JPH0962223A - Power source circuit of liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce turbulence of an image screen at display OFF time and again ON time by arranging a reset circuit to set a display OFF signal on an undisplayed level by detecting ON/OFF and instantaneous cutoff of a power source line, as input of a display OFF circuit of a driving circuit to drive a liquid crystal panel. SOLUTION: A liquid crystal panel 7 is not lighted when a display OFF signal is put on a low level, and is lighted when it is put on a high level. In a reset circuit 1, a threshold value (4.7V) being detecting voltage is set, and the display OFF signal is maintained on a low level for delay time up to becoming operation setting voltage (+5V) from threshold value voltage at power source ON time, and when power source voltage becomes equal to or smaller than the threshold value at power source OFF time, the display OFF signal is put on a low level. When a power source is instantaneously cut off, it becomes a low level, and simlarly to power source ON time, the display OFF signal is maintained on a low level for delay time until the power source voltage becomes the operation setting voltage (+5V).

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,
In particular, the present invention relates to a power supply circuit of a liquid crystal display device that prevents display disturbance due to indetermination of a display off signal due to powering off of a liquid crystal cell in a time-division drive system drive circuit.

[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 illustrating an example of the overall structure of a liquid crystal display device using a liquid crystal display panel to which the present invention is applied. 7 is a liquid crystal display panel, 10a and 10b are drive circuit boards, and 11 is a drive circuit board. Light guide plate, 12 reflection plate, 13 diffusion plate, 14 lamp (cold cathode fluorescent lamp), 14a lamp reflector, 14b lamp cover, 15 intermediate frame, 16 upper frame, 16a cut and raised piece, 16b is a nail, 17 is a lower frame, 17a is a nail receiver, 18 is a spacer, 19 is adhesive tape, 20a, 20b, 21a, 21.
Reference numeral b is a cutout portion, and 22a and 22b are cutout portions.

In FIG. 1, the liquid crystal display panel 7 is set on an intermediate frame 15 on the lower surface of which a light guide assembly including a light guide plate 11, a reflection plate 12 and a diffusion plate 13 and a backlight including a lamp 14 are laminated. The upper frame 16 and the lower frame 17 are sandwiched and fixed.

Cut and raised piece 1 formed on the upper frame 16
6a contacts a ground pad formed on the drive circuit boards 10a and 10b, and the claw 16b engages with a claw receiver 17a formed on the lower frame 17 to fix the upper frame 16 and the lower frame 17.

The upper frame 16 and the liquid crystal panel 7 are fixed with an adhesive tape 19. Cutouts 20a, 20
b is provided at a position symmetrical to a line orthogonal to the central portion of the backlight, and the cutouts 21a and 21b are provided in the longitudinal direction of the lamp 14. Further, notches 22a and 22b are provided in the lower portions of both ends of the lamp 14. The upper frame 16 is made of a steel plate having a thickness of 0.8 mm, for example, and the lower frame 17 is made of a steel plate or aluminum having a considerable thickness of, for example, 0.5 mm.

The liquid crystal display device is sandwiched and fixed by the upper frame 16 and the lower frame 17 in the order shown in FIG. The lamp 14 is installed on one end side of the intermediate frame 15, and the emitted light is directed to the light guide assembly side of the backlight by the lamp reflector 14a. The lamp cover 14b installed on the liquid crystal panel 7 side of the lamp 14 prevents upward light leakage.

The spacer 18 is interposed between the light guide assembly installed in the recess formed in the intermediate frame 15 and the liquid crystal panel 7 to define the display area.

As described above, the upper frame 16 is formed of a stainless thin plate, and the lower frame 17 is formed of an aluminum thin plate. Then, the lamp 14 of the lower frame 17
At least a pair of cutouts 20a, 20b are provided at positions symmetrical to a line orthogonal to the central portion of the lamp 14 in at least the region of the liquid crystal panel 7 in the direction orthogonal to the lamp 14, and the length of the lamp 14 is directly below the lamp 14. At least two cutouts 21a, 21b provided in the direction
And the notch 2 provided at the lower part of both ends of the lamp 14.
2a and 22b are formed.

According to the above structure, the liquid crystal display device can be made thin and lightweight without lowering its rigidity, and at the center of the lamp 14 in at least the region of the liquid crystal panel 7 in the direction orthogonal to the lamp 14. Cutouts 20a and 20b provided at positions symmetrical with respect to a line orthogonal to the cutouts, cutouts 21a and 21b provided in the longitudinal direction of the lamp 14 immediately below the lamp 14, and cutouts provided at lower portions of both ends of the lamp 14. With the notches 22a and 22b, the heat dissipation effect is improved, a uniform temperature distribution is formed on the entire surface of the liquid crystal panel 7, and uneven display is prevented.

Further, since the lamp 14 is driven at a high frequency, a current flows from the lamp 14 to the lower frame 17 via the stray capacitance between the lower frame 17 and the lamp 14. This current is called “leakage current”, but the amount of current that contributes to the lighting of the lamp 14 is reduced by the amount of “leakage current”, resulting in a decrease in brightness.

Further, the lamp 14 generates heat when it is lit for a long time, and the temperature in the vicinity thereof rises with respect to the outside air. Therefore, unless some measures are taken, the heat directly affects the liquid crystal panel 7, The temperature distribution of the liquid crystal panel cannot be made uniform. Therefore, in the above embodiment, a cutout portion is provided,
The occurrence of display unevenness is prevented by preventing a decrease in brightness due to "leakage current" and making the temperature distribution of the liquid crystal panel uniform.

By adding a reset circuit, which will be described later, to the drive circuit of the liquid crystal display panel of the liquid crystal display device having such a structure, it is possible to display a high quality image without any disturbance in the display. FIG. 6 is a circuit diagram illustrating an example of a power supply circuit of a conventional liquid crystal display device. LCD is a liquid crystal panel, COM is a common driver, SEG is a segment driver, SC is a common side signal, SS is a segment side signal, and M is a segment side signal. 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, and R _{7 to} R ₁₁
Is a voltage dividing resistor, C _{11 to} C ₁₅ are smoothing capacitors, and V _{0 to} V ₅
Is the 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.

[0017]

As described in the above-mentioned prior art, the power supply circuit of the conventional 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 so-called power supply indetermination period (indetermination period) becomes large,
During this period, the display off signal is set to H level (+ 5V =
When the display state is set, there is a problem that the display screen is disturbed (such as occurrence of a bright line on the display screen).

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

In the sequence when the drive circuit of the liquid crystal display device is stopped, the voltage for driving the liquid crystal is stopped first, and then various signals and the logic voltage are sequentially stopped. The operation of the general liquid crystal display device at this time is in a non-display state. continue,
The liquid crystal drive voltage stops. However, at this time, the liquid crystal drive voltage is 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 drive circuit, or the stray capacitance of the circuit, that is, the voltage for various signals and logic. Stops.

Therefore, the IC of the drive circuit operates by this residual voltage and applies 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, the threshold value fluctuates due to the residual voltage and the display screen is disturbed.

FIG. 7 is a waveform diagram for explaining the timing of the power-on / off and the display-off signal.
Is a power-on / off waveform, and (b) is a display-off signal (Display-off) waveform.

In the figure, A is (a) power supply (+ 5V).
If the display-off signal (b), which is a signal for controlling display / non-display, is not set to 0 V, that is, the non-display level in the uncertain period A at the time of turning on or off, the above-mentioned display disorder occurs.

The display-off signal (b) is set to a high level (display) after the time t when the voltage reaches a specified voltage when the power is turned on, and is longer than the time when the power is turned off. It must be set to be low level (not displayed) before t.

Conventionally, such a display-off signal has been set on the design side of a target device such as a personal computer or a word processor incorporating a liquid crystal display device.

Therefore, the work on the design side is additionally required, and the load on the design side increases.

An object of the present invention is to solve the above-mentioned problems of the prior art and to reduce the disturbance of the display screen when the display is stopped or relighted on the drive circuit side of the liquid crystal panel. To provide a circuit.

[0029]

In order to achieve the above object, the present invention provides a drive circuit for driving a liquid crystal panel in a power supply circuit of a liquid crystal display device for supplying a drive voltage to the drive circuit of the liquid crystal display panel. The display-off circuit input is provided with a reset circuit for detecting on / off and instantaneous interruption of the power supply line and setting the display-off signal to a non-display level.

[0030]

Based on the configuration of the present invention, the reset circuit is, for example, an PST manufactured by Mitsumi Electric Co., Ltd.
Using the 575 series, it is connected to the display-off circuit input of the drive circuit that drives the liquid crystal panel, monitors the voltage of the power supply line, and detects its on / off and instantaneous interruption.

Based on this detection, after the uncertain period that accompanies the turning on of the power supply, the power supply is surely supplied with the power supply voltage (for example, 5 V).
V), the display-off signal is turned on to the display level, or the display-off signal is offset to the non-display level when the power is turned off.

[0032]

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

FIG. 1 is a circuit diagram for explaining an embodiment of a reset circuit which constitutes a power supply circuit of a liquid crystal display device according to the present invention, in which 1 is a reset circuit, 2 is a reset IC, and 3 is a display off. ) Signal input terminal, 4 +
5V power line, 5 ground (GND) terminal, 6 drive IC
Is a display-off signal output terminal to, and 7 is a liquid crystal panel.

The reset circuit 1 is mounted on a drive substrate of the liquid crystal panel 7 and is inserted into the power supply line 4 to the liquid crystal panel 7, and monitors the voltage of the power supply line 4 to detect its on / off or instantaneous interruption. To do.

FIG. 2 is a waveform diagram for explaining the operation of one embodiment of the reset circuit which constitutes the power supply circuit of the liquid crystal display device according to the present invention and the timing of the power-on / off and the display-off signal. (a) is a power on / off waveform, (b) is a display-off signal (Display-off)
Is the waveform of.

In the figure, the liquid crystal panel 7 is not illuminated (non-display state) when the display-off signal (b) is at low level, and is illuminated (display state) when it is at high level.

The reset circuit 1 sets a threshold value (4.7 V) as a detection voltage, and when the power is turned on, the delay time t from the threshold voltage until the power source voltage surely becomes the operation setting voltage (+5 V) is the display off signal. Is kept at a low level, and when the power supply voltage is below the threshold value when the power is off, the display off signal is immediately brought to a low level.

Further, even when the power supply is momentarily cut off, the display is turned off and the delay time t until the power supply voltage surely reaches the operation setting voltage (+5 V) is the same as when the power supply is turned on. Keep low.

The above time t can be arbitrarily changed by the values of the resistor Rd and the capacitor Cd in FIG. This delay time t is determined by t = 1.3 Cd × Rd + 0.007 (ms).

According to this embodiment, it is possible to reduce the above-mentioned disturbance of the display screen when the display is stopped or relighted on the drive circuit side of the liquid crystal panel.

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. By providing the drive circuit with the reset circuit described above, the disturbance of the screen due to the turning on / off of the power supply can be prevented.

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 turning on / off of the power supply.

[0044]

As described above, according to the present invention,
It is possible to provide a high-quality liquid crystal display device in which the disturbance of the screen due to the on / off of the power source for driving the liquid crystal or the momentary interruption is prevented.

[Brief description of drawings]

FIG. 1 is a circuit diagram illustrating one embodiment of a reset circuit that constitutes a power supply circuit of a liquid crystal display device according to the present invention.

FIG. 2 is a waveform diagram for explaining the operation of a reset circuit which constitutes a power supply circuit of a liquid crystal display device according to an embodiment of the present invention, for explaining the timing of power on / off and a display off signal.

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 an entire liquid crystal display device using a liquid crystal display panel to which the present invention is applied.

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

FIG. 7 is a waveform diagram for explaining power on / off and display off signal timings.

[Explanation of symbols]

 1 reset circuit 2 reset IC 3 display-off signal input terminal 4 + 5V power line 5 ground terminal 6 display-off signal output terminal to drive IC 7 liquid crystal panel 10a, 10b drive circuit board 11 light guide plate 12 reflector plate 13 diffuser plate 14 lamp (Cold cathode fluorescent lamp) 14a Lamp reflector 14b Lamp cover 15 Intermediate frame 16 Upper frame 16a Cut and raised piece 16b Claw 17 Lower frame 17a Claw receiver 18 Spacer 19 Adhesive tape 20a, 20b, 21a, 21b Cutout 22a, 22b Cutout Department.

Claims (1)

[Claims] 1. A power supply circuit of a liquid crystal display device for supplying a drive voltage to a drive circuit of a liquid crystal display panel, wherein a power supply line is turned on / off and momentarily disconnected at a display off circuit input of the drive circuit for driving the liquid crystal panel. A power supply circuit for a liquid crystal display device, which is provided with a reset circuit for detecting a display-off signal to a non-display level.