JPH06202075A - Active matrix type liquid crystal display element and its driving method - Google Patents

Abstract

PURPOSE:To enable faster operation and a high-definition liquid crystal display at high frequencies to be realized by providing a means which controls a potential by a line buffer through a transistor(TR) different from a TR arranged on a display signal line. CONSTITUTION:The line buffer 7 is provided with a reset signal input means through a TR separately from the display signal line 2. A video signal is stored in respective line buffers 7 and transferred to respective display signal lines 2 through TRs. In the remaining blanking period before a next scanning period, the reset signal input means provided to the line buffer 7, i.e., TR is turned on to input a desired potential through it, and all the line buffers 7 are reset to the constant potential. Thus, the respective line buffers 7 are provided with line buffer control means which function independently of the display signal lines 2 and the charge quantities of the line buffers 7 are controlled through them to input the video signal under the optimum condition.

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 element in which a plurality of pixels are arranged vertically and horizontally and a switching element is provided for each pixel to perform active matrix driving.

[0002]

2. Description of the Related Art In recent years, an active matrix driving method has been known as a means for driving a huge number of pixels at a high frequency for the purpose of displaying a high-definition image. This method
A switching element, for example, a transistor is arranged for each pixel, a drain electrode is connected to a pixel electrode of each pixel, a source electrode is connected for each column, and a gate electrode is commonly connected for each row.
The gate electrodes are sequentially turned on by selecting one scanning line at a time, the video signal is input to each display signal line at the same time, and the video signal is input to the pixel electrode via the transistor connected to the selected scanning line to display. It is a method to do.

FIG. 3 shows an example of a drive circuit for an active matrix type liquid crystal display element, and FIG. 4 shows a timing chart thereof.

In the figure, 1 is a scanning signal line, 2 is a display signal line, 3
Is a thin film transistor (hereinafter referred to as “TFT”), 4 is a liquid crystal, 5 is a vertical shift register, 6 is a horizontal shift register, and 7 is a line buffer. In this type of liquid crystal display element, the video signal is accumulated in the line buffer 7 via the transistor whose gate is turned on in response to the signal from the horizontal shift register 6. After the video signals are sequentially accumulated in each line buffer, the pixel side transistors connected to the line buffer 7 are turned on to input the video signals accumulated in each line buffer 7 to all the display signal lines all at once. Forward. At the same time, a scanning signal line 1 is selected, the gate of the TFT 3 connected to the scanning signal line is turned on, the video signal transferred from the line buffer 7 is input to the pixel electrode via the TFT 3, and the next signal is input. It is held as the liquid crystal capacitance C _LC until it is _turned on. This one horizontal scanning is sequentially performed for each scanning signal line to display one screen.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a liquid crystal display device capable of functioning under optimum conditions for driving at a higher frequency in order to realize higher definition image display and a driving method thereof. To provide.

The number of pixels in the display unit is 450 vertical scanning lines ×
There are 600 horizontal scanning lines (display signal lines). When a display panel having this number of horizontal pixels is scanned with an NTSC signal, the time required to charge one horizontal pixel (actually, a line buffer provided for each display signal line) is about 90 [n
sec. ] Only It is necessary to store the video signal charge in the line buffer within this time. The signal charge to be accumulated depends on the display performance of the panel, and in particular, how many gradation levels are required. That is,
If there is a transfer failure of a signal voltage equal to or higher than the signal voltage corresponding to one gradation of the number of gradations to be displayed, the gradation characteristics of the panel can no longer be maintained. In the field of liquid crystal display panels, it is expected that higher gradation will be required in the future. Since the drive voltage amplitude of the liquid crystal does not seem to change significantly, in that case, the signal voltage for identifying one gradation becomes smaller and smaller. For example, when the drive voltage amplitude of the liquid crystal is ± 5 [V], the number of display gradations is 64 when the change in the voltage-transmittance curve of the liquid crystal is taken into consideration.
In order to realize a gradation level, it is necessary to read the difference between the signal voltages of about 20 to 30 [mV].

Consider, for example, a case where all the switching transistors are composed of only P-type MOS. The amplitude of the input signal is 3 to 13 [V]. In this case, the potential of the counter electrode of the liquid crystal cell was set to about 8 [V]. The drive amplitude of the gate pulse that drives the switching transistor that transfers the signal was set to 0 to 15 [V].

Here, when the reset voltage of the line buffer capacity is set to 3 [V] and 8 [V], FIG.
As shown in, the reset voltage causes a difference in the charging rate to the line buffer capacitance, especially when the input signal is small. It is considered that this depends on whether the line buffer capacitance side functions as the drain or the source of the transistor due to the difference in the reset voltage, as described above.

As a result of the above examination, the present inventors have obtained the present invention as a means for realizing a high gradation liquid crystal display panel.

[0010]

SUMMARY OF THE INVENTION The present invention solves the above problems and provides a liquid crystal display device capable of functioning at a higher speed and a driving method thereof. A first invention is a switching device for each pixel. An active matrix type liquid crystal display device using thin film transistors for storing a video signal input to a display signal line according to a signal from a shift register and transferring a Video signal to a pixel through the transistor. An active matrix type liquid crystal display device, characterized in that it has a unit for each display signal line, and the line buffer has means for controlling the potential through a transistor different from the transistor arranged on the display signal line.

A second invention is a method of driving the liquid crystal display device according to the first invention, wherein a video signal is stored in a line buffer provided for each display signal line during a scanning period of one horizontal scanning period, During a part of the subsequent blanking period, the gates of the transistors connected to each line buffer are turned on to transfer the video signals stored in all pixels simultaneously, and the gates of the above transistors are turned off during the remaining period of the blanking period. Then
A method for driving an active matrix type liquid crystal display device, characterized in that a reset signal is input to all line buffers via different transistors connected to the line buffers to keep the potentials of all line buffers constant.

In the present invention, the potential of the line buffer is preferably set to the highest potential within the amplitude of the video signal.

[0013]

[Examples and functions]

(Embodiment 1) FIG. 1 shows a circuit diagram of a liquid crystal display device which is an embodiment of the present invention, and FIG. 2 shows a timing chart thereof.

The reference numerals in the figure are the same as those in FIG. 3, but in the present invention, the reset signal input means is provided in the line buffer 7 separately from the display signal line via a transistor.

In this embodiment, each line buffer 7
It is the same up to the point where the video signal is stored in and the video signal is transferred to each display signal line through the transistor.
In the remaining blanking period before the next scanning period, the reset signal input means provided in the line buffer 7, that is, the transistor in this embodiment is turned on, and a desired potential is input through the reset signal input means to turn on all the line buffers. Reset to constant potential. As a result, the next video signal is input while being held at a constant potential in any line buffer.

In general, the characteristics of a transistor connected to a capacitor depend on the amount of charge stored in the capacitor, and when the capacitor side acts as a drain, that is, when the video signal is input, the charge stored in the capacitor is applied to the Video signal line. When discharging, 90% → 2.74τ for 10% discharging, and conversely 10% →
For 90% charging, if the capacity side acts as a source, 10% → 90% charging requires 7τ. However, τ = C /
Gm (τ is a time constant, C is a load capacitance, and Gm is a transistor conductance).

When the capacitance side is used as the source, it also depends on the potential between the gate and the drain. Therefore, in the present invention, it is most desirable that the capacity side acts as the drain without being affected by the potential between the source and the gate, and the maximum potential, that is, the center of the video signal amplitude is 0, and the P-type MOS is used. Is negative polarity, N-type M
When the OS is used, the capacity may be controlled to the maximum positive potential.

In the present invention, each line buffer has a line buffer potential control means that functions independently of the display signal line, and the charge amount of the line buffer is controlled via this control means to obtain an image. Signals can be input under optimum conditions.

In this way, according to the present invention, the line buffer for inputting the video signal is always controlled to have a constant potential, so that the input can always be performed under optimum and uniform conditions.

Next, the present embodiment will be specifically described. In this embodiment, all switching transistors are NM.
It has an OS configuration.

The size of the transistor for transferring the video signal to each line buffer is W / L = 90/2 [μm]
(W: gate width, L: gate length) and the line buffer capacity value is 12 p [F].

In this embodiment, a single crystal silicon layer (SOI layer) on an insulating layer is used as an active layer forming a transistor.
Was used. Examples of the method for manufacturing such a substrate include those obtained by recrystallizing an amorphous silicon or polycrystalline silicon film deposited on a glass substrate by an energy beam, or a substrate obtained by bonding a single crystal silicon wafer on an insulating film. Can be used. In particular, the latter is effective as a means for obtaining a good-quality single crystal SOI layer, and the substrate prepared by this method was used also in this example.

The input video signal amplitude is 3 to 13
[V], the voltage of the counter electrode of the liquid crystal was 8 [V], and the drive voltage amplitude of the shift register for driving the switching transistor was 0 to 15 [V].

The reset voltage of the line buffer capacity is set to 13 [V].

The liquid crystal display is normally white (white display in a standby state with no input signal).

As a result, in particular, when the input video signal is high, that is, in the display, the gradation property near black display is improved, and a liquid crystal display panel of 60 gradations can be realized as the entire input video signal. It was

It is considered that this is because the line buffer capacity was sufficiently charged when the input video signal was high.

In particular, as a liquid crystal material, TN (Twi
In the case of using a steady nematic liquid crystal, the driving method is so-called AC driving in which a positive and negative signal is input with respect to the center voltage, but in that case, the display is continued with the positive and negative signals being unbalanced. And DC in the liquid crystal molecule
The component is applied, and the liquid crystal molecules are gradually burned in due to the rearrangement of the impurity ions in the liquid crystal, and flicker begins to stand out, and finally a phenomenon occurs in which the liquid crystal molecules completely stop operating.

In the liquid crystal display panel according to the present embodiment, flicker was remarkably reduced, and a long-life panel without liquid crystal burn-in could be realized.

This is because the transfer capability of the video signal on the positive side (high level of the input signal) with respect to the center voltage of the counter electrode is improved, and the positive and negative signals are more balanced. It is believed that there is. (Embodiment 2) As a second embodiment of the present invention, an example of a PMOS configuration is shown in the drive timing chart shown in FIG.

The size of the transistor for transferring the video signal to each line buffer is W / L = 180/2 [μm]
(W: gate width, L: gate length) and the line buffer capacity value is 12 p [F].

In this embodiment, a single crystal silicon layer (SOI layer) on an insulating layer is used as an active layer forming a transistor.
Was used. Examples of the method for manufacturing such a substrate include those obtained by recrystallizing an amorphous silicon or polycrystalline silicon film deposited on a glass substrate by an energy beam, or a substrate obtained by bonding a single crystal silicon wafer on an insulating film. Can be used. In particular, the latter is effective as a means for obtaining a good-quality single crystal SOI layer, and the substrate prepared by this method was used also in this example.

The input video signal amplitude is 3 to 13
[V], the voltage of the counter electrode of the liquid crystal was 8 [V], and the drive voltage amplitude of the shift register for driving the switching transistor was 0 to 15 [V].

Further, the reset voltage of the line buffer capacity is set to 13 [V].

The liquid crystal display is normally white (white display in a standby state with no input signal).

As a result, in particular, when the input video signal is low, that is, as a display, the gradation property near black display is improved, and a liquid crystal display panel of 60 gradations can be realized as the entire input video signal. It was

It is considered that this is because the line buffer capacity was sufficiently charged when the input video signal was low.

In particular, as a liquid crystal material, TN (Twi
When a steady nematic liquid crystal is used, the driving method is so-called AC driving in which a positive and negative signal is input with respect to the center voltage, but in that case, the display is continued with the positive and negative signals being unbalanced. And DC in the liquid crystal molecule
The component is applied, and the liquid crystal molecules are gradually burned in due to the rearrangement of the impurity ions in the liquid crystal, and flicker begins to stand out, and finally a phenomenon occurs in which the liquid crystal molecules completely stop operating.

In the liquid crystal display panel according to the present embodiment, flicker was remarkably reduced, and it was possible to realize a panel having a long life and free from liquid crystal burn-in.

This is because the transfer capability of the video signal on the negative side (low level of the input signal) is improved with respect to the center voltage of the counter electrode, and the positive and negative signals are more balanced. It is believed that there is. (Embodiment 3) As a third embodiment of the present invention, an example different from the embodiment 1 in which the driving timing chart shown in FIG.

The size of the transistor for transferring the video signal to each line buffer is W / L = 400/3 [μm]
(W: gate width, L: gate length) and the line buffer capacity value is 12 p [F].

In this embodiment, a single crystal silicon layer (SOI layer) on an insulating layer is used as an active layer forming a transistor.
Was used. Examples of the method for manufacturing such a substrate include those obtained by recrystallizing an amorphous silicon or polycrystalline silicon film deposited on a glass substrate by an energy beam, or a substrate obtained by bonding a single crystal silicon wafer on an insulating film. Can be used. In particular, the latter is effective as a means for obtaining a good-quality single crystal SOI layer, and the substrate prepared by this method was used also in this example.

The input video signal amplitude is 3 to 13
[V], the voltage of the counter electrode of the liquid crystal was 8 [V], and the drive voltage amplitude of the shift register for driving the switching transistor was 0 to 16 [V].

The reset voltage of the line buffer capacity is set to 13 [V].

The liquid crystal display is normally white (white display in a standby state with no input signal).

As a result, in particular, when the input video signal is high, that is, in the display, the gradation property near black display is improved, and a liquid crystal display panel of 45 gradations can be realized as the entire input video signal. It was

It is considered that this is because the line buffer capacity was sufficiently charged when the input video signal was low.

In particular, as a liquid crystal material, TN (Twi
When a steady nematic liquid crystal is used, the driving method is so-called AC driving in which a positive and negative signal is input with respect to the center voltage, but in that case, the display is continued with the positive and negative signals being unbalanced. And DC in the liquid crystal molecule
The component is applied, and the liquid crystal molecules are gradually burned in due to the rearrangement of the impurity ions in the liquid crystal, and flicker begins to stand out, and finally a phenomenon occurs in which the liquid crystal molecules completely stop operating.

In the liquid crystal display panel according to this example, flicker was significantly reduced, and a long-life panel without liquid crystal burn-in could be realized.

This is because the transfer capability of the video signal on the positive side (high level of the input signal) with respect to the center voltage of the counter electrode is improved, and the positive and negative signals are more balanced. It is believed that there is.

[0051]

As described above, in the present invention, since the transistors connected to each line buffer can be operated under the optimum conditions, it is possible to further increase the number of pixels or the frequency, and to achieve higher definition. High quality image display can be realized.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of a liquid crystal display device of the present invention.

FIG. 2 is a timing chart of the liquid crystal display element shown in FIG.

FIG. 3 is a circuit diagram of an example of a conventional liquid crystal display element.

FIG. 4 is a timing chart of the liquid crystal display element shown in FIG.

FIG. 5 is a diagram showing a relationship between a reset voltage and a charging rate of a line buffer capacity according to the present invention.

[Explanation of symbols]

 1 Scanning Signal Line 2 Display Signal Line 3 TFT 4 Liquid Crystal 5 Vertical Shift Register 6 Horizontal Shift Register 7 Line Buffer

Claims (3)

[Claims] 1. An active matrix type liquid crystal display element using a thin film transistor as a switching element for each pixel, which stores a video signal input to a display signal line according to a signal from a shift register and through a transistor. Each display signal line has a line buffer for transferring a video signal to a pixel, and the line buffer has means for controlling the potential through a transistor different from the transistor arranged on the display signal line. Active matrix liquid crystal display device. 2. A method for driving an active matrix type liquid crystal display element using a thin film transistor as a switching element for each pixel, wherein a video signal is provided in a line buffer provided for each display signal line during a scanning period of one horizontal scanning period. Is stored, the gates of the transistors connected to the respective line buffers are turned on in a part of the subsequent blanking period, and the video signals stored in all the pixels are transferred at the same time. Driving an active matrix type liquid crystal display device characterized by turning off the gate of each of them and inputting a reset signal to all line buffers through different transistors connected to the line buffers to keep the potentials of all line buffers constant. Method. 3. A method for driving an active matrix type liquid crystal display device, wherein the potential held by the reset signal is the highest potential within the amplitude of the video signal.