JP2001272956A - Liquid crystal display device and its driving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a liquid crystal display device keep constant display luminance even under a desired color expression and various temperature surroundings. SOLUTION: In the liquid crystal display device of a field sequential color system, when the color of a light source is changed over and the display state of a liquid crystal display element is changed over by synchronizing it with this, the time-integral value of the luminance of the light source is modulated based on the temperature information or the maximum transmissivity information of the liquid crystal display element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display and a method for driving the liquid crystal display.

[0002]

2. Description of the Related Art In recent years, liquid crystal display elements have been replaced by PCs (personal computers)
, Such as monitors, video camcorder viewfinders, projectors, and various other fields, and many of them use twisted nematic liquid crystals. However, a liquid crystal display device using a twisted nematic liquid crystal has problems such as a low response speed and a narrow viewing angle. On the other hand, a field sequential system without using a color filter has been proposed as a new system of a color liquid crystal display element. This is to turn on the red (R), green (G), and blue (B) light sources sequentially,
By displaying an image corresponding to this on a liquid crystal panel, color display is performed by time mixing. In the case of the field sequential system, a desired color cannot be displayed unless the liquid crystal response is completely completed between the respective color fields. Therefore, a higher liquid crystal response speed is required.

As a liquid crystal mode which solves these problems, for example, in Japanese Patent No. 2681528, Yoshida has proposed a method in which a monostable mode ferroelectric liquid crystal is combined with an active matrix element. This monostable mode ferroelectric liquid crystal has unipolar electro-optical characteristics shown in FIG.
Twisted nematic liquid crystals having ambipolar electro-optical properties and liquid crystals proposed in JP-A-9-50049 show almost the same optical response to both positive and negative voltages,
The shape of the VT curve is called a V-shape. On the other hand, a monostable mode ferroelectric liquid crystal is called a one-sided V-shaped liquid crystal (mode) because it looks like a V-shaped cut in half. Hereinafter, the monostable mode ferroelectric liquid crystal is referred to as a single-sided V-shaped liquid crystal.

In the field sequential system,
FIG. 6 shows an example of a sequence for driving one-sided V-shaped liquid crystal using an active matrix element, and FIG. 7 shows an example of an active matrix element configuration at this time. A driving method will be described. As shown in FIG. 6, the first transistor T of FIG.
A desired voltage corresponding to the gradation information is written to the capacitor C1 in the pixel by sequential scanning by r1 and a second transistor T controlled by a common control line (not shown) for driving the pixel.
applying a voltage to the liquid crystal C _LC by r4. 1 frame is 6
The fields are divided into R, G, B, black display (R), black display (G), and black display (B) in this order. In a black display (Black) field, a voltage having a polarity opposite to that of the voltage applied in the R, G, and B fields is applied so that the D of the liquid crystal layer is changed.
The C component is canceled to prevent deterioration of characteristics. 7, reference numeral 71 denotes a scan electrode driver (X gate drive circuit) for sequentially scanning the gate of the first transistor Tr1, 72 denotes an information electrode (source electrode), and 73 denotes an information electrode driver (source drive circuit).

[0005]

However, FIG.
As shown in the figure, it has been confirmed that the rise response τ _on and the fall response τ _off of the one-sided V-shaped liquid crystal are different. τ
_If both _on and τ _off are sufficiently shorter than one field time, this is not a problem, but otherwise, an effective transmittance difference is caused. In the case of the drive sequence shown in FIG. 6, since the color fields are continuous and one field time is short, the following problem occurs. That is, consider a case where the image data indicating the maximum transmittance is 100%, and for example, the R image data is 50%, the G image data is 100%, and the B image data is 50%. In the case of R, the previous state is black,
In the case of B, since the previous state is white, the response speed is different between R and B even when the same voltage is applied, so that a desired color cannot be expressed. Further, as shown in FIG. 5, as in other liquid crystals, the response speed of one-sided V-shaped liquid crystal has a temperature dependence, and the response speed decreases in a low-temperature environment, causing more significant display degradation. I will. In order to solve this, a method of turning on the light source after the liquid crystal has sufficiently responded can be considered. However, this causes a problem that the display luminance is reduced particularly in a low-temperature environment.

An object of the present invention is to provide a liquid crystal display device and a driving method capable of expressing a desired color and maintaining a constant display luminance under various temperature environments.

[0007]

According to the present invention, there is provided a liquid crystal display device of a field sequential color system, in which the color of a light source is switched.
When switching the display state of the liquid crystal display element in synchronization with this, the time integral value of the light source luminance is modulated based on the temperature information or the maximum transmittance information of the liquid crystal display element. Thereby, a desired color expression can be achieved, and a constant display luminance can be maintained even under various temperature environments.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In a preferred embodiment of the present invention, in a field sequential color type liquid crystal display device, when a color of a light source is switched and a display state of a liquid crystal display element is switched in synchronization with the color, the display is temporarily stopped. By turning off all the light sources, turning on the light sources after the liquid crystal has sufficiently responded, detecting the temperature of the liquid crystal display element, and modulating the time integral value of the light source luminance based on the temperature information, the desired color can be obtained. A constant display luminance is maintained even under expression and various temperature environments. Alternatively, the maximum transmittance information of the liquid crystal display element may be obtained in place of the temperature information, and the time integration value of the light source luminance may be modulated so that the maximum transmitted light amount becomes constant.

As a method of modulating the time integral value (effective value) of the light source luminance, a PWM method in which the light source luminance (instantaneous value) is constant and the lighting time (pulse width) is modulated, and the light source is fixed and the lighting time is constant. And a method in which both the luminance and the lighting time are appropriately controlled using both of these two modulation methods. Also,
When switching the display state of the liquid crystal display element in synchronization with the switching of the color of the light source, it is preferable that the liquid crystal display element displays a black state once.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. [Embodiment 1] FIG. 1 is a timing chart showing a driving sequence including a lighting sequence of a light source according to a first embodiment of the present invention. FIG.
FIG. 1B shows a case in a low temperature environment. FIG. 2 is a circuit diagram showing a pixel driving circuit used in the present embodiment, and FIG. 3 is a timing chart showing details of a driving sequence of the present embodiment. However, in the driving sequence shown in FIG. 1, the same polarity field is continued three times and then the field of the opposite polarity is continued three times.
In, for easy explanation, the polarity is inverted every field.

Vin applies an analog voltage Vdata corresponding to image data and is similar to a source (data) signal of a general active matrix LCD. Vg1
Is a gate signal for writing Vdata to the first stage capacitor C1, which is also a general active matrix LCD.
This is the same as the gate pulse of (liquid crystal display element). Time t
In step 1, Vg1 is set to "H" to turn on the transistor Tr1 and write Vdata (= Vin) to the capacitor C1.

Next, at time t2, Vg2 is set to "Η", transistor Tr3 is turned off, transistor Tr2 is activated, and the voltage of Vdata is transferred to NodeB. Although the voltage actually drops by Vth of the transistor Tr2, it is ignored in this description. Next, at time t3, Vcc is set to "L" and Vg3 is set to "H" (transistors Tr4 and Tr2 are turned on), and the potential of NodeC is reset to 0V. In this case the voltage NodeC-Vcom applied to the liquid crystal layer C _LC by the potential of the desired Vcom (5V in this case) is -5V becomes (one V-shaped) liquid crystal becomes black state. Next, at time t4, Vcc is set to "H".
And set Vg2 to “H” (transistor Tr3 off,
By making the transistor Tr2 active), the voltage V
Data is given to NodeC. At this time, by setting Vcom to a desired (here, 0 V) potential, Vda is applied to the liquid crystal layer.
A voltage of ta-Vcom is applied to make the liquid crystal respond to a desired transmittance state. The operations at times t1 and t2 are performed line by line for each horizontal scanning line, and are performed at times t3 and t2.
The operation in 4 is an operation performed collectively on the entire surface.

As a result, the liquid crystal is reset to the black state at time t3, so that accurate gradation expression can be performed regardless of the state of the previous field. Since the polarity of the reset voltage is also inverted depending on the polarity of the field, no DC component due to the reset remains.

The response speed of the liquid crystal has a temperature dependency as shown in FIG. Therefore, at a low temperature, as shown in FIG. 1B, the light source is turned on when the liquid crystal has sufficiently responded, so that the display information of the immediately preceding color field is not displayed as noise. On the other hand, since the liquid crystal responds in a single time on the high temperature side as compared with the low temperature side, even if the light source is turned on at the same timing as at the low temperature, the information of the immediately preceding color field does not enter. However, when the temperature is low, the liquid crystal response is not sufficiently saturated during the light source lighting period. Therefore, when the light source lighting periods at the low temperature and the high temperature are the same, the display brightness as the liquid crystal display device becomes higher at the high temperature. In order to keep the display brightness constant regardless of the temperature, FIG.
As shown in (2), the light source lighting period at high temperature is shorter than the light source lighting period at low temperature.

The above driving sequence will be described with specific numerical values. As shown in FIG.
The response speed at 0 ° C. is 0% transmittance → 90% transmittance response time τ _on = about 0.8 msec, and 100% transmittance → 10% transmittance response time τ _off = about 0.3 msec. Similarly, the response speed at 10 ° C. is τ _on = about 2 msec, τ _off =
It is about 0.6 msec.

In FIG. 1B, when time t3b = 0, time t3b 'is 2.78 msec (= 1/60/6) when the frame rate is 60 Hz. Since τ _off at 10 ° C. = approximately 0.6 msec, the time t4b for changing from reset to writing is set to, for example, 0.8 msec.
c.

The time t5b at which the light source is turned on is set to 0.
8 msec and t3b 'are 2.78 msec. The brightness of the liquid crystal display element at this time corresponds to the area indicated by S1b including the transient response portion. Similarly, FIG.
Assuming that at time t3a = 0, the time t4a at which the state is changed from the reset to the writing may be a value exceeding about 0.3 msec of the τ _off time at 40 ° C.
Take 0.5 msec. Τ _on at 40 ° C is about 0.8 ms
Since the response time is ec, the response is almost saturated at the time of 1.3 msec.

The lighting time t5a is set to 0.
0.8 which is the same as t5b at 5 msec or 10 ° C
If the time is msec, the luminance difference from the low temperature is large, so that the lighting time t5a is about 1.5 msec. As a result, the brightness S1a of the liquid crystal display element at this time becomes S1b.
Was almost equal to The case of red (R) has been described above. For green (G) and blue (B), S2a = S2b and S3a = S3b by the same method.

On the other hand, the light source also has a temperature characteristic at the upper limit of the emission luminance. FIG. 8 shows the ambient temperature characteristics of the allowable current value of the LED. Since the LED is a semiconductor, its characteristics deteriorate due to an increase in the junction temperature. As a result, as shown in FIG. 8, the higher the temperature, the lower the allowable current value. It can be considered that the allowable current value is almost proportional to the light emission luminance. Therefore, it can be said that the above-described driving method, that is, shortening the lighting time of the light source on the higher temperature side than on the lower temperature side is a method suitable for the temperature characteristics of the light source. The temperature information is obtained by a method such as attaching a device such as a thermistor or a thermocouple to the liquid crystal display element, or directly into the liquid crystal display element in the case of a reflective panel in which a drive circuit of the liquid crystal display element is formed on a silicon wafer. A method of forming a temperature detecting element may be employed.

The temperature information is fetched by these various methods, and a temperature compensation table written in a memory in advance,
That is, the lighting timing of the light source is controlled based on the table in which the temperature information and the lighting timing of the light source have a one-to-one relationship, and the above-described driving sequence is satisfied.

Embodiment 2 FIG. 9 is a timing chart showing a driving sequence according to a second embodiment of the present invention. The pixel circuit is the same as that shown in the description of the first embodiment shown in FIG. 2 or that shown in FIG.

The difference from the first embodiment is that no reset period is provided between adjacent color fields. Since there is no reset period, the display of a certain color field is affected by the information of the immediately preceding color field.In this case, too, the light source is turned on after the liquid crystal responds sufficiently according to the information of the own color field. If lit, there is no display problem. Since this sequence can be performed even with a relatively simple pixel circuit configuration shown in FIG. 7, an improvement in panel yield or an improvement in aperture ratio in a transmissive panel is expected. However, as shown in FIG. 5, since τ _on takes a longer time than τ _off , the drive sequence shown in the first embodiment is more advantageous for obtaining better color reproduction.

Embodiment 3 FIG. 10 shows a liquid crystal according to a third embodiment of the present invention, that is, a V-shaped VT having spontaneous polarization.
It is a figure which shows the optical response of the liquid crystal which shows a (voltage-transmittance) characteristic. FIG. 11 is a timing chart showing the driving sequence of this embodiment, and FIG. 12 is a timing chart for explaining the driving sequence of this embodiment in more detail. The pixel circuit shown in FIG. 2 is the same as that shown in the description of the first embodiment. Here, the description will focus on the differences from the first embodiment.

The operations at times t1 and t2 are the same as in the first embodiment.
It is like. At time t3, Vcc is set to “L” and Vg3
To “H” to reset the potential of NodeC to 0V
You. At this time, by setting Vcom to 0 V, the liquid crystal layer C_LCTo
The voltage NodeC-Vcom becomes 0 V (V-shaped).
The liquid crystal goes to a black state. Next, at time t4, Vcc
Is returned to “H” and Vg2 is set to “H”, whereby the voltage Vd
Data is given to NodeC. At this time, Vcom
(Here, 0 V) to give the liquid crystal layer C _LCTo V
Apply a voltage of data-Vcom, and set the liquid crystal to a desired transmittance.
Respond to conditions. The liquid crystal response is as shown in FIG.
There will always be a black period between the adjacent color fields,
It prevents information between adjacent color fields from being mixed.

As for the lighting of the light source, as in the first embodiment, the light source is turned on when the liquid crystal has sufficiently responded, so that the display information of the immediately preceding color field is not displayed as noise.

As described above, in the field sequential color liquid crystal display device, when the color of the light source is switched and the display state of the liquid crystal display element is switched in synchronization with this, all the light sources are once turned off. By turning on the light source after the liquid crystal has responded sufficiently, detecting the temperature of the liquid crystal display element, and modulating the time integral value of the light source luminance based on the temperature information, the desired color expression and various temperature environments can be obtained. In this case, a constant display luminance is maintained.

In the above-described embodiment, the time integral value of the light source luminance is modulated based on the temperature information. However, the maximum transmittance of the liquid crystal display element is detected, and based on the maximum transmittance information. Alternatively, the time integral value of the light source luminance may be modulated. The maximum transmittance may be obtained by measuring the maximum transmittance of the liquid crystal display element with respect to the temperature in advance, creating a table or an approximate expression, detecting the temperature, and referring to the table or by calculation. The luminance of the group may be detected by a sensor, and the luminance may be calculated from the detected luminance and the gradation information of the pixel or the pixel group. Also,
In the above-described embodiment, an example has been described in which, in order to modulate the time integral value of the light source luminance, the luminance (drive current) of the light source is fixed and the pulse width modulation for modulating the lighting period (pulse width) is employed. Alternatively, luminance modulation for changing the drive current value with a constant pulse width may be used, or luminance modulation and pulse width modulation may be used in combination.

[0028]

As described above, according to the present invention,
In a field sequential color type liquid crystal display device, when the color of the light source is switched and the display state of the liquid crystal display element is switched in synchronization with this, the time of the light source luminance is determined based on the temperature information or the maximum transmittance information of the liquid crystal display element. By modulating the integral value, a constant display luminance can be maintained even in a desired color expression and various temperature environments.

[Brief description of the drawings]

FIG. 1 is a timing chart of driving a pixel according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a pixel driving circuit according to a first embodiment of the present invention.

FIG. 3 is a timing chart showing details of a drive sequence according to the first embodiment of the present invention.

FIG. 4 is a diagram showing electro-optical characteristics of one-sided V-shaped liquid crystal.

FIG. 5 is a diagram showing a temperature characteristic of a response speed of one-sided V-shaped liquid crystal.

FIG. 6 is a timing chart showing a conventional driving sequence.

FIG. 7 is a schematic diagram showing a conventional active matrix element.

FIG. 8 is a diagram showing characteristics of an LED.

FIG. 9 is a timing chart showing a drive sequence according to a second embodiment of the present invention.

FIG. 10 is a diagram showing an optical response of a liquid crystal according to a third embodiment of the present invention.

FIG. 11 is a timing chart showing a driving sequence according to a third embodiment of the present invention.

FIG. 12 is a timing chart showing details of a drive sequence according to a third embodiment of the present invention.

[Explanation of symbols]

Vin: information signal to the column, Vg1: row scanning signal, Vg
2: Transfer signal, Vg3: Black display (reset) signal, Tr
1-4: transistors, C1: first capacitance, C _LC : liquid crystal layer.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G09G 3/20 641 G09G 3/20 641C 641K 642 642C 642L 3/34 3/34 JF Term (Reference) 2H093 NA16 NA33 NA43 NA65 NA80 NC13 NC16 NC34 NC43 NC54 NC57 NC59 NC62 ND17 ND34 ND58 NE07 NF17 5C006 AA15 AA16 AA17 AA22 AC21 AF52 BA12 BB15 BB28 BB29 BC16 EA01 FA19 5C080 AA10 BB05 CC03 DD03 EJ30 EJ30 EE30 EB30

Claims (11)

[Claims] 1. An active matrix type liquid crystal display element for arranging a plurality of pixels in a matrix and displaying an image by row scanning and signal application to a column, a light source for irradiating the display element, A liquid crystal display device of a field sequential color system for switching colors in a time sequential manner and driving means for controlling a transmission or reflection state of the liquid crystal display element in synchronization therewith, and performing color display with a temporal additive color mixture; A liquid crystal display device comprising: a driving unit that detects a temperature of the liquid crystal display element, and modulates a time integration value of a light source luminance based on the detected temperature information. 2. An active matrix type liquid crystal display element for arranging a plurality of pixels in a matrix and displaying an image by scanning a row and applying a signal to a column, a light source for irradiating the display element, A field-sequential color liquid crystal display device comprising a driving means for switching the color in time sequence and controlling the transmission or reflection state of the liquid crystal display element in synchronization therewith, and performing color display with temporal additive color mixing; The driving unit modulates a time integration value of a light source luminance according to a maximum transmittance or a maximum reflectance of the liquid crystal display element to stabilize a maximum transmitted light amount or a maximum reflected light amount of the liquid crystal display element. Liquid crystal display device. 3. The method according to claim 1, wherein the modulation of the time integral value of the light source luminance is modulation by pulse width modulation.
Or the liquid crystal display device according to 2. 4. The liquid crystal display device according to claim 1, wherein the modulation of the time integral value of the light source luminance is modulation by luminance modulation. 5. The liquid crystal display device according to claim 1, wherein the modulation of the time integral value of the light source luminance is a combination of luminance modulation and pulse width modulation. 6. The liquid crystal display device according to claim 1, wherein when switching colors of the light sources, a period is provided in which all light sources are turned off. 7. The liquid crystal display device according to claim 1, wherein the driving unit causes the liquid crystal display element to perform black display once when switching a display state in accordance with a color of the light source. Liquid crystal display device. 8. The liquid crystal display device according to claim 1, wherein the light source is a light source having a characteristic of obtaining higher luminance as the environmental temperature is lower. 9. The liquid crystal display device according to claim 1, wherein the light source is an LED. 10. An active matrix type liquid crystal display element which arranges a plurality of pixels in a matrix and displays an image by row scanning and signal application to a column, and a light source for irradiating the display element. A time-sequentially switching, controlling the transmission or reflection state of the liquid crystal display element in synchronization therewith, a driving method of a field sequential color type liquid crystal display device that performs color display with temporal additive color mixture, A method for driving a liquid crystal display device, comprising: detecting a temperature of the liquid crystal display element; and modulating a time integral value of a light source luminance based on the detected temperature information. 11. An active matrix type liquid crystal display element which arranges a plurality of pixels in a matrix and displays an image by row scanning and signal application to a column, and a light source for irradiating the display element. A time-sequentially switching, controlling the transmission or reflection state of the liquid crystal display element in synchronization therewith, a driving method of a field sequential color type liquid crystal display device that performs color display with temporal additive color mixture, A liquid crystal display device characterized by modulating a time integration value of light source luminance according to a maximum transmittance or a maximum reflectance of the liquid crystal display element to stabilize a maximum transmission light amount or a maximum reflection light amount of the liquid crystal display element. Drive method.