JPH0449712B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION In recent years, liquid crystal electro-optical devices have become extremely popular as display devices for optical switches, calculators, and electronic watches. Furthermore, there is a growing demand for use as a display device for small personal computers and the like. However, with current liquid crystal materials, the duty number that can be driven by multiplex is at most about 1/30, and they lack the ability to display a large amount of information. Display methods devised from this background include: Addressing methods that use nonlinear elements as switching elements Γvaristor address Γmetal-insulator-metal (MIM) element address Γdiode address Γdischarge tube address Active element as a switching element Addressing methods used as: Γ thin film transistor address Γ MOS transistor address Γ triax address Light/thermal/writing method Γ laser thermal writing Γ photoconductor writing Liquid crystal display devices with a large amount of display information, such as dual-frequency addressing methods, have been actively developed. There is.

The present invention relates to a method of driving a liquid crystal electro-optical device using a nonlinear element and an active element of the liquid crystal electro-optical device used as a switching element as a switching element.

More specifically, the present invention relates to a multiplex driving method for suppressing fluctuations in effective voltage applied to pixels of the liquid crystal electro-optical device.

Figure 1 shows a typical MIM among nonlinear elements.
It shows the current-voltage characteristics of the element. Palista,
A reverse series connected diode that utilizes the breakdown voltage of a Pn junction also has a nonlinear current-voltage characteristic similar to that shown in FIG. 1. Further, any element having nonlinear characteristics such as the one shown in FIG. 1, which exhibits high resistance in the low voltage region and low resistance in the photovoltage region, can be used as the switching element.

It is known that when a liquid crystal electro-optical device is constructed using these nonlinear elements, multiplex driving with a higher order of magnitude than general multiplex driving becomes possible. This can be understood as follows.

FIG. 2 is an equivalent circuit diagram for one pixel electrode, which is composed of a liquid crystal capacitance C _LC , a resistance R _LC , an equivalent capacitance C _NL of a nonlinear element, and an equivalent resistance R _NL .
R _NL is determined by the voltage applied to the nonlinear element.
It has low resistance at high voltage and high resistance at low voltage. now,
Consider applying a liquid crystal drive signal to the terminal of the equivalent circuit.

3A to 3C are examples of the 1/50 duty and 1/5 bias method. FIG. 3A shows scanning electrodes 5-1 to 5-
10, a matrix display pixel consisting of signal electrodes 6-1 to 6-50 is shown. Each scanning electrode 5-
1 to 5-50 respectively have scanning signals SCAN1 to SCAN1 to 5-50.
SCAN50 is applied. In addition, each signal electrode 6-1
~6-50 are display signals SIG1~SIG5, respectively.
0 is applied. Here, the display pixels M and N corresponding to the scanning electrode 5-M and the signal electrode 6-N are turned on,
Consider a case where other display pixels (plurality) on the column of signal electrodes 6-N are not lit. The waveform of the scanning signal in this case is shown in FIG. 3B, and the waveform of the display signal is shown in FIG. 3C. In these figures, ts is a scanning period during which a signal is applied to the entire display pixel, and tsel in B of the figure is a selection period of the scanning signal SCANM during which the scanning electrode 5-M is selected. According to C in the same figure, during this selection period tsel, the signal electrode 6-N
has a lighting level V _ON , and has a non-lighting level V _OFF during periods of other scanning signals except SCANM. Therefore, the voltage applied to display pixels M and N is V(M, N)=SCANM− as shown in FIG.
Given by SIGN. The solid line in Figure 4a is the waveform V(M,N) of the applied voltage when the display pixels M and N are at the lighting level, and the solid line in Figure 4b is the voltage of the nonlinear element. The solid line in _c in the same figure shows the voltage waveform V _LC applied to the liquid crystal layer.
That is, the following relationship exists.

V(M,N)=V _NL +V _LC Furthermore, the broken lines shown in a, b, and c of the figure indicate the case where the display pixels M and N are at the non-lighting level.

Next, the concept of operation of the nonlinear element and the liquid crystal layer will be schematically explained based on FIGS. 5a, b, and c. Fifth
Figure a shows the relationship between the applied voltage V _NL and the current value of the nonlinear element, and the nonlinear element has low resistance in the region of 7.
In the region 8, the nonlinear element has high resistance.

Figure b shows the current flow i when the resistance R _NL 4 of the nonlinear element is a low resistance (approximately 0),
Figure c shows the flow of current when the resistor 4 has a high resistance (assumed to be almost infinite).

First, as shown in FIG. 2B, when the nonlinear element enters a low resistance region, a driving voltage is applied to the liquid crystal layer and the liquid crystal layer is charged.

The time constant at this time is expressed by the following equation from the equivalent circuit shown in FIG.

_τ =(C _{LC + C NL} ) _× R _LC C Charge _{the LC} i. At this time, all voltage is applied to the liquid crystal layer.

Next, when the surface pixel V (M, N) enters the non-selection period, it shifts from the low resistance region 7 to the high resistance region. Therefore, R _LC << _{R NL} , and most of the transiently flowing current i flows through R _LC as shown in FIG. 5C. Approximately, the time constant is given by τ=(C _LC +C _NL )R _LC ……(2). In general, it is fully possible to use it in field effect type liquid crystal display panels.

When the display pixels indicated by the broken lines in FIGS. 4a, b, and c are at the non-lighting level, even when V _NL is at its peak, it does not enter the lighting region, so the liquid crystal layer is not charged.
V _LC remains low.

Therefore, the effective value ratio of the lighting level to the non-lighting level of the liquid crystal layer is larger than that of a conventional driving method using a voltage averaging method that does not include a nonlinear element. Therefore, a liquid crystal driving method using a non-linear element enables multiplex driving with a higher number of orders of magnitude, and can realize a larger capacity of a liquid crystal display device.

However, the multiplex drive as described above has a drawback in that the effective voltage applied to the liquid crystal layer varies depending on the state of the display signal during the non-selection period, as described below. 6th
This drawback will be explained based on figures a, b and c.

In this way, non-linear element liquid crystal display devices are capable of increasing the display capacity, but with multiplex drive, the disadvantage is that the effective voltage applied to the liquid crystal layer varies depending on the display signal during the non-selection period. There is. a is the voltage waveform V _LC when only the display signal electrode row 5-M out of the signal electrode row 6-N is lit; b
is the voltage waveform V _L e,c when every other row of the columns 6-N is lit, and is the voltage waveform L _LC when all the columns 6-N are lit. Broken lines indicate display pixels M and N
The solid line is the voltage V(M,N) applied to the liquid crystal layer, and the solid line is the voltage _VLC applied to the liquid crystal layer. Looking at a, b, and c, it can be seen that V _LC varies significantly depending on whether other pixels on the same signal electrode (SIG) are turned on or off. The same thing applies even when the display pixels M and N are not lit.

For this reason, conventionally, the minimum value of the effective voltage of the lighting waveform
E _ON min is larger than the liquid crystal saturation voltage Vsat,
The maximum value E _OFF max of the effective value of the non-lighting waveform was set to be smaller than the threshold value Vth on the liquid crystal display, and a binary display was made. For this reason, non-linear element liquid crystal display devices have been applied only to binary display, and gradation display has been considered impossible. Also, the above E _ON min,
When using E _OFF max as a margin, the required characteristics of the nonlinear element are strict, which poses a manufacturing difficulty. Furthermore, when the saturation voltage is not clear as in the case of a guest-host effect, there is a problem in terms of display quality in that variations in effective values are displayed as variations in contrast.

The present invention was made in view of these drawbacks, and was devised for the purpose of application to gradation display, prevention of contrast variation, and increase of margin by suppressing fluctuations in effective voltage due to display signals. Therefore, if E _ON min and E _OFF max are brought close to an intermediate level, variations in the effective voltage can be suppressed. Another object of the present invention is to divide one scanning period into a plurality of selection levels and non-selection levels, thereby making the discharge of the liquid crystal constant when the switching element is in the OFF state. Therefore, the driving method of the present invention can be applied not only to nonlinear elements but also to active switching elements (TFT, MOS transistor) as switching elements.

Next, the present invention will be explained based on examples.

FIG. 7 compares a conventional driving method B for driving a display panel A consisting of display pixels in a matrix with a driving method C according to the present invention. Same figure A
In the display pixel column 6-O, only the pixel row 5-M is lit, and the display pixel column 6-N is the case where every other pixel row is lit; P is a case where all pixel rows are lit.

In FIG. 7, the 1/50 duty and 1/5 bias method is considered, as in the explanation of the conventional example. In the normal voltage averaging method shown in Figure B, the scanning period T _S is divided in half for AC drive, and then further divided by 50 digits to make a total of 100 divisions (this 1 unit period is 1 scanning period 9). call). Each scanning signal SCAN takes the selection level for one scanning period Tsel once in each half-scanning cycle, and takes the non-selection level for the other scanning periods.

In contrast, in the driving method of the present invention shown in FIG.
Tsel is further divided into a plurality of periods (these are referred to as fine scanning periods), and a scanning signal is created in which only some of the fine scanning periods are at a selection level and the rest are at a non-selection level. The method of dividing one scanning period into fine scanning periods can be selected from various ratios, unequal intervals, and equal intervals, but for the sake of simplicity, the case of equal division into 1/2 will be described.

In the case of this 1/2 equal division, SCANM is the Mth scanning signal, and the apparent duty is 1/10 of half.
It is the same as a 0 duty scanning signal. or,
The display signals applied to the display pixel rows 6-O, 6-N, and 6-P are shown as SIGO, SIGN, and SIGP, respectively. Each display signal is similar to the scanning signal SCANM.
Similar to the scanning signal, one scanning period is divided into multiple parts (in this case, halves), and only some narrow scanning periods (in this case, the first half scanning period) are set to the same level as in the normal voltage averaging method, and the remaining The fine scanning period is made to have opposite levels, that is, the selected level and the non-selected level, and the non-selected level and the selected level. As a result, display pixel rows 6-O, 6-
The display signal waveform applied to each of N and 6-P is
They are SIGO, SIGN, and SIGP. In drive method C of the present invention, the waveform is centered around the reference level and is 1/100
Although the waveform changes appear to be the same as in the case of duty, if we consider the average value of the non-selection period within the half-scanning period, it can be seen that the average value is almost the same in both cases.

8a, b, and c show display pixels (M, O) (M, N) (M,
The voltage applied to the liquid crystal layer V _LC (shown as a solid line) with respect to the voltage applied to each of P) (shown as a broken line) is shown. When compared with the case of the conventional drive method shown in Fig. 6, the V _LC waveform according to the present invention shown in Fig. 8 depicts a discharge waveform that is almost the same except for slight fluctuations caused by the display signal. I understand that. This means that the present invention suppresses fluctuations in the effective voltage of the liquid crystal layer due to display signals.

In this way, the effective voltage applied to the pixel is determined without being affected by ON-OFF on the same signal electrode, so by modulating the peak level of the selection period, the time to take the selection level, and the peak level, it is possible to Gradation display, which was considered impossible with element liquid crystal display devices, is now possible, and the maximum effective voltage in the OFF waveform and the minimum effective voltage in the ON waveform, which were the voltage margins of conventional nonlinear element liquid crystal display devices, have been reduced. , according to the present invention, is provided between specific levels of the OFF waveform and ON waveform, and has the advantage that the margin is expanded. However, in the present invention, even when N-digit multiplex driving is performed, the actual multiplex driving is 2N-digit multiplex driving. Increasing the number of digits was not possible in conventional matrix panels because it would reduce the margin, but in the case of non-linear element liquid crystal display devices, sufficient The equivalent capacitance of the liquid crystal layer up to the level
As long as the C _LC has time to charge, there is no problem even if the number of driving digits increases. In fact, this charging period can be considerably shortened, and depending on the characteristics of the nonlinear element,
A duty of several thousandths is also possible.

In the above embodiments, one scanning period is divided into 1/2 equal parts, but the present invention is not limited to 1/2 equal parts. The point is that there is only time to charge the _CLC to a sufficient level within the fine scanning period in which the voltage reaches its peak voltage value, and there are multiple fine scanning periods in which the voltage reaches its peak voltage within one scanning period. Alternatively, the scanning period may be a narrow scanning period obtained by dividing the scanning period at irregular intervals. However, considering the ease of the drive circuit and the small variation in effective voltage, 1/2 equal division is considered to be the best.

Also, the time when the scanning signal is set to the selected level is ON.
One scanning period does not necessarily have to be a period obtained by equally dividing one scanning period ts into 2N, since it is sufficient that the _CLC is charged to a sufficient level within the waveform selection period. That is, a period less than one scanning period ts
One scanning period can also be a period obtained by dividing xts (0<x≦1) into 2N equal parts.

Figure 9 shows the scanning signals SCAN1 and SCAN8 and the display signal when using the 1/5 bias method, N=8, and x=0.8.
This shows SIG1. In Fig. 9, the display time t _D
is a set of eight scanning periods, the rest period t _P is a time to which no scanning period belongs, and all signal electrodes are also at the non-selection level during this t _P. In this case, _tP
During the period, the display signal always has the non-selected waveform in the present invention, but it may also have the selected waveform.

Further, the selection level and non-selection level described in the present invention are not limited to the selection level and non-selection level of the conventional drive system.

Next, a circuit for generating a drive signal according to the present invention will be described.

FIG. 10 is a panel and panel drive circuit diagram of the liquid crystal display device of the present invention, which is composed of a non-linear element dot matrix liquid crystal panel, a display electrode driver section, a scanning electrode driver section, and a drive signal generation section. A liquid crystal panel is composed of scanning electrodes and display electrodes. The display electrode driver section has J
A stage shift register and J latch circuits connected to each output of the shift register, a level shifter that converts the logic level of the circuit to a liquid crystal display level, and a signal from the level shifter controls the lighting level and non-lighting level of the display signal. It consists of M demultiplexers that change levels. The scanning electrode driver section has the number of scanning electrodes K.
In other words, it is composed of a 2N stage shift register, a level shifter, and K demultiplexers 23 that select or deselect scanning signals according to signals from the level shifter. The drive signal generating section 14 is composed of a demultiplexer ~〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓]

FIG. 10 will be explained in detail using the timing charts of FIGS. 11 and 12a and 12b.

_φS in FIG. 11 is a transfer clock of the shift register, and display data DATA is transferred from left to right by _φS . When J pieces of data for one line are transferred, the clock pulse of the latch circuit
CLl becomes high level and data is latched from the shift register to the latch circuit. The data is level-shifted and input to the control terminal of the multiplexer.In the multiplexer, the display signal from the drive signal generator 14 is input by the display data DATA signal.
DON and DOFF can be switched. As shown in Figure 11, the shift register of the scanning electrode driver 13 transfers the pulses synchronized with the latch clock pulse _CLC and the display data DATA by J/2 units (in the case of an odd number of J, the maximum transfer rate does not exceed J/2). An integer or the smallest integer larger than J/2), the scanning clock CL _SC of the logical sum of the pulses generated when the clock pulse is completed,
Data Dscan, which becomes high only once in one cycle, is input as data. In the shift register, only the odd-numbered stages out of the 2K stage outputs are connected to the level sister. Therefore, the outputs SC ₁ , SC ₂ , SC _{3 ,} etc. of each odd stage of the shift register are as shown in Figure 12-a. The signal passes through a level sister,
Supplied to the demultiplexer. In the demultiplexer, the scanning signal selection signal SCON and non-selection signal SCOFF are switched by the signal DSCAN.

The resistors 〓〓〓〓～〓〓 are made by dividing the voltage between -V and 5V from the -5V value. The demultiplexer, 〓〓, switches the level of the scanning signal according to the frequency signal φf of the AC drive of the liquid crystal, selects the scanning electrode SCON, and deselects the scanning electrode.
Create SCOFF signal. Demultiplexer〓〓〓〓〓〓〓〓
are the selection signal DSEL and non-selection signal in the conventional drive method of the display electrode according to φf.
Create DNSEL. 〓〓, 〓〓 are demultiplexers required in the present invention, and in the multiplexers 〓〓, 〓〓, the signal 1/2CL _SC , which is the frequency of CL _SC divided by 1/2, is
DSEL and DNSEL are switched by . In this way, the display electrode signals DON and DOFF (FIG. 12b) are formed.

FIG. 13 is an example of a control circuit that generates the necessary clock pulses for the drive circuit of the present invention. In this embodiment, J=160, K=120, 6-bit binary counter, NOR gate, RS flip-flop, inverter, D-type flip-flop, NOR gate〓〓〓、〓
〓、
It consists of a 6-bit binary counter and an AND gate. Clock pulse φs supplied to the shift register of the display electrode driver 12
is counted by a 6-bit binary counter 〓〓, and when J/2 = 80 counts, the gate detects this and sets SR-FF. RS-FF is
It is immediately reset in synchronization with the rise of φs.
The output CL _SC is input to the RESET terminal of the counter 〓 〓 and also becomes the input of the D type FF 〓 〓. The D clip flop divides the frequency of CL _SC by 1/2, creates a signal 1/2 CL _SC , and supplies it to the D input of the D type FF. signal 1/2
CL _SC is differentiated by D-type FF〓〓NOR gate〓〓,
This becomes a signal CLe having a period of one scanning period, and is supplied to the clock pulse input terminal of the latch circuit.
The counter uses the clock as CL _SC and counts the clock pulses CL _SC from the RS flip-flop 32. When it counts 239 pulses, the output of the AND gate becomes high. This high signal is delayed by the D-type FF, and the gate It is differentiated by 〓. The signal delayed by the DFF 39 becomes a delayed signal D _SCAN and is supplied to the shift register 21 of the scan electrode driver. Also, the pulse differentiated by the gate 〓〓 is D
It becomes a clock pulse of type FF〓〓, and every half period
It is supplied to the demultiplexers 24 to 26 of the drive signal generation section as an AC drive signal φf whose high and low levels change.

As described above, in the driving method of the liquid crystal electro-optical device of the present invention, the absolute value of the average value of the voltage applied to the pixel electrode during the non-selection period within one frame period is made almost equal for most of the pixel electrodes. , the minimum value of the effective voltage applied to the liquid crystal in the lighting waveform E _ONnio and the maximum value of the effective voltage applied to the liquid crystal in the non-lighting waveform
This brings E _ONnax closer to an intermediate level, making it possible to apply it to gradation display, prevent variations in contrast, and increase margins. In other words, it is possible to provide a liquid crystal electro-optical device that can display uniform gradations over the entire panel image.

[Brief explanation of the drawing]

FIG. 1 shows the characteristics of a typical nonlinear element. FIG. 2 is an equivalent circuit of a non-linear element liquid crystal display device. FIG. 3 is an example of driving waveforms for a liquid crystal panel using a conventional nonlinear element. FIG. 4 shows the operation of the non-linear element liquid crystal display device. a is an example of an applied waveform, b is a voltage waveform applied to the nonlinear element, and c is a voltage waveform applied to the liquid crystal layer. FIG. 5 schematically shows the operational concept of a non-linear element liquid crystal display device. a indicates the ON region and OFF region of the nonlinear element,
b indicates when the nonlinear element is turned on, and c indicates when it is turned off. FIG. 6 shows a pixel applied voltage waveform (broken line) and a liquid crystal layer applied waveform (solid line) of a non-linear element liquid crystal display device. a is pixel ON at the time, other pixels on the same signal electrode
When OFF, b is alternately ON, when OFF, c is when all pixels are ON. FIG. 7 depicts a comparison between an example of the drive waveform according to the present invention and a conventional drive waveform. A is a display example of a liquid crystal panel, B is a conventional example, and C is an example of the present invention. The area inside the box is the voltage waveform applied to the pixel at the intersection of the scanning electrode and the signal electrode.
In SIGO, the pixel was ON at that time, and other pixels on the same signal electrode
Signal waveform when OFF, SIGN is alternately ON,
The signal waveform in the case of OFF, SIGP is the signal waveform in the case that all pixels on the same signal electrode are ON. FIG. 8 shows the voltage waveform applied to the liquid crystal layer when the present invention is applied. a is the relevant pixel ON, other pixels on the same signal electrode
When OFF, b is mutually ON; when OFF, c is when all pixels are ON. FIG. 9 is an example of a drive waveform when a pause period _tP according to the present invention is provided. FIG. 10 is a driving circuit diagram of the liquid crystal display device of the present invention. FIG. 11 and FIGS. 12a and 12b are timing charts. FIG. 13 is an example of a control circuit of the drive circuit of the present invention.

Claims (1)

[Claims] 1. Liquid crystal electro-optics that drives a liquid crystal display panel in which a liquid crystal is sandwiched between a pair of transparent insulating substrates and a plurality of nonlinear elements and a plurality of pixel electrodes are formed on at least one of the transparent insulating substrates in a two-frame AC mode. A method for driving a liquid crystal electro-optical device, wherein the absolute value of the average value of the voltage applied to the pixel electrode during a non-selection period within one frame period is substantially equal for each pixel electrode. Method.