JPS6361677B2 - - 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 performs gradation display using two-frequency dynamic driving.

SUMMARY OF THE INVENTION An object of the present invention is to obtain a well-balanced display of shading in a liquid crystal display that performs gradation display.

Liquid crystal display devices 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 X-Y matrix displays or display elements for televisions, dynamic It is undecided to adopt a drive system. 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 ON /}
V _OFF ) is required to be large. The following equation gives V _ON /V _OFF in the optimal state in the voltage averaging method.

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, Vsa/Vth = 1.45. When driving as above, V _OFF = Vth
V _ON is shown in Figure 1, where n = 200 and 100 are represented by A and B, respectively. 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, good contrast cannot be expected in gradation display using the conventional voltage averaging method due to problems with the characteristics of liquid crystal. The two-frequency dynamic drive eliminates this drawback, that is, the inability to obtain a large voltage ratio as shown in equation (1), and is a gradation display using the two-frequency dynamic drive. The principle of two-frequency dynamic drive and the gradation display method of the present invention will be described below.

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 △ε _L is the value of ε ₁₁ −ε⊥ at that frequency, which is positive. . Also, _H is a higher frequency than _c ,
Δε _H is the dielectric anisotropy at that time and 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 2, if a voltage is applied at a frequency _L , it will light up at a certain voltage, but in the case of a frequency _H , it will not light up and the liquid crystal molecules, which are oriented laterally, will be further oriented laterally. Adds the power to 2 took advantage of this phenomenon.
It is a frequency method dynamic drive.

FIG. 3 shows an example of an applied signal used in two-frequency dynamic driving. R is a timing signal in which a high frequency ( _H ) rectangular wave is applied during the selection period (i.e. period Ts in Figure 3), and a low frequency ( _L ) rectangular wave is applied during the non-selection period (i.e. period T _N in Figure 3). The voltage to which the rectangular wave is applied is ±V. This figure shows a waveform with a 1/8 duty ratio. C _ON and C _OFF are lit respectively,
These are non-lighting data signals, and are in phase and anti-phase with respect to the high frequency of the timing signal, respectively.
Its voltage is ±mV. m is a proportionality constant.
G _ON or G _OFF , which is a composite waveform of the timing signal R and the lighting data C _ON or non-lighting data C _OFF of the data signal, is applied to the liquid crystal pixel.

That is, G _ON indicates a lit state within the selection period _Ts , and G _OFF indicates a non-lit state within the same period. In this figure, one selection period
Only the T _s signal is shown, but depending on the input data, lighting data may be changed in other selection periods.
A mixed signal of C _ON and non-lighting data C _OFF is applied to the liquid crystal as a data signal.

Here, the effective voltages applied to the liquid crystal due to the signal voltages G _ON and G _OFF are respectively V _ON and V _OFF , and the forces applied to the liquid crystal at that time are ε _OX and ε _OX F _OFF . ε ₀ is the permittivity of vacuum.

To find F _ON and F _OFF at G _ON and G _OFF , F _ON = n-1/n△ε _L V ² −n-1/n｜△ε _H ｜m ² V ²
−1/n｜△ε _H ｜V ² (1−m ² )……(2) F _OFF =n−1/n△ε _L V ² −n−1/n｜△ε _H ｜m ² V ²
-1/n|△ε _H |V ² (1+m ² )...(3). n is the number of digits. In the case of FIG. 3, n=8.

Figure 4 shows the voltage-contrast curve of the liquid crystal display element when frequency _L is used. V _T and V _s are voltages that provide 10% and 90% contrast, respectively. Therefore, at least it can be obtained from equations (3) and (2)
F _OFF and F _ON must correspond to values below and above this voltage, respectively. Here, F _OFF is set to always obtain 10% contrast. In other words, F _OFF = △ε _L V ² _T (= constant) ...(4) Therefore, from equations (4) and (3), nm ² +2m+1−△ε _L / | △ε _H | {n− 1-n(V ² /V)}=0...(5) The following relationship is obtained. In other words, m is set so as to satisfy equation (5). Here, equation (3) gives 10% contrast.

Therefore, V _OFF =V _T ...(6).

Next, equation (2) becomes F _ON =F _OFF +4m・|△ε _H |/nV ² ...(7), and from this becomes.

Then, is obtained. However, m must satisfy equation (5). It can be seen from equation (9) that the voltage (V _ON /V _OFF ) can be increased by increasing V.
This converts equation (1) in the conventional voltage averaging method to n digits
This is an epoch-making achievement compared to the previous case, where it was impossible to increase the voltage ratio because it was a function of Next, gradation display using this two-frequency method dynamic drive will be described.

FIG. 5 shows an example of an applied signal used in the two-frequency dynamic gradation display according to the present invention. At the arrows (↓), each signal is synchronized. R is similar to the selection signal shown in FIG.
C ₀ , C ₁ . . . C ₁₀ are data signals, and 0 and 2 indicate gradations starting from the one with the lowest contrast.
C ₀ and C ₁₀ have the same waveforms as C _OFF and C _ON in FIG. 3, respectively. These data signals apply voltages such as C ₀ , C _{1 .} . . C ₁₀ to the liquid crystal pixels. The waveforms at times other than selection are the same as described above with respect to FIG.

In FIG. 5, there are 11 gradations, but this is because there are 10 gradations in R. By increasing this number, the number of gradations can be increased. Also, if you decide every other gradation, that is, in Fig. 5,
If C ₀ is followed by C ₂ , C ₄ , C ₆ , C ₈ , and C ₁₀ , six gradations can be obtained. Furthermore, Ca and Cb in FIG. 6 represent the same gradation as _C2 in FIG. 5, and at this time, signals such as Ga and gb are applied to the liquid crystal pixels. This is because the gradation is determined by the number of matches between the high frequency phase of the selection signal and the phase of the data signal. For example, the phase of C ₀ is 0, C ₁ is 1, C ₂ is 2, and C ₁₀ is 10.

Next, find the effective voltages of the applied signals G ₀ , G ₁ ....

First, let p be the number of high frequency peaks of the selection signal. In FIG. 5, p=10. On the other hand, let q be the number of waves in the data signal that are in phase (0 for C ₀ , 1 for C ₁ , etc.). This corresponds to gradation. Let F _q be the force applied to the liquid crystal at this time, then F _q = n-1/n△ε _L V ² −n-1/n | △ε _H | m ² V ² |
△ε _H |/nV ² {q/p(1-m) ² +p-q/p(1+
m) ² } ...(10). In the case of q=0 and q=p, respectively (3)
It corresponds to equation (2). Using the conditions of equations (4) and (5), is obtained.

Equation (11) holds true in the range of 0≦q≦p (q=0, 1, 2...). As is clear from equation (11), the effective voltage _{V q} applied to the liquid crystal depends on the value of q.
The value of is calculated.

FIG. 7 shows the value of V _q for each gradation.
The conditions in this figure are when the number of digits (n) is 100, that is, the duty ratio is 1/100, and the frequencies _L and _H are 200 Hz and 30 kHz, respectively. At this time, △ε _L = 0.764,
Δε _H =−1.08. Also, the voltage (V) of the digit selection signal is 31V, and the voltage (mV) of the data signal is
24.7V, and the ratio (m) of both voltages is set to 0.80. Since the frame period is 30Hz, the number of high frequency peaks of the digit selection signal is 20, which is the maximum
It was possible to display 21 gradations. As you can see from Figure 7, the gradation and effective voltage are almost proportional,
It has a slightly upward curve. FIG. 8 shows the relationship between gradation and contrast at this time. Since the difference in contrast between each gradation was almost constant, a good display with well-balanced shading could be obtained.

Next, an example in which the number of digits is 200 and 400 will be described. When n is 200, _L is set to 200Hz, _H is set to 30KHz, and the frame period is set to 30Hz, which is the same as above. At this time, digit selection signal voltage 43.2V, data signal voltage
By setting it to 35.4V, we were able to obtain the same effective voltage as shown in Figure 7. However, the number of gradations is 11, n
= half of the case of 100.

Furthermore, in the case of n=400, the frame period is set to 25Hz _L
When 200Hz, _H = 50KHz, and the number of gradations is 11,
Digit selection signal voltage 59.6V, data signal voltage 47.7V
By doing so, it was possible to obtain an effective voltage substantially similar to that shown in FIG. At this time, although the contrast decreased overall, the gradation display was normal.
This decrease in contrast appears to be due to the fact that the selection time for each pixel is too short. However, the fact that a large scale gradation display of n=400 can be performed is revolutionary.

As described above, the present invention provides a liquid crystal display device in which a plurality of data signal electrodes and digit selection signal electrodes are disposed within a pair of substrates and a liquid crystal is enclosed, and when selecting the digit selection signal, a high frequency signal is applied to the selected digit selection signal. The data signal within one selection period that is applied to the digit selection signal and is applied to the data signal electrode is formed from a high frequency signal that is in opposite phase to the high frequency signal and a high frequency signal that is in phase with the high frequency signal, and is formed from a high frequency signal that is in phase with the high frequency signal. Since the number of pulses of the high-frequency signal is determined in accordance with the gradation level of the data signal, gradation display can be realized using only a binary level power supply voltage, and therefore, it is possible to realize a gradation display using only a binary level power supply voltage. Gray scales can be displayed with a simple circuit design without requiring a voltage circuit. Furthermore, since the number of high-frequency pulses directly corresponds to the gradation, fine control can be obtained depending on the presence or absence of one wavelength of these high-frequency pulses, making it possible to obtain fine gradation control with large contrast. It has the effect of

[Brief explanation of drawings]

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 an example of a two-frequency method dynamic drive waveform. FIG. 4 is a diagram showing a voltage-contrast curve. FIGS. 5 and 6 are diagrams showing an example of the dynamic drive waveform of the gradation display two-frequency method used in the present invention. FIG. 7 is a diagram showing the effective voltage values for each gradation obtained by the present invention. FIG. 8 is a diagram showing the contrast for each gradation obtained by the present invention.

Claims (1)

[Claims] 1. In a liquid crystal display device in which a plurality of data signal electrodes and digit selection signal electrodes are arranged within a pair of substrates and a liquid crystal is sealed, when the digit selection signal is selected, a high frequency signal is applied to the digit selection signal. and when the digit selection signal is not selected, a low frequency signal is applied, and the data signal within one selection period that is applied to the data signal electrode is a high frequency signal having the opposite phase to the high frequency signal and the high frequency signal. and a high-frequency signal of the same phase, and the number of pulses of the high-frequency signal of the opposite phase is determined corresponding to the gradation level of the data signal,
A liquid crystal display device characterized in that a data signal is in phase opposite to the high frequency signal when lit, and a data signal is in phase with the high frequency signal when not lit.