JPH11271707A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To speedily discharge electric charges accumulated in a capacitive component and to prevent deterioration of a liquid crystal material and degradation in picture quality of printing by switching the input/output potentials to and from a driving circuit to specific potentials when a power-off or abnormal state is detected. SOLUTION: A power detecting circuit 1 when detecting a power-off state outputs signals SW1 and SW2 for turning off switch circuits 2 to 5 (logical level 0) to respective driving circuits. When the signal SW1 enters the state of the logical level 0, the switch circuit 4 holds the reference signal input to a signal line driver 8 at the ground potential, so the signal voltage inputted from the signal line driver 8 to a pixel electrode of a liquid crystal panel 6 becomes 0 V (ground potential). Further, the switch circuit 3 connects the counter electrode of the liquid crystal panel 6 to the ground potential, so the counter electrode potential becomes 0 V. Therefore, electric charges accumulated in the liquid crystal capacitors of the respective pixels and capacitors between signal line counter electrodes can be all discharged.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a liquid crystal display device.

[0002]

2. Description of the Related Art A liquid crystal display (LCD) is an OA such as a personal computer, a word processor, and an EWS.
In addition to the display of the device, it is widely used in portable devices such as calculators, electronic books, display devices for electronic notebooks, portable TVs, and mobile phones. This is because the liquid crystal display device is different from other display devices, for example, a CRT (Cathode Ray Tube) or a PRT.
This is because it has features that it is small, has low power consumption, and has high display quality as compared with a DP (Plasma Display Panel) or the like.

[0003] However, for example, an LCD having a switching element such as a thin film transistor (TFT) in a pixel, particularly an FLC (Ferroelectric).
Liquid Crystal) and AFLC (Anti Ferroelectric Liq
In the case of using a liquid crystal material having a spontaneous polarization and having a memory property such as uid Crystal), since the display signal is held in the pixel, the operation ends with the display signal held in the pixel when the power is turned off. In some cases. Therefore, when the power supply is cut off, the impedance of the liquid crystal driving circuit becomes higher than that in a normal driving state, that is, a state in which an image is displayed, and the charge of the display signal stored in the pixel remains for a long time. I will. As a result, there is a problem that the display quality is deteriorated such as the deterioration of the liquid crystal material and the burn-in of the screen as when the liquid crystal is driven by DC.

FIG. 10 shows a block diagram of a conventional liquid crystal display device. The liquid crystal display device has an image signal (DATA)
And a synchronization signal (SYNC) for displaying the image signal on the liquid crystal panel and a power supply (Pwr) for operating a circuit in the liquid crystal display device. In liquid crystal display devices,
Based on these signals, control signals for operating the respective drive circuits by the display controller 7 are created.

In the signal line driver 8, a display signal (SIG), a synchronization signal (STH),
A clock signal (CPH) is input, and an output control signal (OE)
X), a reference signal (Vre
With reference to f), a display signal corresponding to the captured display signal (SIG) is supplied to the liquid crystal panel 6. The gate driver 9 sequentially supplies a scanning signal to the liquid crystal panel 6 according to a scanning start signal (STV), a clock signal (CPV), and an output control signal (OEY) from the display controller 7. The counter electrode drive circuit 11 supplies a counter electrode drive signal Vcom to the liquid crystal panel 6 according to a display polarity control signal (POL) from the display controller 7.

A signal line driver 8, a gate driver 9, a counter electrode driving circuit 11, a reference signal generating circuit 12
DC / DC for circuits other than logic circuits
The converter 10 converts a power supply voltage (Pwr) supplied from outside into a power supply voltage required in each circuit.

Considering the case where the power supply (Pwr) supplied to the liquid product display device shown in FIG. 10 is turned off and the circuit operation stops, the moment when the power supply voltage (Pwr) is turned off is shown in FIG. The equivalent circuit is as shown. Note that FIG.
The equivalent circuit inside the liquid crystal display panel 6 of the above equivalent circuit shows one pixel of the liquid crystal display panel 6, and in the actual liquid crystal panel 6, there are many pixels composed of TFTs and the like.

In the example shown in FIG. 11, after the power is turned off, the electric charge accumulated in the liquid crystal capacitance Clcp of the pixel and the capacitance Clcs between the signal line opposing electrodes is reduced by the respective drivers connected to the liquid crystal panel. (The output impedance Zx1 of the signal line driver 8, the output impedance Zy1 of the gate driver 9, and the output impedance Zcom1 of the common electrode driving circuit). There is an output impedance of the DC converter 10, and the electric charge accumulated in the liquid crystal capacitance Clcp of the pixel and the capacitance Clcs between the signal line opposed electrodes is not discharged instantaneously. Whether the TFT is in the ON state or the OFF state is undefined depending on the scanning state of the liquid crystal panel 6.
When T is off, the charge of the display signal written in the liquid crystal capacitance Clcp of the pixel is hardly discharged.

A state in which the power supply is completely shut off after a lapse of a while after the power supply is turned off, that is, the power supply of the drive circuit is completely turned off.
FIG. 12 shows an equivalent circuit in the case of volts. As shown in FIG. 12, the pixels of the liquid crystal panel 6 are connected to the ground through the output impedance of each driver even when the power supply is completely shut off, and the electric charge remaining in the capacitance Clcs between the signal line opposed electrodes passes over time. However, the charge remaining in the liquid crystal capacitance Clcp of the pixel is discharged even after the charge of the signal line counter electrode capacitance Clcs is discharged because the TFTs of all the pixels inside the liquid crystal panel 6 are turned off. It will remain for a while.

As described above, when electric charge remains in the pixel electrode capacitance Clcp or the capacitance Clcs between the signal line opposing electrodes inside the liquid crystal panel, a phenomenon equivalent to a case where the liquid crystal is driven by direct current for a long time is a phenomenon. There arises a problem that the reliability is reduced due to the deterioration and the image quality is reduced due to the occurrence of the burn-in phenomenon.

[0011]

As described above, in the conventional liquid crystal display device, when the power supply is turned off (or when an abnormality occurs in the power supply), the power is accumulated in the capacitance component in the liquid crystal panel. Since the remaining electric charges remain for a while without being discharged, there is a problem that deterioration of image quality such as deterioration of a liquid crystal material and burn-in occurs.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and when a power supply is cut off, electric charges accumulated in a capacitance component in a liquid crystal panel are quickly discharged to cause deterioration or burn-in of a liquid crystal material. It is an object of the present invention to provide a liquid crystal display device capable of preventing image quality deterioration such as the above.

[0013]

A liquid crystal display device according to the present invention comprises a liquid crystal panel having a plurality of pixels each having a liquid crystal layer interposed between a first electrode and a second electrode facing the first electrode. A first drive circuit for supplying a display signal voltage to the first electrode, a second drive circuit for supplying a counter voltage to the second electrode, and detection means for detecting an off state or an abnormal state of a power supply And switching the input to the first drive circuit and the output from the second drive circuit to a predetermined potential when the detection unit detects an off state or an abnormal state of the power supply. And a first switching means for discharging the electric charge stored in the capacitance component.

According to the present invention, when an off state or an abnormal state of the power supply is detected, the input potential to the first drive circuit and the output potential from the second drive circuit are switched to predetermined potentials. Since the electric charge accumulated in the capacitance component of each pixel can be quickly discharged, it is possible to prevent deterioration of the image quality such as deterioration of the liquid crystal material and burn-in. In particular, the above effects can be remarkably exerted for a liquid crystal display device using a liquid crystal material having spontaneous polarization (spontaneous polarization or spontaneous polarization induced by applying an electric field) such as FLC and AFLC. .

In an active matrix type liquid crystal display device in which a switching element is provided for each pixel, a first electrode corresponds to a pixel electrode, a second electrode corresponds to a counter electrode, and a first drive circuit corresponds to a signal. The line drive circuit and the second drive circuit correspond to a counter electrode drive circuit. Further, it is preferable that the input potential to the first drive circuit and the output potential from the second drive circuit switched by the first switching means be the same potential (preferably a ground potential).

Further, in the above invention, each of the pixels of the liquid crystal panel is selected after the input to the first drive circuit and the output from the second drive circuit are switched to a predetermined potential by the first switching means. And a second switching means for switching an input to a third driving circuit (corresponding to a scanning line driving circuit) to a predetermined potential (preferably a ground potential). By adopting such a configuration, it is possible to more effectively discharge the charge accumulated in the capacitance component of each pixel.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a block diagram of a liquid crystal display device according to the first embodiment of the present invention.

The liquid crystal display device shown in FIG. 1 includes an image signal (DATA), a synchronization signal (SYNC) for displaying the image signal on a liquid crystal panel, and a power supply voltage (Pwr) for operating a circuit in the liquid crystal display device. ) Is entered. In the liquid crystal display device, a control signal for operating each drive circuit by the display controller 7 is created based on these signals.

In the signal line driver 8, a display signal (SIG), a synchronizing signal (STH),
A clock signal (CPH) is input, and an output control signal (OE)
X), a reference signal (Vre
With reference to f), a display signal corresponding to the captured display signal (SIG) is supplied to a pixel electrode in the liquid crystal panel 6 through a signal line. The gate driver 9 sequentially supplies a scanning signal to each TFT in the liquid crystal panel 6 according to a scanning start signal (STV), a clock signal (CPV), and an output control signal (OEY) from the display controller 7. The counter electrode drive circuit 11 supplies a counter electrode drive signal Vcom to a counter electrode in the liquid crystal panel 6 according to a display polarity control signal (POL) from the display controller 7.

A signal line driver 8, a gate driver 9, a counter electrode driving circuit 11, a reference signal generating circuit 12
DC / DC for circuits other than logic circuits
The power supply voltage (Pwr) supplied from outside is converted by the converter 10 into a power supply voltage required in each circuit.

In the liquid crystal display device of this embodiment, a power supply voltage (Pwr) supplied from the outside is input to the power supply detection circuit 1 in addition to the DC / DC converter 10. By monitoring the fluctuation of the power supply voltage in the power supply detection circuit 1, the power supply off state or the power supply voltage abnormal state is detected. The output impedance of each drive circuit of the liquid crystal panel 6 is reduced by controlling the switch circuits 2 to 5 connected to each drive circuit based on the detection result of the power supply detection circuit 1.

FIG. 2 shows a configuration example of the power supply detection circuit 1.
In the circuit of FIG. 2, the power supply voltage (Pwr) is changed by the resistors R1 and R2.
And divide the divided voltage values into reference voltage values VZ1 and VZ.
2 to detect the fluctuation of the power supply voltage. That is, the divided power supply voltage value is equal to the reference voltage value VZ1.
, And VZ2, it is detected as power-off. When power-off is detected, a signal SW for turning off the switch circuits 2 to 5 (zero the logical level)
1 to SW2 are output to each drive circuit. Also, the signal SW3
Is a signal supplied to the display controller 7. The display controller 7 always turns on the scanning start signal (STV) supplied to the gate driver 9 (logical level 1) by this signal SW 3, and sets all the TFTs inside the liquid crystal panel 6.
The gate of is turned on.

FIG. 3 shows a configuration example of the switch circuits 2 to 5. The switch circuit used for the drive circuit has a function of selecting one of the two inputs even when the power is turned off, such as a relay, and is preferably a switch having sufficiently low impedance even when the power is off. . Further, the selection input of the switch circuit needs to be configured to select the ground potential when the power is off.

Referring to FIG. 4, the switch circuits 2 to 5 are turned off (logic level 0), the gates of all the TFTs in the liquid crystal panel 6 are turned on, and writing for reducing the charge held in each pixel electrode to zero is performed. 3 is a timing chart of the signals SW1 to SW3 when the operation is performed. The reference voltage values VZ1 and VZ2 in the power supply detection circuit 1 are set so that VZ1>VZ2> the minimum voltage at which the logic circuit can operate. Power supply voltage (Pwr)
Power supply voltage (Pwr) inside the liquid crystal display device so that the voltage drop of
It is necessary to set the capacity of the decap 13 to the computer.

FIG. 5 shows a state in which the signal SW1 for controlling the switch circuits 2 to 5 is at the logical level 0 and the signal SW1 is at the logical level 1, that is, the gates of all the TFTs in the liquid crystal panel 6 are turned on. 4 shows an equivalent circuit of a drive circuit in a case where a signal writing for reducing the charge held in a pixel electrode to zero is performed.
FIG. 5 shows an equivalent circuit for one pixel in the signal line driver 8 and the gate driver 9.

In the case of the circuit shown in FIG. 5, the logic circuit is operating, and although the analog voltage (Vana) and the gate voltage (Vg) slightly vary, the signal line driver 8 and the gate driver 9 can operate. It is. When the SW1 signal is at the logical level 0, the reference signal input to the signal line driver 8 is set to the ground potential by the switch circuit 4, so that the display signal SIG input from the display controller 7 to the signal line driver 8 is provided. Irrespective of the above, the signal voltage input from the signal line driver 8 to the pixel electrode of the liquid crystal panel 6 becomes 0 volt (ground potential). SW1
When the signal is at the logical level 0, the counter electrode of the liquid crystal panel 6 is connected to the ground potential by the switch circuit 3, so that the counter electrode potential becomes 0 volt (ground potential). Further, when the SW1 signal is at the logical level 0, the SW3 signal is also at the logical level 1 at the same time, so that the display controller 7 always turns on the scanning start signal (STV) supplied to the gate driver 9 (logical level). 1), the gates of all the TFTs inside the liquid crystal panel 6 are turned on.

As described above, all the TFTs inside the liquid crystal panel
Is turned on, and the signal line potential and the counter electrode potential become the ground potential. Therefore, the liquid crystal capacitance C of each pixel
All the charges accumulated in lcp and the capacitance Clcs between the signal line opposed electrodes can be discharged.

Next, FIG. 6 shows an equivalent circuit in the case where both the SW1 and SW2 signals are at the logic level 0. In this case, since both the signal line driver 8 and the gate driver 9 are connected to the ground potential through the switch circuit, the impedance between each terminal of the liquid crystal panel 6 and the ground can be reduced. Accordingly, the power supply (Pwr) voltage suddenly drops, and the SW2 signal remains at the logic level 0 while the SW2 signal is
Even if the signal does not have sufficient time for the logic level 1 state, that is, even if the residual charge in the liquid crystal panel 6 cannot be sufficiently discharged, the impedance between each terminal of the liquid crystal panel 6 and the ground is reduced. Can be
The charge retention time can be made shorter than before. Further, the impedance between the liquid crystal panel and the driving circuit can be made lower than that of a liquid crystal driving circuit having a normal configuration, so that the effect of preventing static electricity is improved.

Next, a second embodiment of the present invention will be described with reference to a block diagram shown in FIG. 7 in which a case where power-off information is supplied from outside the liquid crystal display device in addition to detecting a power-off state. I do.

In the example of FIG. 7, the power-off signal P
off (logic level 0). This example is shown in FIG.
The difference from the above example is the configuration of the power supply detection circuit 1. FIG. 8 shows a circuit configuration of the power supply detection circuit 1 used in the liquid crystal display device shown in FIG. That is, in the configuration example of the power supply detection circuit 1 illustrated in FIG. 8, the power supply off state is detected by the power supply detection circuit 1 itself, and the power supply off signal Poff is also detected by the power supply detection circuit 1. Therefore,
In this example, the power supply fluctuation or an external power supply off signal Po
The impedance of the liquid crystal driving circuit is reduced by any one of ff.

Next, a third embodiment of the present invention will be described with reference to FIG. In this example, the display signal SIG supplied from the display controller 7 to the signal line driver 8 is switched to a value corresponding to the signal of the ground level, thereby making the residual charge in the liquid crystal panel zero.

In the configuration example of FIG. 9, when the SW1 signal is at the logic level 0 and the SW2 signal is at the logic level 1 (see FIG. 4), the display controller 7 uses the SW3 signal (logic level 1) to control the gate. Scan start signal (STV) supplied to driver 9 is always on (logic level 1)
In addition, all the logic signals such as the display signal SIG, the synchronization signal STH, the clock signal CPH, and the output control signal OEX supplied to the signal line driver 8 are always set to the ground level (or a value that becomes the display signal closest to the ground level). Fixed to. As a result, the gates of all the TFTs inside the liquid crystal panel are turned on, and the signal line potential and the counter electrode potential become the ground potential.

Therefore, it is possible to discharge all the electric charges stored in the liquid crystal capacitance Vlcp and the signal line facing electric capacitance Vlcs of each pixel. Further, since the number of switch circuits can be reduced, a low-impedance circuit of the liquid crystal drive circuit can be realized at low cost.

In each of the above embodiments, when a power-off state or the like is detected, in addition to the switching of the input to the signal line driver and the gate driver to a predetermined potential as described in the above embodiments, the signal line driver and the gate Apply another input to the driver to a specified potential (ground potential)
May be switched. In addition, the present invention can be variously modified and implemented without departing from the spirit thereof.

[0035]

According to the present invention, when an off state or an abnormal state of the power supply is detected, the input or output of the drive circuit is switched to a predetermined potential, whereby the electric charge stored in the capacitance component of each pixel is changed. Can be quickly discharged, and it is possible to prevent deterioration in image quality such as deterioration of the liquid crystal material and burn-in.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a configuration example of a power supply detection circuit shown in FIG. 1;

FIG. 3 is a diagram illustrating a configuration example of a switch circuit illustrated in FIG. 1;

FIG. 4 is a diagram showing operation timings of the power supply detection circuit according to the first embodiment of the present invention.

FIG. 5 is a diagram showing an equivalent circuit at the time of writing a ground potential in the first embodiment of the present invention.

FIG. 6 is a diagram showing an equivalent circuit of the liquid crystal drive circuit when the power is off according to the first embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of a liquid crystal display device according to a second embodiment of the present invention.

FIG. 8 is a diagram showing a configuration example of a power supply detection circuit shown in FIG. 7;

FIG. 9 is a block diagram showing a configuration of a liquid crystal display device according to a third embodiment of the present invention.

FIG. 10 is a block diagram showing a configuration of a liquid crystal display device according to the related art.

FIG. 11 is a diagram showing an equivalent circuit at the moment when the power is turned off in the liquid crystal display device according to the related art.

FIG. 12 is a diagram showing an equivalent circuit of a liquid crystal driving circuit when a power supply of a liquid crystal display device according to the related art is turned off.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Power supply detection circuit 2, 3, 4, 5 ... Switch circuit 6 ... Liquid crystal panel 7 ... Display controller 8 ... Signal line driver 9 ... Gate driver 10 ... DC / DC converter 11 ... Counter electrode drive circuit 12 ... Reference signal generation circuit 13. Power supply voltage bypass capacitor

Claims (2)

[Claims] 1. A liquid crystal panel having a plurality of pixels each having a liquid crystal layer sandwiched between a first electrode and a second electrode facing the first electrode, and supplying a display signal voltage to the first electrode. A first drive circuit, a second drive circuit for supplying a counter voltage to the second electrode, detection means for detecting a power-off state or an abnormal state, and a power-off state or an abnormal state by the detection means Is detected, by switching the input to the first drive circuit and the output from the second drive circuit to a predetermined potential, thereby discharging the charge accumulated in the capacitance component of the liquid crystal layer. 1. A liquid crystal display device comprising: 2. The method according to claim 1, wherein said first switching means switches an input to said first drive circuit and an output from said second drive circuit to a predetermined potential, and then selects each pixel of said liquid crystal panel. 2. The liquid crystal display device according to claim 1, further comprising second switching means for switching the input to the third drive circuit to a predetermined potential.