JP2630133B2 - Driving method of liquid crystal panel, its driving circuit and display device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art A ferroelectric liquid crystal has a layer structure, and switches when spontaneous polarization perpendicular to the long axis of liquid crystal molecules is arranged in the direction of an electric field. FIG. 5 is a cross-sectional view of the ferroelectric liquid crystal panel, in which the liquid crystal molecules 50 are always inclined at a fixed angle with respect to the layer normal.
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).
5A, the direction of the upper electrode is aligned with the direction of the upper electrode, and FIGS. Since the liquid crystal molecules are arranged substantially horizontally on the electrode surface, the alignment states of FIGS. 5A, 5A and 5B are maintained even after the electric field is removed. For example, when the polarizing axis of the polarizer is aligned with the molecular long axis direction (FIG. 5A) by the birefringence effect with the panel interposed between orthogonal polarizing plates (FIG. 5)
(A) shows a black display, and (FIG. 5) and (b) show a bright display. When the thickness between the electrodes (the thickness of the liquid crystal layer) is about 2 μm (FIG. 5), (b) shows a white display.

[0005] Since the direction of polarization is reversed to reverse the state shown in FIG. 5A to FIG. 5B, charge transfer occurs twice as large as the spontaneous polarization of the ferroelectric liquid crystal.

FIG. 6 shows an example of a driving waveform diagram of a conventional ferroelectric liquid crystal. A scan voltage 60 is applied to the scan electrode and a signal voltage 61 is applied to the signal electrode. At this time, a pixel applied voltage 62 which is a difference between the scan voltage and the signal voltage is applied to the pixel. After resetting to a black state (white state) by a reset pulse 65, a certain scan 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 the pixel is inverted to a white state (black state). And the reset state is maintained. FIG. 7 is also a conventional example of the driving waveform of the ferroelectric liquid crystal, and shows only the pixel applied voltage. In this case, the scanning is divided into two fields, and the image is written by writing (inverting or holding) white (black) in the first field and black (white) in the second field. In either case (FIG. 6) or (FIG. 7), since the threshold voltage of the ferroelectric liquid crystal panel changes depending on the pulse width, the threshold voltage of the panel falls between the selection voltage and the half selection voltage. Needs to set the pulse width.

[0005]

The reversal current flowing when reversing the spontaneous polarization flows from the driving LSI through the scanning electrode and the signal electrode. However, when the pixel size is large, the panel length is long, or the liquid crystal is 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 pixels on the scan electrode are inverted by the reset pulse, the charging time is constant,
Although it is sufficient to increase the pulse width, there is no problem. However, when selecting the scanning electrode, the amount of charge changes greatly depending on the number of ON pixels, so that an appropriate pulse width differs depending on the pattern. Therefore, with a fixed pulse width, the contrast differs depending on the display pattern, and display unevenness occurs. The limit of the reversal current is limited to the driving LSI, especially the scanning LSI
The maximum output current, or routing electrode resistance and connection resistance of the mounting part from the LSI output terminal to the scanning electrode,
And the current value obtained by dividing the output voltage by the resistance value obtained by adding the output resistance of the LSI and the resistance value.

[0006]

In order to solve the above-mentioned problems, a liquid crystal panel driving method according to 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, and pixels in one group are selectively inverted by one scan, and pixels in groups other than the one group are inverted in the scan. And writing of pixels of each group is sequentially performed by the predetermined number of scans.

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

Further, the display device of the present invention comprises a liquid crystal panel having a ferroelectric liquid crystal interposed between the opposing scan electrode and signal electrode, a scan driver for supplying a drive voltage from the scan electrode and the signal electrode, and a signal driver. The value obtained by dividing twice the sum of the spontaneous polarization amounts of the ferroelectric liquid crystal of the pixels on the arbitrary scanning electrodes of the liquid crystal panel by the maximum output current of one output terminal of the scanning driver LSI is obtained by the scanning. 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 electrode selection time. is there.

[0009]

The charge Q required for reversing the spontaneous polarization is determined by the area S of the pixel to be reversed and the spontaneous polarization Ps of the liquid crystal per unit area.
Twice the product of Assuming that 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 for inverting the liquid crystal when the charging current is sufficient are determined by the liquid crystal material. If the current is limited, the pulse width T must be further increased to T + τ. Must. In the case of matrix driving of binary display, 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 the voltage is sufficiently larger than V1, there is a width (margin) in the driving conditions under which the contrast is constant. a is preferably about 3 to 5 in bias ratio. For example, if the bias ratio is 4, the half-selection voltage is の of the selection voltage, so that the voltage margin is less than (1/2) V1. The actual margin depending on the steepness of the threshold characteristic 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 a pulse width when the voltage is fixed. Therefore, when the charging time τ becomes larger than the margin ΔT of the pulse width T,
Contrast unevenness occurs.

The area of the pixel to be inverted differs depending on the display pattern. In the conventional driving method, the case where all the pixels on the scanning electrode are inverted is the maximum, and the case where the inverted pixel is 0 is minimum. Therefore, in the driving method of the liquid crystal panel of the present invention, the maximum value of the charging time τ is suppressed by limiting the number of pixels that are inverted by one selection, thereby preventing display unevenness. Specifically, the pixels on the scanning electrodes are divided into several groups,
For one selection scan pulse, a selection voltage is applied (written) to only one group of pixels in accordance with data, and only a half-selection voltage is applied to the other groups. And write to each group sequentially.

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 scanning electrode to increase the maximum charging current Imax, thereby reducing the charging time and preventing display unevenness.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific examples will be described in detail with reference to the drawings.

Embodiment 1 (FIG. 1) is a configuration 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 a chiral smectic C phase having a spontaneous polarization of 24 nC / cm ² is sandwiched by a thickness of 2 μm. There are a scanning electrode 4 on the upper substrate 2 and a signal electrode 5 on the lower substrate 3, both of which have 32 lines, a width of 2.45 mm and a pitch of 2.5 mm.
And made of indium tin oxide. S on each electrode
An insulating film made of iO ₂ and an alignment film of an organic polymer are formed thereon, and rubbing treatment is performed to align the liquid crystal.

A characteristic diagram (FIG. 2) is obtained by applying the conventional drive waveform (FIG. 6) to the pixels on one scanning electrode of the liquid crystal panel 1 and measuring the relationship between the pulse width and the amount of transmitted light. The measurement is
It is performed four times by changing the number of pixels to be inverted.
Pixels, 3b are 8 pixels, 3c is 16 pixels, and 3d is 32 pixels. The solid line is the transmitted light amount at the selected voltage, and the dotted line is the transmitted light amount at the half-selected voltage. However, the bias ratio is 1/4 and the selection voltage is fixed at 20 volts. The pulse width of the constant 3a pulse is approximately 50 μsec to 70 μsec.
In seconds, there is a margin of about 20 μs. As the number of inversion pixels increases, the characteristics (shown in the figure) move almost in parallel, and 3d
Is about 26 μs later 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. Irregularities occurred. The first reason is that, since the number of pixels to be inverted is different, the inversion charge of the 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 μs.

Second, LSIs differ depending on the applied voltage.
Is changed, and the way in which the waveform is rounded due to 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, so that the voltage waveform is more distorted when the selection voltage is applied to more pixels. It can be said that. 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)

If so, it shows the same response. here,
T and T 'indicate the voltage application time. Solving this assuming T = τ results in approximately T ′ = 1.4T, which is smaller than the ratio of the time constants. Since the dielectric constant of the ferroelectric liquid crystal according to the present 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.
When charging progresses and the difference in output resistance disappears, the difference due to the applied voltage disappears. For this reason, 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 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 shown in FIG. 1 includes a driving circuit for realizing this driving method.

An output terminal of a 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 ohms. The scanning driver LSI 6 performs scanning based on the synchronization signal of the timing signal generator 10, and corresponding data is sent from the image memory 8 to the signal driver LSI 7 through the AND elements 9a and 9b. Scanning is LSI
6 is performed twice consecutively, and at the time of the first scanning, 1 is sent to the AND element 9a corresponding to the pixels (16) of the group A, and the image data is transferred 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.

In the second scan, image data is sent only to the group B, and a correct image is written on the liquid crystal panel 1 in the second scan. The voltage is 20 volts and the pulse width is 65-70
At μ seconds, display unevenness disappeared. In addition, scan 3
When the number of times is increased, the condition range in which the contrast is constant is widened, and the influence of temperature change and the like can be avoided.

In this embodiment, the specific driving waveform is in accordance with the waveform of the conventional driving method shown in FIG. 6, and the scanning is performed twice after applying the reset pulse. The two scans do not necessarily mean that the second scan is required to be performed after all the scan electrodes are selected for the first time, and means a scan for selecting a certain scan electrode twice. The drive waveform is not limited to the waveform shown in FIG.

The scanning driver L used in this embodiment is
The maximum output current of SI is 4mA, but a commercially available driver LS
In the case of I, the maximum output current is often about 1.7 mA, so
The present invention is particularly effective when the spontaneous polarization on the scanning electrode is 20 nC or more. In this embodiment, the maximum supply current to the scanning electrode is determined by the rating of the driver LSI. However, the mounting connection resistance and the wiring resistance of the electrode from the LSI output terminal to the pixel portion of the scanning electrode are reduced. If it is larger, the maximum supply current is defined by the value obtained by dividing the output voltage of the LSI by the sum of the resistors.

(Embodiment 2) FIG. 3 shows a configuration diagram of a display device provided with a driving circuit for realizing the liquid crystal panel driving method 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 inversion pixels. Accordingly, display unevenness has been eliminated by a driving method in which the driving pulse width is set to an optimum pulse width in accordance with image data sent to the signal driver LSI.

More specifically, data is sent serially 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 obtained by dividing 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 is variable.

However, the length of one selection period (1H) was fixed, and the scanning and signal driver were set to the same potential during the remaining period when the output pulse was short. According to the present invention, display unevenness has been eliminated and the driving margin has been 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. Since the current is limited, the charging time depends on the display pattern. It became longer and the display became uneven. Therefore, in such a case, in order to increase the supply current to the scanning electrodes, in this embodiment, the outputs of a plurality of scanning driver LSIs are connected to one scanning electrode.

As shown in FIG. 4, the two output terminals of the LSI are short-circuited on the flexible substrate, and the image data is short-circuited by the flip-flop 20 so that the same data is sent twice. The output of the terminal outputs 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. However, since the maximum output current of the entire LSI is 8 mA, the effect is not further 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 generated in a liquid crystal panel using a ferroelectric liquid crystal material having a large pixel size and a large spontaneous polarization. .

[Brief description of the drawings]

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

FIG. 2 is a characteristic diagram of a 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 present invention.

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

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

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Liquid crystal panel 2 Upper substrate 3 Lower substrate 4 Scan electrode 5 Signal electrode 6 Scan driver LSI 7 Signal driver LSI 8 Image memory 9a, 9b AND element 10 Timing signal generator 11 Integrator 12 Switch 13 Minute period 14 Flip-flop

Claims (2)

(57) [Claims] In a method of driving a liquid crystal panel in which a ferroelectric liquid crystal is sandwiched between opposing 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, inhibit 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 scans. I
Ferroelectric liquid crystal of a group of pixels on any of the scanning electrodes
Twice the total amount of spontaneous polarization of
When the value divided by the supply current is t, the selection of the scanning electrodes is performed.
T is smaller than the drive margin T where the contrast of the selection time is constant.
A method for driving a liquid crystal panel , wherein the predetermined number is set to a minimum integer value that decreases . 2. A driving method of a liquid crystal panel according to claim 1, wherein t is less than 30% of the selection time of the scanning electrodes.