FIG. 9 is a simplified block diagram of a circuit for providing temperature compensation in accordance with the present invention. A is a thermistor installed near the liquid crystal display panel, which detects the temperature. B is a reference power source. C is a voltage dividing resistor hole, and the voltage at B is divided by the thermistor A and the voltage dividing resistor C to be output. This output is amplified by the amplifier D, and the A/F-1 and F-2
A D converter performs analog-to-digital conversion,
This code is converted in G-1 and G-2. G
-1 and G-2 are code conversion devices, which respectively have digit selection signal voltage (V) and data signal voltage (mV).
Output. G-1 and G-2 are D/A converters, which convert the outputs of F-1 and F-2 into digital-to-analog converters and input them to the drive signal power supplies of H-1 and H-2. H-1 and H-2 are respectively digit selection signals,
This is the power source for data signals. As a result, the voltage of the digit selection signal and the voltage of the data signal can be set to the values shown in FIG. 8, 8-1 and 8-1, respectively, and satisfactory temperature compensation can be achieved.

As described above, in the present invention, the digit selection signal is formed by a high frequency signal during the selection period and a low frequency signal during the non-selection period, and the data signal is formed by a high frequency signal having the opposite phase to the high frequency signal and a high frequency signal having the same phase as the high frequency signal. The application time ratio of the opposite-phase high-frequency signal and the in-phase high-frequency signal is variable, and the digit selection signal voltage and the data signal voltage are independent of each other with respect to temperature changes of the liquid crystal. Because of variable control, the liquid crystal always maintains a constant maximum voltage ratio VON/
Since it is possible to obtain VOFF, it has the effect of always maintaining the maximum gradation level regardless of temperature changes.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a voltage-contrast curve of a liquid crystal. FIG. 2 is a diagram showing the dielectric dispersion characteristics of the liquid crystal composition used in the present invention. FIG. 3 is a diagram showing drive signal waveforms for displaying gradations using two-frequency dynamic drive. Fourth
The figure shows the contrast for the main gradation. Figures 5 and 6 are diagrams showing the contrast for each gradation when temperature compensation is not performed, and Figures 5 and 6 are diagrams showing cases where the temperature is lower and higher than the set temperature, respectively. be. FIG. 7 shows the change in dielectric anisotropy (Δε) with respect to temperature change. 7-1 is Δε _L when frequency _L = 200Hz, 7-2 is frequency _H = 30KHz
This is the value of |Δε _H | when . FIG. 8 is a diagram showing an example of the relationship between temperature and drive voltage for temperature correction obtained in the present invention. 8-1 is the digit selection signal voltage (V), and 8-1 is the value of the data signal voltage (mV). FIG. 9 is a simple block diagram of the temperature compensation circuit used in the present invention. A is a thermistor, B is a reference power supply, C is a voltage dividing resistor, D is an amplifier, E is an A/D converter, F-1, F-2 are code converters, G-1, G-2 are D/A converter,
H-1 and H-2 are power supply units for a digit selection signal and a data signal, respectively.

Claims (1)

[Claims] 1 Liquid crystals whose dielectric anisotropy changes depending on frequency and exhibit positive dielectric anisotropy at frequencies lower than the crossover frequency c and negative dielectric anisotropy at frequencies higher than the crossover frequency c are facing each other. In a liquid crystal display device in which a digit selection signal is applied to one of the electrodes and a data signal is applied to the other electrode, the digit selection signal is a high frequency signal during the selection period and a low frequency signal during the non-selection period. The data signal is formed by a high frequency signal, and the data signal is composed of a high frequency signal having an opposite phase to the high frequency signal and a high frequency signal having the same phase as the high frequency signal, and the application time ratio of the high frequency signal having the opposite phase and the high frequency signal having the same phase is A liquid crystal display device characterized in that the digit selection signal voltage and the data signal voltage are each independently and variably controlled based on a signal from the same temperature detection means in response to a temperature change of the liquid crystal. .