JPH04195025A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To improve display quality and reliability by providing a discharging path for making the potential of the common electrode of the liquid crystal display panel of active matrix configuration provided with a driving circuit the same as the potential on an opposed picture element electrode side in terms of AC at the time of cutting off a power source. CONSTITUTION:A scanning line, a signal line, a TFT transistor and the picture element electrode are formed on an LCD substrate. Next, a common electrode is provided to supply voltage Vcom to positive and negative picture element signals. Then, the voltage is outputted through a buffer amplifier BA. A resistance R3 having a large resistance value is provided between the output of the buffer amplifier BA and the ground potential point of a circuit. Thus, the holding voltage of each picture element is rapidly discharged by connecting the common electrode side to the ground potential of the circuit at the time of cutting off the power source.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a liquid crystal display device, for example, a thin film (TPT) display device.
) This relates to a technology that is effective for use in liquid crystal display panels with an active matrix configuration using transistors.

[Conventional technology]

Regarding active matrix liquid crystal devices equipped with TPT, for example, see Nikkei McGraw-Hill, 1984.
"Nikkei Electronics 1, page 211, dated September 10, 2016.

[Problem to be solved by the invention]

In conventional liquid crystal display devices, technological development is being carried out in the direction of increasing panel impedance in order to improve reliability. The inventor's research has revealed that increasing the panel impedance in this way causes the following problems. That is, when the power is cut off, each pixel of the liquid crystal display panel accumulates the pixel signal just before the power is cut off, and if the panel impedance becomes high as described above, it will take several hours to discharge the signal.

This not only causes poor display quality as the previous screen remains shallow when the power is turned on again, but also lacks reliability as DC voltage is applied to the liquid crystal during power off, shortening the life of the liquid crystal. A problem arises.

An object of the present invention is to provide a liquid crystal display device with improved display quality and reliability.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[Means to solve the problem]

A brief overview of typical inventions disclosed in this application is as follows.

That is, for a liquid crystal display device equipped with an active matrix liquid crystal display panel and a drive circuit, the common electrode of the liquid crystal display panel is set at the same potential as the opposite pixel electrode side when AC power is cut off. Provide a discharge path.

[For production]

According to the above-mentioned means, since the signal voltage of each pixel is quickly discharged when electricity is cut off, display quality and reliability can be improved.

[Embodiment] FIG. 1 shows a block diagram of an embodiment of a liquid crystal display device according to the present invention. In the figure, the liquid crystal display panel LCD includes a plurality of scanning lines G1 to G480 extending in the horizontal direction and a plurality of signal lines DI extending in the vertical direction.
or have Dn.

One pixel PX consisting of a TPT transistor and a pixel electrode is arranged at each intersection of the scanning line and the signal line.

Roughly speaking, a liquid crystal display panel LCD has scanning lines, signal lines, TFT transistors and pixel electrodes provided at each intersection, formed on a single transparent glass substrate.
It is formed by providing a common electrode as shown by the dotted line in the same figure on a transparent glass substrate facing this, and sealing liquid crystal between them. Although not particularly limited, a color filter, a blank matrix pattern for light shielding, and the like are formed on the common electrode side.

In the above-mentioned scan', the odd-numbered scanning line electrodes Gl, , G3 .
They are sequentially brought into a selected state by DL. The remaining even-numbered scanning lines G2. G'4...G480 are sequentially brought into a selected state by the second scanning line drive circuit GDR arranged on the right side. That is, the above two scanning line drive circuits GDL
By alternately selecting the scanning lines by GDR and GDR, scanning lines G1 to G480 are sequentially selected. These scanning line drive circuits GDL and GDR have their operations controlled by selection signals S1 and G2, and are composed of a shift register and a drive circuit that perform a shift operation in response to a synchronization signal (not shown), although not particularly limited thereto. Note that in the figure, one scanning line drive circuit GDL and GDR are arranged on the left and right sides of the liquid crystal display panel LCD.
The present invention is not limited to two independent scanning line drive circuits. That is, the scanning line drive circuit G
The DL and GDR may be configured by I/n semiconductor integrated circuit devices. Alternatively, the scanning line electrode of the liquid crystal display panel may be divided into a plurality of parts, and a plurality of semiconductor integrated circuit devices having the above-mentioned circuits GDL and GDR may be used corresponding to each divided scanning line electrode.

Pixel signals are supplied to the signal lines DI to Dn by a signal line drive circuit DD. This signal line drive circuit DD
is a serial/parallel converter with a launch function that converts pixel data VD supplied serially into parallel data and holds it.
White/white according to the parallel conversion circuit S/P and its retained information
It has a drive circuit that drives the signal line with a signal level corresponding to the black hell. Through the serial/parallel conversion operation, pixel data for one horizontal line is serially input, converted into parallel data, and output to each of the signal lines DI to Dn.

In synchronization with the parallel output of such pixel signals, the scanning lines are sequentially selected and each signal line D to D is selected as described above.
The pixel signal inputted through the pixel signal is written to the pixel electrode via the TPT transistor turned on according to the selection level of the scanning line, and is held for one field. Due to AC driving of the liquid crystal, in the next field a pixel signal with reversed polarity is formed and written to the same pixel electrode.

The timing control circuit TG receives the synchronization signal 5YNC and outputs the shift clock CL for the serial/parallel conversion, the selection signals Sl and S2 that put the scanning line drive circuits GDL and GDR into operation, and the shift operation (not shown). generate the necessary timing signals.

For positive and negative pixel signals as above, the midpoint voltage V c
om is supplied to the common electrode. The voltage v com applied to the common electrode is formed by voltage dividing resistors R1 and R2 provided between the voltage Vcc and the voltage -Vss, and is outputted via the buffer amplifier BA configured as a portage follower. A resistor R3 having a relatively large resistance value is provided between the output of the buffer amplifier BA and the ground potential point of the circuit to reduce power consumption. This resistor R3 acts to connect the common electrode side to the ground potential of the circuit when electricity is cut off, and to quickly discharge the held voltage of each pixel.

FIG. 2 shows an equivalent circuit diagram focusing on one pixel.

The liquid crystal display panel LCD has a high panel impedance between the two electrodes, and the buffer amplifier BA also has a large output impedance when the power is cut off. Therefore, if the resistor R3 that forms the discharge path is not provided, the signal voltage held in the pixel PX will remain for a relatively long time. In this embodiment, by inserting the resistor R3 as described above, the common electrode side is connected to the ground potential point of the circuit. On the other hand, the pixel electrode side is connected to the signal line through the TFT l-transistor, and this TPT transistor has a resistance value sufficient to form a discharge meridian even in the off state, and the signal vA electrode is connected to the signal line through the TFT transistor. A ground potential point of the circuit is substantially provided through the circuit DD and the like. As a result, a discharge path as shown by the arrow in the figure is formed, and the signal voltage held in the pixel PX is quickly discharged. It is desirable to select the resistance value of the resistor R3 so that the discharge time is about 1 second at most.

FIG. 3 shows an equivalent circuit diagram of an embodiment in which an additional capacitor ICD is provided to improve the retention characteristics of pixel signals.

Since the pixel electrode alone has a small capacitance value and cannot sufficiently hold the pixel signal, an additional capacitor CD is provided in parallel to the pixel PX. In this case, in order to obtain a large capacitance value due to a small size, the additional capacitance is formed on the glass substrate on which the pixel electrode is provided. Therefore, a similar half-circuit amplifier BA' is also provided on the common electrode side of the additional capacitance CD. Corresponding to this, a discharge resistor R4 is also provided on the common electrode side of the additional capacitor CD.

FIG. 4 shows an equivalent circuit diagram of another embodiment in which an additional capacitor CD is provided to improve the retention characteristics of pixel signals.

In this embodiment, the wiring for the common electrode is omitted by connecting the additional capacitor CD to the previous scanning line. In this case, a discharge resistor R4 is provided for each drive circuit ODA that drives the scanning line. however,
Since the additional capacitance is not connected to the scanning line G480 located at the lowest position, the discharging resistor R4 may be omitted.

FIG. 5 shows a circuit diagram of another embodiment of the discharge path.

When fixed resistors R3, R4, etc. are used as the discharge path as in the above embodiment, this resistor R is
A DC current constantly flows through 3, R4, etc., increasing current consumption. In other words, if you try to shorten the discharge time,
Power consumption also increases accordingly.

Therefore, in this embodiment, a variable resistance circuit is used to construct the discharge path.

When the potential V com of the common electrode is set to a negative potential, a resistor R is connected between the circuit ground potential GND and the negative voltage -Vss.
IO and R11 are provided to turn on the PNP transistor Q1 when the voltage -Vss is applied. This PNP ) transistor Ql collector output is PNP
l-supply to the base of the transistor Q2, and the transistor Q
1 is in the on state, it is supplied to the base of the transistor Q2 at a high level, such as the ground potential of the circuit, to turn off the transistor Q2. A resistor R14 is provided between the emitter and collector of the transistor Q2, and load resistors R12 and R13 are provided between the collectors of the transistors Q1 and Q2 and the terminal P, respectively. The resistors RIO, R11, R12, and R14 are set to large resistance values in order to reduce current consumption. And resistance R
13 is set to a relatively small resistance value so that the holding voltage is quickly discharged when the power is cut off. In the embodiment shown in FIG. 1 or 2, the terminal P is connected to the common electrode of the liquid crystal panel or the output terminal of the buffer amplifier BA, and in the embodiment shown in FIG. In the embodiment shown in FIG. 4, the output terminal of BA and the output terminal of buffer amplifier BA'' are respectively connected to the common electrode of the liquid crystal panel or the output terminal of buffer amplifier BA and the output terminal of the scanning line or drive circuit.

In this embodiment circuit, since the power supply voltage -Vss is supplied during display operation, the transistor Q1 is in the on state and the transistor Q2 is in the off state as described above. As a result, the small current caused by the large resistors RIO and R11 does not flow between the ground potential of the circuit and the power supply voltage -VSS.

Further, since series resistors R13 and R14 are inserted between the terminal P and the ground potential point of the circuit, a minute current does not flow. This makes it possible to reduce power consumption during display operation. Then, when the power is cut off, the transistor Q1 is turned off, and the transistor Q2 is turned on due to the voltage held at the terminal P. As a result, the terminal P is quickly discharged to the ground potential of the circuit through the transistor Q2 and the resistor 13.

Among the above-mentioned discharge paths, those using resistive elements include those built into the liquid crystal display panel or the driving semiconductor integrated circuit device, as well as those provided on the mounting board on which the driving semiconductor integrated circuit device is mounted. You can also do this. Further, as described above, the variable resistance circuit made of a transistor resistance element may be built into the driving semiconductor integrated circuit device or provided on the mounting board.

The effects obtained from the above examples are as follows. That is, (1) for a liquid crystal display device having an active matrix liquid crystal display panel and a drive circuit, the common electrode of the liquid crystal display panel is brought to the same potential as the opposite pixel electrode side when the power is cut off using an alternating current; By providing a discharge path, the signal voltage of each pixel is quickly discharged when the power is cut off, and no DC voltage is applied to the liquid crystal (this improves display performance and reliability without alignment deterioration). You can get the effect that you can.

(2) By incorporating the resistor elements that make up the discharge path into the display panel or drive semiconductor integrated circuit device, it is possible to improve display quality and reliability without increasing assembly man-hours or mounting space. This has the effect of being able to.

(3) A power cutoff detection transistor is provided as a discharge path, and the resistance value of the discharge path is set to high resistance while being displayed and low resistance when the power is cut off, thereby suppressing an increase in current consumption.
The effect is that display quality and reliability can be improved.

Although the invention made by the present inventor has been specifically explained based on Examples above, this invention is not limited to the above-mentioned Examples, and it should be noted that various changes can be made without departing from the gist of the invention. Not even. For example, a negative voltage -Vss for display is supplied to the gate of a depletion type MO3FET, the depletion type MO3FET is turned off by the negative voltage -Vss during display operation, and the negative voltage -Vss is turned off when the power is cut off. The depletion type MO5FET may be turned on by utilizing the ground potential of , and used as the discharge path as described above. In this case, the holding voltage of the pixel cell can be quickly discharged without increasing current consumption due to the simple configuration.

In this way, the circuit constituting the discharge path can take various embodiments.

Further, the signal line of the liquid crystal display device may be supplied with a gradation multi-value signal corresponding to an analog television signal or a digitally converted television signal. That is, the driving circuit may be any circuit as long as it drives an active matrix liquid crystal display panel.

This invention can be widely used in liquid crystal display devices.

〔Effect of the invention〕

A brief explanation of the effects obtained by typical inventions disclosed in this application is as follows. That is, for a liquid crystal display device equipped with an active matrix liquid crystal display panel and a drive circuit, a discharge is performed to bring the common electrode of the liquid crystal display panel to the same potential as the opposite pixel electrode side when the voltage is cut off using an alternating current. By providing a path, the signal voltage of each pixel is quickly discharged when the power is cut off, and no DC voltage is applied to the liquid crystal, so it is possible to improve display performance and improve reliability without alignment deterioration. .

[Brief explanation of the drawing]

FIG. 1 is a block diagram of essential parts showing an embodiment of a liquid crystal display device according to the present invention, FIG. 2 is an equivalent circuit diagram of an embodiment focusing on one pixel cell in the liquid crystal display device, and FIG. 4 is an equivalent circuit diagram showing another embodiment of the liquid crystal display device; FIG. 5 is an equivalent circuit diagram showing another embodiment of the liquid crystal display device focusing on one pixel cell; The figure is a circuit diagram showing another embodiment of the discharge path provided in the liquid crystal display device. L CD...Liquid crystal display panel, PX...pixel, GDL7
．． GDR...Scanning line drive circuit, DD (S/P)...Signal line drive circuit, TG...Timing control circuit, G1-0
480...Scanning line, D1-Dn...Signal line, R1-R1
3...Resistor, T...TPT transistor, BA, BA'
・・Hasofa amplifier, GDA・・Drive circuit, CD・
・Additional capacitance, Ql, Q2...transistor.

Claims (1)

[Claims] 1. A liquid crystal display panel with an active matrix configuration;
1. A liquid crystal display device comprising: a drive circuit; and a discharge path that brings the common electrode of the liquid crystal display panel to the same potential as the opposing pixel electrode when power is cut off using an alternating current. 2. A patent characterized in that the pixel electrode is provided with an additional capacitor, and a discharge path similar to the above is also provided on the common electrode side of the additional capacitor corresponding to the common electrode side of the liquid crystal display panel. A liquid crystal display device according to claim 1. 3. The electrode corresponding to the common electrode side of the additional capacitor is connected to an adjacent scanning line, and correspondingly, a discharge path similar to the above is provided on the scanning line side as well. A liquid crystal display device according to claim 2. 4. The liquid crystal display device according to claim 1, 2, or 3, wherein the discharge path is built in a liquid crystal display panel or a drive circuit.