JPS6116990B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field] The present invention relates to a liquid crystal display device, and more particularly to an applied signal for dynamic driving.
It relates to dynamically driving a liquid crystal display element (abbreviated as TN method) using a two-frequency method. [Interlocking technology] 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 picture elements such as XY matrix displays, a dynamic drive method is adopted to reduce the number of electrical connection terminals with the drive circuit and to simplify the circuit itself. It is a prerequisite to do so. Furthermore, in order to extend the life of the liquid crystal display element, it is necessary to perform AC driving, so the currently used dynamic driving method for the liquid crystal display element is a method called the voltage averaging method. FIG. 1 shows an example of drive waveforms for the voltage averaging method. R is a timing signal, C is a data signal, and the difference between the two (R-C) is applied to the liquid crystal pixel. In this case, the ratio of the effective voltage value in the selected state (V _ON ) to the effective voltage value in the non-selected state (V _OFF ) is √:1 (n is the number of digits: duty factor). It reaches its maximum when the reciprocal of the ratio), becomes. According to the formula, the larger n becomes, the more V _ON /V _OF
It can be seen that _F becomes smaller. Figure 2 shows the relationship between voltage and contrast in the TN method. Vth is a voltage that shows a contrast of 10% of the saturation value, and V _S a is a voltage that shows a contrast of 90% of the saturation value. Therefore, at least in order to turn on and off, it is necessary to turn on at a voltage higher than Vsa and turn off at a voltage lower than Vth. Therefore, the value of V _ON /V _OFF in the formula is Vsa /
Must be greater than or equal to Vth. However, so far this condition has not been satisfied and sufficient contrast has not been obtained for n=8. As described above, in the conventional voltage averaging method, when multi-digit dynamic driving is performed, the contrast ratio between the lit state and the non-lit state decreases, and this becomes more significant as the number of digits increases. On the other hand, as a display method, there is a liquid crystal display method using two-frequency drive. FIG. 3 shows the frequency characteristics of the dielectric constant of the liquid crystal according to this driving method. FIG. 3 shows the vertical and horizontal dielectric constants of liquid crystal molecules, respectively. _Let ε L and ε H be the relative permittivity in the vertical direction at a frequency (f _H ) where ε and ε⊥ have the same value, respectively, then ε _L > _ε⊥ , ε _H <ε⊥, and the dielectric anisotropy (Δε =ε−ε⊥) are positive and negative, respectively. Conventionally, in this dual-frequency drive liquid crystal display, the signal applied to the timing signal electrode is low frequency f _L when selected, f _H when not selected, and the signal applied to the data signal electrode is f when turned on. An anti-phase signal of _L was applied, and an in-phase signal of f _L was applied when the lamp was not lit. However, in such a driving method, as the number of time divisions increases by increasing the number of digits of the matrix, the time at the time of selection becomes shorter;
If the low frequency signal f _L is set within this short selection time, the frequency of this signal will exceed the crossover frequency f _C , causing the problem that the low frequency signal will no longer perform its original function. Moreover, the low frequency signal f _L is
It is used when selecting a timing signal, and when the superimposed signal applied to the liquid crystal is not selected, it is used to control whether to turn on or off by changing the phase of the data signal with respect to this low frequency signal _f L. The period of f _L at is f _L
The drawback was that the effective voltage varied between 2f _L and 2fL. [Purpose] The present invention is to fundamentally improve the above-mentioned drawbacks of conventional liquid crystal display devices, and even if the number of digits in matrix display is increased, the decrease in V _ON /V _OFF values and contrast can be minimized. The present invention provides an innovative liquid crystal display device that does not require any interference. [Embodiment] FIG. 4 shows the applied signals of the two-frequency dynamic drive used in the present invention. R is a timing signal applied to the timing signal electrode; when selected, a high frequency rectangular wave is applied, and when not selected, a low frequency wave is applied. C _ON and C _OFF are ON and OFF data signals applied to the data signal electrodes, respectively, and are in-phase and OFF, respectively, with respect to the waveform when R is selected.
This is a reverse phase signal. As a result, the liquid crystal pixels have F _ON ,
A voltage is applied like F _OFF , and when a high frequency is applied at the time of selection, it becomes an OFF state, and when the applied voltage is 0, it becomes an ON state. Since this method uses ON-OFF timing in the opposite way to the conventional method, we will refer to it as the NTM method (Negatiue timing method). Here, we will discuss the principle of this NTM. First, let us define the frequencies of high and low frequencies as f _H
f _L , and the anisotropy of permittivity (Δ
ε) are respectively set as Δε _H (<0) and Δε _L (>0). Considering static drive, let Vsa (see Figure 2) when f _L is applied be V _L . Then f _H −f
Mixed waveform of _L (F _ON when not selected in Fig. 4,
Vsa in static drive of _FOFF waveform)
When V _H is a mixed waveform of f _H -f _L , V _H of a mixed waveform of f H -f L corresponds to V _L in static driving at a frequency f _L. Therefore, this f _H −f _L mixed waveform is shown in Fig. 4 as F _ON ,
If added when F _OFF is not selected, the square of the effective value of static conversion will be V, where n is the number of digits.
It becomes _L ²・n-1/n. Next, considering the waveform at the time of selection, since it is 0V when ON, the effective voltage is naturally also 0V. On the other hand, when OFF, a voltage of V _H is applied with a waveform of frequency f _H , so the value of the square of the effective voltage is V _H
² /n, and since Δε _H has a negative value at f _H , it acts negatively on the overall voltage. Therefore, the following formula can be obtained from this. The formula is the effective voltage in the ON state, and the formula is the effective voltage in the OFF state. Here, V _ON in the formula is
Since it will inevitably become smaller due to V _L , V _O
V _L must be set so that _N is higher than Vsa. , from the formula, the ratio of these two voltages is becomes. What should be noted here is that this voltage ratio does not depend only on the number of digits (n) as in the conventional case, but also on the voltage (V _H ). In other words, this shows that the voltage ratio increases as the applied voltage (V _H ) increases. (V _L is a value determined by determining f _L and is a constant in this case.) Here, in the equation

[Formula] When V _{O N} /V _OFF →∞, and

〔effect〕

As described above, the present invention has a dielectric anisotropy that changes depending on frequency, has a positive dielectric anisotropy at a frequency f _L lower than a certain frequency f _C , and has a positive dielectric anisotropy at a frequency f _H higher than a certain frequency f _C. A twisted nematic liquid crystal composition having negative dielectric anisotropy is horizontally aligned between a pair of substrates, with a timing signal electrode formed on one of the substrates and a data signal electrode formed on the other substrate. In a liquid crystal display device comprising: means for applying a high frequency f _H signal when the timing signal electrode is selected, and applying a low frequency f _L signal when the timing signal electrode is not selected, and means for applying the high frequency f _H signal when the data signal electrode is turned on; Since a means is provided for applying a signal in phase with the signal of _fH , and a signal of opposite phase to the signal of high frequency fH when the light is not turned on, the number of timing signal electrodes increases and the number of time divisions increases. Even if the selection time becomes shorter, it will not become impossible to turn on like in the past, and it will always be possible to maintain constant lighting or non-lighting, and the period of the superimposed signal will not fluctuate when not selected. The effective value can be kept almost constant, and therefore, even if the number of timing signal electrodes increases, a desired contrast can always be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a drive waveform of a conventional voltage averaging method. FIG. 2 is a diagram showing the relationship between AC voltage and contrast. FIG. 3 is a diagram showing the frequency dispersion of the dielectric constant of the liquid crystal composition used in the present invention. FIGS. 4 and 5 are diagrams showing an example of the dual frequency drive system (NTN) used in the present invention. Figure 6 shows the drive method of the present invention.
ON waveform when performing 16-digit dynamic drive,
It is a figure which shows the voltage-contrast curve of an OFF waveform.

Claims (1)

[Claims] 1 Dielectric anisotropy changes depending on frequency, with positive dielectric anisotropy at frequencies f _L lower than a certain frequency f _C and negative dielectric anisotropy at frequencies f _H higher than the frequency f _C A liquid crystal in which a twisted numeristic liquid crystal composition having a characteristic is horizontally aligned and sealed between a pair of substrates, a timing signal electrode is formed on one of the substrates, and a data signal electrode is formed on the other substrate. In the display device, means for applying a high frequency f _H signal when the timing signal electrode is selected, and applying a low frequency f _L signal when the timing signal electrode is not selected;
A liquid crystal display characterized by comprising means for applying a signal having the same phase as the high frequency f _H signal when the data signal electrode is lit, and applying a signal having the opposite phase to the high frequency f _H signal when the data signal electrode is not lit. Device.