JPH068936B2 - Driving method of liquid crystal matrix panel - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving method of a liquid crystal matrix panel capable of displaying a large amount of information with good contrast.

Configuration of Conventional Example and its Problems Conventional liquid crystals are widely used for calculators, watches, and dot matrix displays because of their low voltage, low power consumption, and thin shape. For example, in a dot matrix display of twisted nematic liquid crystal (hereinafter abbreviated as TN liquid crystal), a method called a voltage averaging method shown in FIGS. 1 (a) to 1 (d) is used as a driving method for obtaining optimum visibility. It is commonly used.

It is known that the optical characteristics of the TN liquid crystal depend on the effective value voltage applied to the liquid crystal, and the effective value voltage applied to the on cell A and the off cell B (or C) in FIG. 1 (a).
It is indispensable to maximize the ratio of the effective value voltage applied to the display in order to obtain a display with good contrast. It should be noted that the shaded portions in the figure are cells that are turned on by the image to be displayed. Maximum value of RMS voltage ratio R when the number of scanning electrodes is N Is easily guided. That is, even if the optimum driving method is used, N = 64, 128, and 256 decrease as R = 1.134, 1.093, and 1.065, respectively, and the contrast and viewing angle deteriorate. In terms of characteristics, the number of scanning lines is practically N = 64 or N.
In reality, the upper limit is 128.

On the other hand, in order to remove the above restriction, a driving method called a two-frequency method shown in FIGS. 2 (a) to (g) has also been proposed. Here, a special liquid crystal material as shown in FIG. 3 is used, which has a positive dielectric anisotropy at a low frequency and a negative dielectric anisotropy at a high frequency. When a low frequency voltage is applied, the liquid crystal molecules are oriented in the direction of the electric field, and the liquid crystal molecules are oriented vertically with respect to the high frequency electric field in which the dielectric anisotropy is negative, so that display contrast is achieved.

In this method, as shown in FIGS. 2 (a) to (d) and FIGS. 2 (e) to (g), two driving methods, a low frequency selection method and a high frequency selection method, have been proposed. In the latter case, the period is low frequency, the non-selection period is high frequency, and vice versa. In either case, good contrast can be achieved even when N is large, regardless of the ratio of the effective value voltage applied to the ON and OFF cells. It

However, this method requires the use of a liquid crystal material having a strong frequency dependence of dielectric anisotropy, and since it is difficult to obtain a liquid crystal material exhibiting a high positive dielectric anisotropy at a low frequency, the driving voltage becomes high. However, since the dielectric anisotropy usually has a considerable temperature dependency at the same time, the temperature compensating circuit becomes complicated, and it has not been widely used.

An object of the present invention is to provide a liquid crystal for high information display capable of displaying a large number of characters, figures, images, etc. by slightly modifying the structure of a conventional liquid crystal matrix panel and adopting a driving method suitable for the structure of this panel. A method for driving a matrix panel is provided.

Structure of the Invention The present invention provides a predetermined first period and a predetermined period for each scan electrode and each signal electrode of a liquid crystal matrix panel having N scan electrodes and M signal electrodes (N and M are positive integers). A method of driving a liquid crystal matrix panel, in which each voltage of 2 periods is applied n times (n is a positive integer), and in the first period, voltage V ₀ ( V ₀ is a real number) and voltage 0 and voltage are applied to the non-selected scan electrodes. Repeatedly applying a high frequency voltage, voltage 0 is applied to the signal electrode in the on-cell and voltage (2 /
a) V ₀ is applied respectively, and in the second period, voltage ₀ is applied to the selected scan electrode of each liquid crystal cell of the liquid crystal matrix panel.
And a voltage _{V 0} to the scan electrode not selected (1-2 / a) V
_While repeatedly applying high frequency voltage of ₀ ,
By applying the signal voltage V _{0 in the} on-cell and the voltage (1-2 / a) V ₀ in the off-cell to the signal electrode,
It is characterized by driving a liquid crystal matrix panel.

Description of Examples The optical characteristics of a TN liquid crystal cell are said to correspond to the effective value of the voltage normally applied to the cell.

However, a rectangular wave AC voltage (for example, 60H) is applied to the liquid crystal cell.
z) is applied to maintain a predetermined black level (here, since a TN liquid crystal cell with crossed Nicols F is used, a voltage of 0
It is clear that when the voltage value is kept as it is and the frequency is increased while the voltage value is kept as it is, the brightness becomes almost zero voltage at several KHz as shown in FIG. If a material whose dielectric anisotropy of liquid crystal decreases with frequency, such a thing naturally occurs,
It was confirmed that the frequency dependence of the dielectric anisotropy of the used liquid crystal was constant in the above frequency range. That is, it is difficult to say that the optical characteristics of the liquid crystal cell usually correspond to the effective value as a whole panel. Considering the cause of this with an electrical model, it becomes as follows.

The electrical equivalent circuit of the picture element portion corresponding to the intersection of the scanning electrode and the signal electrode can be represented as shown in FIG. 5 when simplified. When an AC voltage of V is applied between the scan electrode 1 and the signal electrode 2,
The effective voltage V _L applied to the liquid crystal cell is It is represented by. Where ω is the applied AC voltage and the angular frequency, R ₁ is the electrode resistance inserted in series to the picture element, the alignment film, the color filter film or the series resistance component additionally introduced, and C, R ₂ is the equivalent parallel capacitance and resistance of the crystal layer.

As is clear from this, the higher the frequency becomes, the lower the net voltage applied to the liquid crystal layer becomes. And it is extremely difficult in practice to make the series resistance small enough to be ignored.

Therefore, in the present embodiment, a method of positively utilizing this frequency dependency that a cell normally has for improving visibility is adopted. In that case, in the signal waveform by the conventional voltage averaging method as shown in FIG. 1, the frequency components in the selection period and the non-selection period are the same, and the characteristics of FIG. 4 cannot be effectively utilized. However, if a high frequency component is applied during the non-selection period of the on-cell and the off-cell so that only a small voltage is applied to the liquid crystal during this period, the difference in the voltage between the on-cell and the off-cell during the selection period appears more significantly and the contrast becomes higher. It will be improved. On the other hand, however, it is necessary to avoid applying a DC voltage component to the liquid crystal in terms of life. Under such conditions, the liquid crystal drive waveform is considerably limited. The signal waveforms in Table 1 satisfy these conditions to some extent.

That is, as shown in the left column of Table 1, in the first period, the DC voltage V ₀ (V ₀ is a real number) is applied to the selected scan electrode of each liquid crystal cell of the liquid crystal matrix panel and to the scan electrode not selected. High frequency signal voltage 0 And a voltage 0 is applied to the signal electrode while the signal voltage is applied to the signal electrode. , And the second period, the scan voltage 0 is applied to the selected scan electrode of each liquid crystal cell of the liquid crystal matrix panel, and the high frequency signal voltage V _{0 is} applied to the non-selected scan electrodes.
When And the signal voltage Vo is applied to the signal electrode in the on-cell and the signal voltage is applied to the signal electrode in the off-cell. By applying each of them, Fig. 1 (a) and Fig. 2 (a)
Image pattern as shown in (on-cell is indicated by the shaded area)
In this case, at the intersection of each scan electrode, display electrode and XY matrix of the liquid crystal matrix panel shown in FIG.
The signals as shown in the right column of the table are applied.

When a signal waveform as shown in Table 1 is applied between scanning and signal electrodes, effective value voltages V _on and V _off applied between the electrodes of the ON picture element and the OFF picture element are given by the following equations.

Therefore, the effective voltage ratio R applied between the scanning and signal electrodes is And R is Takes the maximum value at. In the voltage averaging method It takes 2N even though the scanning electrode is N
Although it is only -1 effective value voltage ratio, the actual voltage ratio applied to the liquid crystal layer is large because the high frequency component described above causes the voltage component consumed by the series resistance to increase.
The net effective voltage ratio is the ratio obtained by the voltage averaging method. Can be higher.

For the sake of simplicity, Table 1 shows the case of driving by the so-called two-field AC method, but it may be driven by the in-field AC method or the multi-field AC method.

In any case, in order to effectively utilize the present invention, it is desirable that the matrix or cell of the liquid crystal exhibits remarkable characteristics as shown in FIG. 4, and in terms of the liquid crystal material, the dielectric anisotropy is up to several tens KHz. Ordinary liquid crystal for TN cell that does not change (in this case, some series resistance is required in series with the liquid crystal layer), liquid crystal whose dielectric anisotropy decreases with frequency, but whose sign does not change even at high frequencies, or A liquid crystal whose sign is changed at high frequency, such as a liquid crystal for driving two frequencies, is used. The latter liquid crystal does not necessarily require a series resistance for the liquid crystal layer.

As described above, from the characteristics of the liquid crystal itself or the configuration of the cell,
Alternatively, if the characteristics as shown in FIG. 4 or the characteristics having greater frequency dependency are contributed to the panel by using the characteristics of both, the driving method of the present application can be effectively utilized, and the high frequency component voltage in the non-selection period can be used. Can be prevented from being applied to the liquid crystal layer, and the RMS voltage is Thus, for N = 64, 128, 256, R = 1.195, 1.134, 1.093, and an effective value voltage ratio corresponding to twice the number of scanning lines of the voltage averaging method is obtained. It is expected.

EFFECTS OF THE INVENTION The present invention focuses on the frequency dependence of optical characteristics that generally occurs in a normal TN cell, modifies a signal waveform based on the conventional voltage averaging method, and a signal waveform in which a high frequency component is superimposed in a non-selection period. Therefore, better visibility can be expected with the same number of scanning lines without increasing the cost on the panel or driving driver, and the display information amount of the matrix panel for high information display can be expected. It contributes to the increase and the improvement of visibility.

[Brief description of drawings]

1 (a) to (d) are diagrams for explaining a conventional voltage averaging method, and FIGS. 2 (a) to (g) are diagrams for explaining a conventional two-frequency method, and FIG. Is a diagram showing the frequency dependence of the dielectric anisotropy of a liquid crystal material suitable for use in a drive panel of the dual frequency method, FIG.
FIG. 5 is a diagram showing frequency-dependent characteristics of optical characteristics of the liquid crystal cell when the liquid crystal cell has a series resistance component, and FIG. 5 is an electrical equivalent circuit diagram of the liquid crystal cell for explaining the characteristics of FIG. R ₁ ... series resistance, R ₂ ... equivalent parallel resistance of the liquid crystal layer,
C: Equivalent parallel capacitance of the liquid crystal layer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshio Tachimichi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Sadakazu Yamagami 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co.

Claims (1)

[Claims] 1. N scan electrodes and M signal electrodes (N, M).
Is a positive integer) for each scan electrode and each signal electrode of the liquid crystal matrix panel having a predetermined first period and a second period.
A driving method of the liquid crystal matrix panel, wherein each voltage of a period is applied n times (n is a positive integer), wherein a voltage V is applied to a selected scanning electrode of each liquid crystal cell of the liquid crystal matrix panel in the first period. ₀ (V ₀ is a real number)
To the scan electrodes that are not selected Repeatedly applying a high frequency voltage, voltage 0 is applied to the signal electrode in the on-cell and voltage (2 /
a) V ₀ is applied respectively, and in the second period, voltage 0 is applied to the selected scan electrode of each liquid crystal cell of the liquid crystal matrix panel, and voltage V ₀ and (1-2 / a) V ₀ are applied to the non-selected scan electrodes. And the signal voltage V ₀ is applied to the signal electrode in the on-cell and the voltage (1-2 / a) V is applied in the off-cell.
_A method of driving a liquid crystal matrix panel, characterized in that the liquid crystal matrix panel is driven by applying ₀ respectively.