JPH0594152A - Method for driving liquid crystal panel and driving circuit and display device thereof - Google Patents

Abstract

PURPOSE:To prevent the irregularity of display from occurring by dividing picture elements on a scanning electrode to plural groups, restricting the number of picture element inverted by a scanning selection pulse and setting a maximum charging time within the margin of pulse width. CONSTITUTION:The output terminal of a scanning driver LS16 is connected to a scanning electrode 4 and a signal driver LS17 is connected to a signal electrode 5 by connecting resistance through a flexible substrate. Then, scanning is executed by the driver LS16 based on the cycle signal of a timing signal generator 10 and corresponding data is sent to the driver LS17 from an image memory 8 through AND elements 9a and 9b. The scanning is executed twice by the driver LS16. At the first scanning time, '1' is sent to the AND element 9a corresponding to the picture element of the group A and the image data is delivered to a signal LSI. However, '0' is sent to the AND element 9b corresponding to the group B and only an off-voltage is impressed on the picture element. At the second scanning time, the image data is sent to only the group B and an image is written on a liquid crystal panel 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferroelectric liquid crystal panel, and more particularly to a driving method, a driving circuit and a display device for a liquid crystal panel having a large pixel size.

[0002]

2. Description of the Related Art Ferroelectric liquid crystals have a layered structure, and spontaneous polarization perpendicular to the long axis of liquid crystal molecules is aligned in the direction of an electric field to switch. FIG. 5 is a sectional view of the ferroelectric liquid crystal panel. The liquid crystal molecules 50 are always inclined at a constant angle with respect to the layer normal, and therefore the cone 51 is formed.
Move on the side of the cone indicated by. When a voltage is applied between the electrodes 52 and 53, the spontaneous polarization 54 points in the direction of the electric field (FIG. 5).
The upper electrode direction is aligned in (a) and the lower electrode direction is aligned in (FIG. 5) and (b). Since the liquid crystal molecules are aligned substantially horizontally on the electrode surface, the alignment states of (FIG. 5) (a) and (FIG. 5) (b) are maintained even if the electric field is removed. When the panel is sandwiched by the polarizing plates which are orthogonal to each other, and the polarization axis of the polarizer is aligned with the molecular long axis direction of (FIG. 5) (a) by the birefringence effect (FIG. 5),
(A) is displayed in black, and (FIG. 5) (b) is displayed in bright. When the thickness between the electrodes (thickness of the liquid crystal layer) is around 2 μm (FIG. 5) (b) is white display.

Since the direction of polarization is reversed in order to reverse the state of (FIG. 5) (a) to (FIG. 5) (b), a charge transfer of twice the amount of spontaneous polarization of the ferroelectric liquid crystal occurs.

FIG. 6 shows an example of a drive waveform diagram of a conventional ferroelectric liquid crystal. A scanning voltage 60 is applied to the scanning electrodes and a signal voltage 61 is applied to the signal electrodes. At this time, a pixel application voltage 62, which is the difference between the scanning voltage and the signal voltage, is applied to the pixels. After resetting to a black state (white state) by a reset pulse 65, a certain scanning electrode is selected by a selection pulse 66,
When the signal voltage at that time is the on-voltage 67, the selection voltage 69 exceeding the threshold voltage is applied to the pixel and is inverted to the white state (black state), and when the off-voltage 68, it is less than or equal to the threshold voltage. A half-select voltage 70 is applied to hold the reset state. (FIG. 7) is also a conventional example of the drive waveform of the ferroelectric liquid crystal, showing only the pixel applied voltage. In this case, scanning is divided into two fields, and writing (inversion or holding) is performed on white (black) in the first field and black (white) in the second field to write an image. In both cases (Fig. 6) and (Fig. 7), the threshold voltage of the ferroelectric liquid crystal panel changes depending on the pulse width, so that the threshold voltage of the panel should be between the selection voltage and the half selection voltage. Then, it is necessary to set the pulse width.

[0005]

The reversal current flowing when reversing the spontaneous polarization flows from the driving LSI through the scan electrodes and the signal electrodes, but when the pixel size is large, the panel length is long, or the liquid crystal is liquid crystal. When the spontaneous polarization of is extremely large, the amount of charge to be charged becomes large and the charging time becomes long. When all the pixels on the scan electrodes are inverted by the reset pulse, the charging time is constant, so
Although it suffices to lengthen the pulse width, there is no problem. However, when the scan electrode is selected, the amount of charged electric charge largely changes depending on the number of ON pixels, and therefore the appropriate pulse width differs depending on the pattern. Therefore, with a fixed pulse width, the contrast varies depending on the display pattern, resulting in display unevenness. The limit of the reverse current is the drive LSI, especially the drive LSI on the scanning side.
Output current, or the lead-out electrode resistance from the LSI output terminal to the scan electrode or the connection resistance of the mounting part,
Or a resistance value obtained by adding output resistances of the LSI and a current value obtained by dividing the output voltage, whichever is smaller.

[0006]

In order to solve the above problems, a liquid crystal panel driving method of the present invention is a liquid crystal panel driving method in which a ferroelectric liquid crystal is sandwiched between opposing scanning electrodes and signal electrodes. Pixels on an arbitrary scan electrode are divided into a predetermined number of groups, pixels in one group are selectively inverted by one scan, and in the scan, pixels in groups other than the one group are inverted. Is prohibited, and the pixels of each group are sequentially written by the predetermined number of times of scanning.

As another means for solving the problem, the driving method of the liquid crystal panel of the present invention is optional in the driving method of the liquid crystal panel in which the ferroelectric liquid crystal is sandwiched between the scanning electrodes and the signal electrodes which face each other. The pulse width is changed according to the number of pixels to be inverted on the scan electrode.

Further, the display device of the present invention includes a liquid crystal panel in which a ferroelectric liquid crystal is sandwiched between a scanning electrode and a signal electrode facing each other, a scanning driver and a signal driver for supplying a driving voltage from the scanning electrode and the signal electrode. The value obtained by dividing twice the total spontaneous polarization amount of the ferroelectric liquid crystal of the pixel sandwiched between the arbitrary scan electrodes of the liquid crystal panel by the maximum output current of one output terminal of the scan driver LSI is the scan. The problem is solved by connecting the plurality of output terminals of the scan driver LSI to the scan electrodes and supplying the same signal data to the plurality of output terminals, which is 20% or more of the selection time of the electrodes. is there.

[0009]

The charge Q required for reversing the spontaneous polarization is the area S of the pixel to be reversed and the spontaneous polarization Ps of the liquid crystal per unit area.
Is twice the product of When the maximum value of the charging current is Imax, the charging time τ = Ps · S / Imax.

The threshold voltage Von and the pulse width T at which the liquid crystal is inverted when the charging current can be sufficiently obtained are determined by the liquid crystal material. When the current is limited, the pulse width T must be further increased to T + τ. I have to. In the case of binary display matrix driving, the threshold voltage of the liquid crystal is sufficiently smaller than the selection voltage V1, and the half selection voltage (1-2 / a)
Since the contrast is provided when it is sufficiently larger than V1, there is a width (margin) in the driving condition where the contrast is constant. The bias ratio of a is preferably about 3 to 5. For example, if the bias ratio is 4, the half-selected voltage becomes 1/2 of the selected voltage, and the voltage margin becomes less than (1/2) V1. The actual margin, which depends on the steepness of the threshold characteristics and the drive waveform, varies depending on the panel configuration (liquid crystal material, alignment film, insulating film), but is about 20 to 40%. The margin can be expressed by the pulse width when the voltage is fixed. Therefore, when the charging time τ becomes larger than the margin ΔT of the pulse width T,
Uneven contrast occurs.

The area of the pixel to be inverted varies depending on the display pattern, and in the conventional driving method, the case where all the pixels on the scan electrode are inverted is the maximum, and the case where there is no pixel to be inverted is the minimum. Therefore, in the liquid crystal panel driving method of the present invention, the maximum value of the charging time τ is suppressed and the display unevenness is prevented by limiting the number of pixels to be inverted by one selection. Specifically, the pixels on the scan electrodes are divided into several groups, and 1
With respect to the number of selective scanning pulses, only one group of pixels is applied (written) with a selective voltage in accordance with the data, and only the half-selective voltage is applied to the other groups. Select once and write to each group sequentially.

Further, in the liquid crystal panel driving method according to another aspect of the present invention, the pulse width of the selective scanning pulse is changed according to the number of pixels to be inverted to prevent display unevenness.

Further, in the display device of the present invention, a plurality of driver outputs are connected to one scan electrode to increase the maximum charging current Imax to reduce the charging time and prevent display unevenness.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific examples will be described in detail below with reference to the drawings.

(Embodiment 1) (FIG. 1) is a block diagram of a display device of the present invention. The liquid crystal panel 1 includes an upper substrate 2 and a lower substrate 3.
A ferroelectric liquid crystal of the chiral smectic C phase having a spontaneous polarization of 24 nC / cm ² is sandwiched between them with a thickness of 2 μm. There are scanning electrodes 4 on the upper substrate 2 and signal electrodes 5 on the lower substrate 3, both of which have 32 electrodes, a width of 2.45 mm, and a pitch of 2.5 mm.
It consists of indium tin oxide. S on each electrode
An insulating film made of iO ₂ and an organic polymer alignment film are formed on the insulating film, and a rubbing treatment is performed to align the liquid crystal.

FIG. 2 is a characteristic diagram in which the relationship between the pulse width and the amount of transmitted light is measured by applying the conventional driving waveform (FIG. 6) to the pixel on one scanning electrode of the liquid crystal panel 1. The measurement is
The number of pixels to be inverted is changed four times, and 3a is 1
In the case of 8 pixels for 3b, 16 pixels for 3c, and 32 pixels for 3d, the solid line indicates the amount of transmitted light at the selection voltage, and the dotted line indicates the amount of transmitted light at the half-selection voltage. However, the bias ratio was set to 1/4 and the selection voltage was fixed at 20 volts. The pulse width with a constant contrast of 3a is about 50 μs to 70 μs.
There is a margin of about 20 μs in seconds. As the number of inverted pixels increases, the characteristics (shown in the figure) move almost in parallel.
Is about 26 μs slower than 3a.

In the conventional driving method shown in FIG. 6, the change in the charging time depending on the number of inversion pixels is larger than the pulse width margin, so that no matter how the driving pulse width is set, the contrast varies depending on the display pattern. There was unevenness. The first reason for this is that since the number of pixels to be inverted is different, the inversion charge of spontaneous polarization becomes large and the charging time becomes long. When all the pixels are inverted, the charge flowing due to the inversion of the spontaneous polarization is 24 nC / cm ² × 2 × 0.245 ² cm ² × 32 = 92.2 nC, which is the maximum of one output terminal of the scan driver LSI of this embodiment. Output current is 4mA, charging time is 92.2nC
/ 4 mA = 23 μsec.

Secondly, due to the difference in applied voltage, the LSI
It can be said that the output resistance of is changed and the way the waveform is rounded by the capacitance of the paraelectric component is different. When a current of 4 mA which is the maximum output current flows and the voltage further rises, the output resistance of the driver LSI equivalently increases. Therefore, when the selection voltage is applied to more pixels, the voltage waveform is more blunt. Can be said. Since the response speed of the ferroelectric liquid crystal is almost proportional to the product of the applied voltage and the applied time, if the time constant of the charging voltage waveform is doubled,

[0019]

[Equation 1]

When satisfied, they show the same response. here,
T and T'represent the voltage application time. Solving this with T = τ gives approximately T ′ = 1.4T 2, which is smaller than the ratio of the time constants. Since the dielectric constant of the ferroelectric liquid crystal of this embodiment with respect to the paraelectric term is about 3.5, the capacitance C of the pixel on one scanning line is about 3.5 nF. Output resistance is 10v /
When 4 mA = 2.5 kΩ, the time constant is 8.75 μsec.
As charging progresses and the difference in output resistance disappears, the difference due to applied voltage disappears. Therefore, the difference due to the second factor is about several microseconds in this embodiment,
The difference due to spontaneous polarization is large.

Therefore, in the liquid crystal panel driving method of the present invention, the pixels on the scanning electrodes are divided into a plurality of groups, the number of pixels to be inverted by the scanning selection pulse is limited, and the maximum charging time is within the pulse width margin. By doing so, display unevenness is prevented. The display device of FIG. 1 includes a drive circuit that realizes this drive method.

An output terminal of the scan driver LSI 6 is connected to the scan electrode 4, and a signal driver LSI 7 is connected to the signal electrode 5 via a flexible substrate with a connection resistance of several tens of Ω. The scanning driver LSI 6 performs scanning based on the synchronization signal of the timing signal generator 10, and the corresponding data is sent from the image memory 8 to the signal driver LSI 7 through the AND elements 9a and 9b. Scan is LSI
6 is performed twice in succession, 1 is sent to the AND element 9a corresponding to the pixels (16 pixels) of the group A at the time of the first scanning, the image data is passed to the signal LSI, and the AND of the group B is performed.
0 is sent to the element 9b and only the off voltage is applied to the pixel.

Image data is sent only to the group B in the second scan, and a correct image is written in the liquid crystal panel 1 in the second scan. The voltage is 20 volt and the pulse width is 65-70.
The display unevenness disappeared at μ seconds. In addition, scan 3
When the number of times was set to more than one time, the condition range where the contrast was constant was widened and the influence of temperature change could be avoided.

In this embodiment, the specific driving waveform is similar to the waveform of the conventional driving method shown in FIG. 6, and scanning is performed twice after applying the reset pulse. The twice scanning does not necessarily need to perform the second scanning after selecting all the scanning electrodes in the first scanning, and means scanning in which a certain scanning electrode is selected twice. Further, the drive waveform is not limited to the waveform shown in FIG.

Further, the scanning driver L used in this embodiment
Maximum output current of SI is 4mA, but commercially available driver LS
In the case of I, the maximum output current is mostly 1.7mA, so
The present invention is particularly effective when the spontaneous polarization on the scanning electrode is 20 nC or more. Further, the maximum supply current to the scan electrode is determined by the rating of the driver LSI in this embodiment, but the mounting connection resistance from the LSI output terminal to the pixel portion of the scan electrode and the wiring resistance of the electrode are When large, the maximum supply current is defined by the output voltage of the LSI divided by the sum of the resistances.

(Embodiment 2) FIG. 3 shows a block diagram of a display device provided with a drive circuit for realizing the driving method of the liquid crystal panel of the present invention. As shown in (Example 1), in a ferroelectric liquid crystal panel having a large pixel size, the threshold pulse width greatly depends on the number of inverted pixels. Therefore, the display unevenness is eliminated by the drive method in which the drive pulse width is set to the optimum pulse width according to the image data sent to the signal driver LSI.

Specifically, data is serially sent from the image memory 8 to the signal driver LSI 7 immediately before a certain scanning electrode is selected, and at the same time, the data is sent to the integrator 11
Add with. The output of the integrator 11 is output to the switch 12, and the switch 12 selects and outputs the input from the frequency divider 13 that divides the basic clock from the timing signal generator according to the output of the integrator 11, This is the signal driver L
The width of the output pulse that is input to the SI 7 and the scan driver LSI 6 becomes variable.

However, the length of one selection period (1H) was fixed, and the scanning and signal drivers were set to the same potential for the remaining period when the output pulse was short. According to the present invention, display unevenness is eliminated and the driving margin is greatly increased.

(Embodiment 3) In (Embodiment 1), the rated maximum output current of the scan driver LSI is the maximum supply current to the scan electrodes, and the current is limited. Therefore, the charging time depends on the display pattern. It became longer and displayed unevenly. Therefore, in such a case, in order to increase the current supplied to the scan electrodes, in this embodiment, the outputs of the plurality of scan driver LSIs are connected to one scan electrode.

As shown in the configuration diagram of FIG. 4, the two output terminals of the LSI are short-circuited on the flexible substrate, and the image data is short-circuited so that the flip-flop 20 sends the same data twice. The terminals output the same voltage. As a result, the charge time of the charge due to the reversal of the spontaneous polarization was halved, and the difference in the charge time depending on the display pattern was 11.5 μsec, which was within the margin, and the display unevenness disappeared. However, since the maximum output current of the entire LSI is 8 mA, the effect will not be improved even if three or more terminals are short-circuited.

[0031]

As described above, according to the present invention, in particular, it is possible to improve the contrast unevenness depending on the display pattern which occurs in a liquid crystal panel using a ferroelectric liquid crystal material having a large pixel size and a large spontaneous polarization. ..

[Brief description of drawings]

FIG. 1 is a configuration diagram of a display device according to a first embodiment of the present invention.

[Fig. 2] Characteristic diagram of ferroelectric liquid crystal panel

FIG. 3 is a configuration diagram of a display device according to a second embodiment of the present invention.

FIG. 4 is a configuration diagram of a display device according to a third embodiment of the invention.

FIG. 5 is a sectional view of a ferroelectric liquid crystal panel.

FIG. 6 is a drive waveform diagram of a conventional ferroelectric liquid crystal panel.

FIG. 7 is a drive waveform diagram of a conventional ferroelectric liquid crystal panel.

[Explanation of symbols]

 1 liquid crystal panel 2 upper substrate 3 lower substrate 4 scanning electrode 5 signal electrode 6 scanning driver LSI 7 signal driver LSI 8 image memory 9a, 9b AND element 10 timing signal generator 11 integrator 12 switch 13 minutes cycle 14 flip-flop

Claims (11)

[Claims] 1. In a method of driving a liquid crystal panel in which a ferroelectric liquid crystal is sandwiched between opposed scanning electrodes and signal electrodes, pixels on an arbitrary scanning electrode are divided into a predetermined number of groups and one scanning is performed. To selectively invert the pixels in one group, prohibit the inversion of the pixels in the groups other than the one group in the scanning, and sequentially write the pixels in each group by the predetermined number of times of scanning. A method for driving a liquid crystal panel, which is characterized in that 2. When t is a value obtained by dividing twice the total amount of spontaneous polarization of the ferroelectric liquid crystal of one group of pixels on any scan electrode by the maximum supply current to the scan electrode. , T from the fluctuation width T where the contrast of the scanning electrode selection time is constant
The method of driving a liquid crystal panel according to claim 1, wherein a predetermined number is set so that 3. The method of driving a liquid crystal panel according to claim 2, wherein t is 30% or less of the scanning electrode selection time. 4. A group for inhibiting inversion according to claim 1, wherein a scan driver is connected to a scan electrode and a signal driver is connected to a signal electrode, and data of an image memory is input to the signal driver through a logic element, the logic element inhibiting reversal according to claim 1. A drive circuit for a liquid crystal panel, which sends an off signal to a signal driver connected to a signal driver, and sends image memory data to a signal driver connected to a group which selectively inverts. 5. A liquid crystal panel having a ferroelectric liquid crystal sandwiched between opposing scanning electrodes and signal electrodes, and a drive circuit for the liquid crystal panel according to claim 4, wherein pixels on arbitrary scanning electrodes of the liquid crystal panel are provided. A value obtained by dividing twice the total amount of spontaneous polarization of the ferroelectric liquid crystal by the maximum supply current to the scan electrode is larger than the constant fluctuation width T of the contrast during the selection time of the scan electrode. Display device. 6. The display device according to claim 5, wherein the maximum supply current can be defined by the maximum output current of the output transistor of the scan driver. 7. The display device according to claim 6, wherein the total amount of spontaneous polarization of the ferroelectric liquid crystal of the pixel on any scanning electrode is 20 nanocoulombs or more. 8. A liquid crystal panel in which a ferroelectric liquid crystal is sandwiched between opposed scan electrodes and signal electrodes, a scan driver for supplying a drive voltage from the scan electrodes and signal electrodes and a signal driver, and the liquid crystal panel in the liquid crystal panel A value obtained by dividing twice the total spontaneous polarization amount of the ferroelectric liquid crystal of a pixel on an arbitrary scan electrode by the maximum output current of one output terminal of the scan driver LSI is constant in the contrast during the selection time of the scan electrode. The display device is characterized in that the plurality of output terminals of the scan driver LSI are connected to the scan electrodes and the same signal data is supplied to the plurality of output terminals. 9. In a method of driving a liquid crystal panel in which a ferroelectric liquid crystal is sandwiched between opposed scan electrodes and signal electrodes, the pulse width is changed according to the number of pixels to be inverted on an arbitrary scan electrode. Characteristic liquid crystal panel driving method. 10. A scan driver LSI for the scan electrode,
A signal driver LSI is connected to the signal electrode, and the data of the image memory is sequentially transferred to the signal driver LSI, and at the same time, the transferred data is input to an integrator and the result of integrating the data on one scanning electrode is a voltage-pulse. 10. A drive circuit for a liquid crystal panel which realizes the drive method according to claim 9, wherein the drive circuit is input to a width converter and the output of the voltage-pulse width converter is used as a synchronization signal for the scanning driver LSI and the signal driver LSI. 11. A liquid crystal panel having a ferroelectric liquid crystal sandwiched between opposed scan electrodes and signal electrodes, and a drive circuit for the liquid crystal panel according to claim 10, comprising a pixel on any scan electrode of the liquid crystal panel. A display characterized in that a value obtained by dividing twice the total amount of spontaneous polarization of the ferroelectric liquid crystal by the maximum supply current to the scan electrode is larger than the constant fluctuation width T of the contrast of the selection time of the scan electrode. apparatus.