JPS6258512B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a driving method for a liquid crystal display device that enables high duty.

[Prior art]

Conventionally, liquid crystal display devices have been widely used in desktop electronic calculators and electronic clocks, but in recent years, as microcomputers have become lower in cost and higher in performance, small, thin, and low power consumption liquid crystal display microcomputers and portable microcomputers have become popular. Applications to terminals have become possible. For such uses, the panel area is large and it is necessary to display many characters and pictures at the same time. For example, for word processors, it is necessary to simultaneously display 20 lines of 64 alphanumeric characters. Multiplex drive is essential at this time, but the drive duty at this time is 1/160. On the other hand, the response speed of the liquid crystal itself is slow, and as a result, the duty that can be driven is currently limited to 1/16 using the standard voltage averaging method, which is an order of magnitude worse than the requirement. Therefore, as a means to improve this driving duty, a method has been proposed in which the liquid crystal is driven via a nonlinear element or a switching element to increase the driving margin. Nonlinear elements include metal-insulator-metal elements, varistor elements, diode elements, etc., and switching elements include thin film transistors made of compound semiconductors and amorphous silicon. Figure 1 shows the typical electrical characteristics of a nonlinear element, and the rising voltage of the current near use is called the threshold value Vth. FIG. 2 is a schematic diagram of a matrix display using this nonlinear element. Write selection signals T ₁ to _{T n} during time division driving called timing lines
and n×m display dots connected to data signals D ₁ -D _o for writing display data into selected cells, respectively. Here, nonlinear element 1 and liquid crystal element 2
are arranged in series between the timing line and the data line to form a display cell 3. FIG. 3a shows the configuration of one cell, and b shows its equivalent circuit. Here R _LC , R _NL
are the equivalent resistance of liquid crystal and nonlinear element, C _LC ,
C _NL is the equivalent capacitance of the liquid crystal and the nonlinear element, respectively, and the nonlinear element is expressed as a variable resistance R _NL . FIG. 4 shows respective waveforms of applied driving waveforms based on the conventional voltage averaging method in order to explain the operating principle. During the selection time, that is, the write time tw, write signals are written to the liquid crystal from the data line Di according to ON and OFF, respectively. When ON, the potential difference between Di and Ti is large, and when OFF, it is small,
The equivalent capacitance C of the liquid crystal via the equivalent resistance of the nonlinear element
A voltage is applied to _{the LC} . When this selection is made, R _NL <R
It has become _LC . Next, when the writing is finished,
The potential between Di-Tj becomes smaller, so R _NL
> R _LC , and the written voltage is stored as a charge in C _LC and discharged according to the time constant of R _LC・C _LC , but this time constant is usually 10 m sec or more, so it will be discharged again within 10 m sec. When a write operation is performed, the liquid crystal operates as if a certain potential were always applied even during the non-selection period. Regarding the effective value of the voltage V _LC applied between the electrodes of the liquid crystal at this time, the effective value V _ONR when ON is higher than the lighting voltage of the liquid crystal,
Also, if the effective value V _OFFR when OFF is less than the non-lighting voltage, ON-OFF control of the liquid crystal can be performed successfully and a display suitable for the purpose can be obtained. In the commonly used matrix display that does not use such elements, a voltage is applied only when a selection is made, but by using nonlinear elements in this way, the voltage applied when a selection is made can be maintained even when the selection is not made. This is the principle by which drive duty can be improved. The operating principle of the switching element is almost the same. FIG. 5 shows an example of the operation when a drive waveform based on the conventional voltage averaging method is applied to such a matrix display. To drive the liquid crystal with alternating current, the polarity is reversed every half of the frame period T _F . Here, sections A and B indicate a case where the light is lit or not lit only during the selected period, and sections C and D indicate a case where the light is lit or not lit during both the selected period and the non-selected period. Drive is V-
Since it is a 5V system, the selection level in the timing signal is 5V _L or 0, the non-selection level is V _L or 4V _L , the lighting level in the data signal is 0 or 5V _L , and the non-lighting level is 2V _L or 3V _L. It is.

Here, since the writing voltage V _N at the time of selection actually applied to the liquid crystal is a lighting signal, it is slightly smaller than the voltage Vth lower than the applied voltage 5V _L. That is, V _N
5V _L −Vth. Next, at the _same time as switching to non-selection, the signal on the data line changes to a non-lighting signal, so the potential _of V _M in FIG. _L −Vth)
Since this is larger than the Vth of the nonlinear element, the nonlinear element has a low resistance and is easily discharged toward the nonlinear element. Therefore, when only one cell is lit and the other cells are not lit, the effective ON value V _ONR becomes quite small as shown in the figure. On the other hand, if only one cell is not lit and the other cells are lit, the data line has a Di
(OFF) is applied, the write voltage V _F is
V _F =3V _L −Vth. After this, when switching to non-selection, the voltage applied to the nonlinear element becomes V _F - V _L , that is, (2V _L - Vth), which is lower than Vth, so the nonlinear element becomes high resistance, and only the liquid crystal discharges. As shown in the figure, the write voltage is maintained, so the effective value is high. As a result, V _ON in sections A and B
_R /V _OFFR is smaller than 1, which is a reversal phenomenon, that is, the lamp does not turn on when it should be turned on, but it turns on when it is not lit, which is really bad.

On the other hand, in sections C and D, all cells are in a lit or non-lit state. First, when lighting, up to V _N is written as before, but even if the data line is switched to non-selection, the lighting signal is maintained, so the voltage applied to the nonlinear element (5V _L −Vth − V _L = 4V _L −
Vth) and since this voltage is lower than Vth, the nonlinear element maintains a high resistance and therefore C _LC (ON)
remains at a high voltage for half a frame. On the other hand, when the lights are off, there is not much difference between the A and B sections. Therefore, in this case, the ratio of the effective value V _ONR when the lamp is on and the V _OFFR when the lamp is not lit is larger than 1, and the lighting and non-lighting can be well controlled.

[Problems with conventional technology]

As mentioned above, in the conventional liquid crystal driving method using non-linear elements, if the data signal is lit or not lit only when selected, or if the ratio of the number of lit and non-lit signals is different, the data signal is selected or not selected. Full lighting of the hour,
There is a problem in that it is difficult to obtain a perfect display because the effective value applied to the liquid crystal changes compared to when all lights are off.

The present invention overcomes the above problems, and by increasing the effective value when the light is on and the effective value when the light is off, it is possible to always obtain stable lighting and non-lighting regardless of the display method. Accordingly, it is possible to provide a liquid crystal display device that can be used.

〔Example〕

Here, let us consider the effective voltage applied to the liquid crystal during lighting in the conventional driving method. In Fig. 5, the effective value of V _ON during lighting is in sections A, B, C,
There is a big difference in section D. This is because in the A and B sections, the potential of the data line turns on only the selected pixel and the others are not lit, while in the C and D sections, the selected pixel and non-selected pixels are also in the lit state. Even if the data is written up to V _N , at the beginning of the holding period until the next write, the voltage applied to the nonlinear element is (V _N +V _L ) in the A, B, and C periods.
In sections C and D, the equivalent resistance of the nonlinear element is as low as (V _N -V _L ), and therefore the equivalent resistance of the nonlinear element is lower in sections A and B, so that the written charge is discharged through the nonlinear element. By putting it away, A and B as shown in the diagram
In this section, the effective value at the time of lighting decreases. On the other hand, when the light is not lit, at the beginning of the holding section, it becomes (V _F + V _L ) in sections A and B, and (V _F - V _L ) in sections C and D, and the value of V _F is low, so compared to when it is ON. The resistance of the nonlinear element is always high, and the effect of discharging or charging the stored charge is small. Therefore, in the drive waveform based on the conventional voltage averaging method, regarding the equivalent resistance R _E of the nonlinear element in the holding interval, the resistance R _E (V _N +V _L ) when only the nonlinear element is lit and the others are not lit is as follows: Since the resistance R _E (V _F −V _L ) is smaller than the resistance R E (V F −V L ) when all pixels are not lit, the effective voltage during lighting during the liquid crystal retention time is reduced.

FIG. 6 shows the basic drive waveform of the present invention. In this embodiment, the potential difference V _NL when the data signal Di is lit and the potential difference V _FL when it is not lit are set to different potential differences with respect to the non-selection potential of the timing signal Tj, and the potential difference between these data lines is It is shifted to the opposite side of the selection potential of the timing line. The simplest example of this drive method is V _FL
= 0.

FIG. 7 shows an example of the drive waveform in this case. The pixel voltages for these timing signals and data signals are as shown in the figure. At this time, the equivalent resistance of the nonlinear element is R _E (V _N ) R _E (V _F - V _O ), and it becomes ON.
The ratio of the effective value of OFF can be quite large.

Here, the relationship between the voltage applied to the nonlinear element and Vth will be explained.

FIGS. 9a to 9d are diagrams for explaining this relationship. First, FIGS. 9a and 9b show the relationship between the voltage applied to the nonlinear element and the threshold value V _th of the nonlinear element in a conventional liquid crystal driving method. First, in FIG. 3A, the voltage waveform V _M at the point V _M shown in FIG. 3 is shown. Here, it is assumed that at the time of selection, the data signal Di is lit at ₁ level. Next, the level of the data signal Di during the non-selection period is compared between the lighting level ON ₂ and the non-lighting level OFF. Consider an arbitrary point 0,0' after switching from selection to non-selection. When the non-selection period is at the lighting level ON ₂ , the potential difference actually applied to the nonlinear element at the 0 point is A. On the other hand, when the non-selection period is at the non-lighting level OFF, the potential difference actually applied to the nonlinear element at the 0' point is B. When applying each potential difference A and B in figure a to the VI characteristic diagram of the nonlinear element in figure b, it is clear from figure b that the potential difference A is smaller than the threshold level Vth of the nonlinear element. , the completely nonlinear element remains in the OFF state. However, the potential difference B
exceeds this threshold value Vth, so leakage occurs from the nonlinear element.

In this way, conventionally, after the lighting state in the selection period, there is a large difference in the voltage applied to the nonlinear element depending on whether the non-selection period is continuous lighting or non-lighting. Since the threshold voltage is exceeded, the resistance of the nonlinear element decreases and leakage occurs.

Next, the same explanation regarding the driving method of this embodiment will be given based on FIGS. 9c and 9d. First, in c of the same figure, at the time of selection, the data signal Di is at the lighting level _ON1 . Next, the level of the data signal Di during the non-selection period is compared between the ON ₂ level of lighting and the OFF level of non-lighting, as in the conventional case. However, as described above, this embodiment differs from the conventional one in that ON ₂ has a potential difference V _NL and OFF has a potential difference V _FL with respect to the non-selection level. Consider an arbitrary point 0,0' after switching from selection to non-selection. Lighting level during non-selection period
In the case of ON ₂ , the potential difference applied to the nonlinear element at the 0 point is A', and on the other hand, when the non-selection period is at the non-lighting level OFF, the potential difference applied to the nonlinear element at the 0' point is B'. This potential difference A′,
When B' is applied to the nonlinear element VI characteristic diagram shown in FIG. d, both potential differences A' and B' have values lower than the threshold value Vth of the nonlinear element.

As described above, in this embodiment, after the lighting state in the selection period, the voltage applied to the nonlinear element is always below the threshold value, regardless of whether it is continuously lit or not in the non-selection period. Since the completely OFF state is maintained, sufficient effective voltage can be maintained in the liquid crystal.

Here, when the threshold value V _TH of the nonlinear element is low, when the frame switches, that is, when the polarity is reversed, the effective value decreases a little at the time of switching, as shown in Figure 7, V _LC (ON). do. This means that the resistance value of the nonlinear element varies from R _E (V _N ′) to R _E (V _N ′+V
_G ) by changing to To prevent this, frame inversion is performed in long cycles. The frame reversal operation is originally performed to extend the life of the liquid crystal. Therefore, if the polarity is uniformly reversed over time, it is sufficient to lengthen the cycle and write to the same polarity many times. The thing to be careful about at this time is that if the inversion cycle becomes too long, the afterimage effect on the eyes will disappear.
The flashing will be distorted and the quality of the display will be degraded.

FIG. 8 shows a waveform as a further specific example of the present invention, which makes it easier to control ON-OFF without reducing the effective value during ON by performing multiple write operations. Write operations of the same polarity are performed n times during a half period of the frame period Tf. By doing this, by reversing the polarity every time you write,
It is possible to prevent the effective value from decreasing at the time of frame switching. At the frame polarity switching point (point P), large flashing appears to the eye, but it is hidden by flashing caused by fine writing operations during that time and is not perceived visually. Therefore, long-period driving of several seconds to several hours is also possible.

〔effect〕

As described above, the present invention includes a pair of substrates filled with liquid crystal, and a plurality of pixel electrodes, timing lines, and data lines arranged in a matrix on the substrates.
A nonlinear element is connected to the pixel electrode, a data signal is applied to one electrode of the pixel electrode from a data line via the nonlinear element, and a timing signal is applied to the other electrode of the pixel electrode via a timing line. In a liquid crystal display device in which a voltage is applied to the data line, a potential difference when the data line is lit is different from a potential difference when the data line is not lit based on the non-selection potential of the timing line, and the potential difference of the data line is opposite to the selection potential of the timing line. Since the data signal is shifted to the side, the effective value when the data signal is lit can be significantly increased without increasing the effective value when the data signal is not lit, so it does not depend on how it is displayed. This makes it possible to control stable lighting and non-lighting at all times, making it possible to realize high-resolution liquid crystal displays.

[Brief explanation of the drawing]

Fig. 1 is a characteristic diagram of a nonlinear element, Fig. 2 is an overall diagram of a liquid crystal display device arranged in a matrix, Fig. 3 is an equivalent circuit diagram of a liquid crystal display device, and Fig. 4 is a diagram of a conventional liquid crystal display device. 5 is an operation waveform diagram of a conventional liquid crystal display device, FIG. 6 is a diagram showing driving waveforms of one embodiment of the liquid crystal display device of the present invention, and FIG.
8 is a waveform diagram showing another embodiment of the present invention, FIG. 9 is a waveform diagram showing another embodiment of the present invention, and FIG.
- d are diagrams showing a comparison between the present invention and a conventional example.

Claims (1)

[Claims] 1 A liquid crystal is sealed in a pair of substrates, a plurality of pixel electrodes, timing lines, and data lines are arranged on the substrates in a matrix, a nonlinear element is connected to the pixel electrode, and the nonlinear element In a liquid crystal display device in which a data signal is applied from a data line to one electrode of the pixel electrode via a timing line, and a timing signal is applied to the other electrode of the pixel electrode via a timing line, A liquid crystal characterized in that a potential difference when a data line is lit is different from a potential difference when the data line is not lit based on a selection potential, and the potential difference of the data line is shifted to the opposite side of the selection potential of the timing line. Display device.