JPH08220508A - Power supply circuit for liquid crystal display - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a power supply circuit for a liquid crystal display device, which forcibly eliminates the residual voltage generated when the liquid crystal display device is stopped and reduces the disorder of the display screen when the display is stopped or relighted. To do. A power supply circuit for a liquid crystal display device for supplying a drive voltage to a drive circuit DV of a liquid crystal display panel LCD,
Between the power supply of the drive voltage and the drive circuit DV, a voltage detection circuit 1 that detects that the voltage of the power supply has dropped below a predetermined value, and a switch that operates a discharge circuit with the detection output of the voltage detection circuit 1. The circuit 2 and the discharge circuit 3 for discharging the electric charge of the capacitor on the drive circuit side by the switch circuit 2 are provided.

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 device, and more particularly to a power supply circuit for a liquid crystal display device used as a time-division drive type high voltage drive high capacitance type power supply.

[0002]

2. Description of the Related Art A simple matrix type liquid crystal display device, which is one type of liquid crystal display device, has a TN or STN liquid crystal layer between at least two transparent substrates such as glass substrates on which electrodes and alignment films are formed. A power supply circuit for sandwiching the above electrodes, one of which is a common electrode and the other of which is a segment electrode, supplies independent bias voltage and power supply voltage.

FIG. 5 is a developed perspective view for explaining a structural example of this type of liquid crystal display device, in which 34 is a drive circuit (IC),
35 is a printed circuit board on which circuit elements such as a drive circuit are mounted,
36 is a cold cathode fluorescent lamp, 37 is a light guide, 38 is a reflector, 3
Reference numeral 9 is a diffusion plate, 41 is a frame, 42 is a frame-like body, 43 is a claw, 44 is a notch, 45 is a recess, 46 is a tongue piece, 47 is a small edge, and 62 is a liquid crystal cell.

In the figure, I for driving the liquid crystal cell 62
C34 is mounted on a frame-shaped printed board 35 having a window portion for fitting the liquid crystal panel 62 in the center.

The printed circuit board 35 in which the liquid crystal panel 62 is fitted is fitted in a window portion of a frame-shaped body 42 formed by plastic molding, a metal frame 41 is placed on the window-shaped body 42, and its claws 43 are formed on the frame-shaped body 42. The frame 41 is fixed to the frame-shaped body 42 by bending it into the formed notch 44.

The cold cathode fluorescent lamps 36 disposed at the upper and lower ends of the liquid crystal panel 62, and the light guide 37 made of an acrylic plate for uniformly irradiating the liquid crystal display cells 60 with the light from the cold cathode fluorescent lamps 36. A reflecting plate 38 formed by applying a white paint to a metal plate, and a milky white diffusing plate 39 for diffusing the light from the light guide 37 are fitted into the window portion from the back side of the frame body 42 in the order shown. .

An inverter power supply circuit (not shown) for lighting the cold cathode fluorescent lamp 36 is provided in a recess (not shown in the drawing) provided on the back side on the right side of the frame 42 in the figure. It is stored in the opposite position).

Diffusing plate 39, light guide 37, cold cathode fluorescent lamp 3
6 and the reflection plate 38 are fixed by bending a tongue piece 46 provided on the reflection plate 38 into an edge 47 provided on the frame-shaped body 42.

FIG. 6 is a circuit diagram for explaining an example of a conventional power supply circuit for a liquid crystal display device, where LCD is a liquid crystal panel and C is a liquid crystal panel.
OM is a common driver, SEG is a segment driver,
SC is a common side signal, SS is a segment side signal, M is an AC signal, V _OP is a power supply voltage, GND is a ground potential, V _DD is a logic circuit power supply voltage, OP is a buffer amplifier, R ₇
To R ₁₁ are voltage dividing resistors, C _{11 to} C ₁₅ are smoothing capacitors, and V ₀
˜V ₅ is a bias voltage.

In the figure, the power supply voltage V _OP is _divided by the voltage dividing resistor R
_The voltage is divided by _{7 to} R ₁₁ , and bias voltage V ₀ , V ₂ , V ₃ , to the segment driver SEG via the buffer amplifier OP.
The V _5, bias voltage V ₀ to the common driver COM, V
₁ , V ₄ , V ₅ are distributed. The capacitor C ₁₁
˜C ₁₅ are for smoothing the bias voltages V _{0 to} V ₅ .

Further, the power supply voltage V _DD for the logic circuit is commonly supplied to the segment driver SEG and the common driver COM.

As a disclosure of the prior art relating to this type of power supply circuit, there is JP-A-4-121786.

[0013]

As described in the above-mentioned prior art, the conventional power supply circuit for a liquid crystal display device is of a high capacitance type in which the capacitance of the capacitor is increased to improve the image quality.

In such a high capacitance type power supply circuit, the time constant when the power supply is stopped becomes large, and the display screen is displayed when the liquid crystal display device is stopped or restarted due to the residual voltage due to this time constant. There was a problem of disturbance.

When the cause of this was verified, the following facts were found.

In the sequence when the drive circuit of the liquid crystal display device is stopped, the voltage for driving the liquid crystal (about 30 V) is stopped first, and then various signals and logic voltages are sequentially stopped. The operation of the general liquid crystal display device at this time is in a non-display state. Then, the liquid crystal driving voltage is stopped.

At this time, however, the liquid crystal driving voltage is subjected to the next operation in the state where the voltage remains due to the time constant of the capacitance of the capacitor, the capacitor of the driving circuit, or the stray capacitance of the circuit. The logic voltage stops.

Therefore, the IC of the drive circuit is operated by this residual voltage to apply the residual voltage to the liquid crystal panel. Therefore, the residual voltage is confined inside the liquid crystal panel.

As a result, when the liquid crystal display device is stopped or relighted (redisplayed), the threshold value changes due to the residual voltage, and the display screen is disturbed.

An object of the present invention is to provide a power supply circuit for a liquid crystal display device, which forcibly eliminates the residual voltage generated when the liquid crystal display device is stopped and reduces the disturbance of the display screen when the display is stopped or relighted. To do.

[0021]

In order to achieve the above object, the present invention provides a driving power supply with a discharging circuit, detects a driving voltage when a liquid crystal display device is stopped, and supplies various signals and a logic power supply. Before stopping, the remaining drive voltage is eliminated by operating the discharge circuit of the drive power supply when the drive voltage reaches about 2V, for example.

That is, a first aspect of the present invention is a power supply circuit for a liquid crystal display device for supplying a drive voltage to a drive circuit DV of a liquid crystal display panel LCD, wherein the power supply for the drive voltage and the drive circuit are used. The voltage detection circuit 1 for detecting that the voltage of the power supply has dropped to a predetermined value or less during DV, the switch circuit 2 for operating the discharge circuit by the detection output of the voltage detection circuit 1, and the switch circuit 2 Discharge circuit 3 for discharging the electric charge of the capacitor on the drive circuit side
And characterized in that:

In a second aspect of the present invention, the voltage detection circuit 1 includes a Zener diode ZD connected in parallel to the power source, and the switch circuit 2 is turned on by a detection output of the voltage detection circuit 1. The discharge circuit 3 includes a MOSFET that is turned on by the detection output of the voltage detection circuit 1, and the detection output of the voltage detection circuit 1 detects a decrease in the power supply voltage below a predetermined value. The transistor Tr of the switch circuit 2 is turned off, and the MOSFET of the discharge circuit 3 is turned on by this off output to discharge the electric charge of the capacitor on the drive circuit side.

Incidentally, in place of the Zener diode ZD, the transistor Tr and the MOSFET in the above-mentioned structure of the present invention, an element having a function equivalent to these can be adopted.

[0025]

In the configuration of the first invention, the voltage detection circuit 1 is connected in parallel between the drive voltage power source and the drive circuit DV, and detects that the voltage of the power source has dropped below a predetermined value.

The switch circuit 2 gives a control signal for the discharge operation to the discharge circuit by the detection output of the voltage detection circuit 1.

The discharge circuit 3 grounds the capacitor on the drive circuit side according to the control signal from the switch circuit 2 to discharge the electric charge.

In the configuration of the second aspect of the invention, the Zener diode ZD constituting the voltage detection circuit 1 connected in parallel to the power supply detects that the power supply voltage has become equal to or lower than a predetermined value.

Transistor T constituting switch circuit 2
r is turned on by the detection output of the voltage detection circuit 1, and the MOSFET forming the discharge circuit 3 is turned on by this on signal to discharge the electric charge of the capacitor on the drive circuit side.

[0030]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a circuit configuration diagram for explaining one embodiment of a power supply circuit for a liquid crystal display device according to the present invention. 1 is a voltage detection circuit, 2 is a switch circuit, 3 is a discharge circuit, LCD is a liquid crystal panel, DV is a driver, V _LCD1 is a power supply voltage, V _LCD2 is a power supply voltage actually supplied to the driver, ZD is a Zener diode, R ₁ , R ₂ , R ₃ , R ₄ are resistors, Tr is a transistor, Q is a MOSFET, C ₁ , C ₂ and C ₃ are capacitors.

In the figure, the power supply voltage V _LCD1 is converted into the power supply voltage V _LCD2 actually supplied to the driver.

A voltage detection circuit 1, a switch circuit 2 and a discharge circuit 3 are connected in parallel between the power supply voltage V _LCD1 and the driver DV.

The voltage detection circuit 1 is a Zener diode ZD
And a resistor R ₁ and a resistor R _{2 connected} in series, and the Zener voltage of the Zener diode ZD is set to 2V, for example.

The switch circuit 2 is composed of a series circuit of a resistor R ₃ and a transistor Tr, the base of the transistor Tr is connected between the resistors R ₁ and R ₂ of the voltage detection circuit 1, and the collector serves as an output terminal.

The discharge circuit 3 includes a resistor R ₄ and a MOSFET.
It consists of a series circuit of (Q), and connects the collector of the transistor Tr to the gate.

The drive voltage V _LCD2 is supplied to the drive circuit DV from the drain side of the MOSFET (Q).

The resistors R ₁ and R ₂ are resistors for setting the operating voltage of the Zener diode ZD and the transistor Tr, and the resistors R ₃ and R ₄ are resistors for controlling overcurrent.

In this structure, when the power supply of the liquid crystal display device is cut off and the drive circuit is stopped, when the voltage detection circuit 1 monitoring the power supply voltage V _LCD1 detects 2V, the transistor Tr of the switch circuit 2 is turned on. . When the transistor Tr is turned on, the MOSFET (Q) of the discharge circuit 3
Is turned on, and the line of the drive voltage V _LCD2 is dropped to the ground.

As a result, the capacitor C of the liquid crystal display device
₁ , the residual voltage generated when the liquid crystal display device is stopped because the electric charge of the capacitor C _{2 of the} drive circuit or the stray capacitance C ₃ is discharged to the ground, and the display screen when the display is stopped or relighted Turbulence is reduced.

FIG. 2 is an explanatory diagram of a voltage sequence when the driving voltage of the liquid crystal display device is stopped. A solid line a is a decrease curve of the conventional driving voltage V _LCD2 , and a broken line b is the driving voltage V of this embodiment. reduction curve of _{LCD 2,} V _{LCD 1} is the power supply voltage, V _{LCD 2} is a power supply voltage actually supplied to the driver, V _CC logic power supply, GND is a ground potential.

In the figure, when the liquid crystal display device is performing a display operation, the Zener diode ZD of the voltage detection circuit 1 is off, so that the switch circuit 2 is inactive and the discharge circuit is inactive.

When the liquid crystal display device is stopped at t ₁ , the power supply voltage V _LCD1 gradually decreases and the Zener diode ZD of the voltage detection circuit 1 becomes conductive at time t ₂ after T time.

Along with this, the transistor Tr of the switch circuit 2 is turned on and the MOSFET (Q) of the discharge circuit 3 is turned on.
Is turned on, and the line of the drive voltage V _LCD2 is dropped to the ground.

As a result, the drive voltage V _LCD2 is forcibly discharged to the ground potential by the charge of the capacitor C ₁ , the capacitor C _{2 of the} drive circuit, or the stray capacitance C ₃ .

As a result, the disorder of the display screen is reduced.

FIG. 3 is a block diagram of a liquid crystal display device to which this embodiment is applied. 34 is a drive IC, 48 is a control IC, 49 is a microprocessor unit, and 62 is a control unit.
Is a liquid crystal panel, and 64 is a liquid crystal module.

In the figure, the result calculated by the microprocessor 49 is driven by the driving IC 34 through the control LSI 48 to drive the liquid crystal display module.

FIG. 4 is a perspective view of a laptop personal computer as an example of a liquid crystal display device to which this embodiment is applied.
Since the power supply circuit according to the present embodiment is applied to the liquid crystal module 63, it is possible to obtain a high-quality display in which the display is not disturbed due to the power interruption.

[0050]

As described above, according to the present invention,
Since the electric charge remaining in the drive circuit or the like when the liquid crystal driving power supply is stopped can be forcibly removed, the display is not disturbed when the power is stopped or relighted, and a high-quality liquid crystal display device can be provided. .

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram illustrating an embodiment of a power supply circuit for a liquid crystal display device of the present invention.

FIG. 2 is an explanatory diagram of a voltage sequence when the driving voltage of the liquid crystal display device is stopped.

FIG. 3 is a block diagram of a liquid crystal display device to which an embodiment of the present invention is applied.

FIG. 4 is a perspective view of a laptop personal computer as an example of a liquid crystal display device to which an embodiment of the present invention is applied.

FIG. 5 is a developed perspective view illustrating a structural example of a liquid crystal display device.

FIG. 6 is a circuit diagram illustrating an example of a conventional power supply circuit for a liquid crystal display device.

[Explanation of symbols]

1 voltage detection circuit 2 switch circuit 3 discharge circuit supply voltage ZD zener diode R ₁ actually supplied to the LCD liquid crystal panel DV driver V _{LCD 1} power supply voltage V _{LCD 2 driver, R 2, R 3, R} 4 the resistance Tr transistor Q MOSFET C ₁ , C ₂ , C ₃ capacitors.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshimitsu Matsudo 3300 Hayano, Mobara-shi, Chiba Hitachi, Ltd. Electronic Device Division

Claims (2)

[Claims] 1. A power supply circuit for a liquid crystal display device for supplying a drive voltage to a drive circuit of a liquid crystal display panel, wherein the voltage of the power supply drops below a predetermined value between the power supply of the drive voltage and the drive circuit. And a switch circuit for operating the discharge circuit by the detection output of the voltage detection circuit, and a discharge circuit for discharging the electric charge of the capacitor on the drive circuit side by the switch circuit. Power supply circuit for liquid crystal display device. 2. The voltage detection circuit according to claim 1, wherein the voltage detection circuit includes a Zener diode connected in parallel to the power source, the switch circuit includes a transistor that is turned on by a detection output of the voltage detection circuit, and the discharge circuit includes the transistor. A MOSFET that is turned on by the detection output of the voltage detection circuit 1 is turned on, the transistor of the switch circuit is turned off by the detection output of the voltage detection circuit that detects a decrease of the power supply voltage below a predetermined value, and the discharge is performed by the off output. Circuit mosfet
A power supply circuit for a liquid crystal display device, which is configured so that the electric charge of the capacitor on the drive circuit side is discharged by turning on the.