JPH04371999A - Power supply circuit for driving liquid crystal display panel - Google Patents

Abstract

PURPOSE:To minimize the disturbance of a display screen and the impressing of a DC voltage to liquid crystal by short-circuiting the power supply line between a liquid crystal display panel and a power source, and the ground when electric power supply is stopped. CONSTITUTION:When the electric power supply to the liquid crystal display panel 19 is stopped, a short-circuit means (4th transistor) 16 short-circuits the power supply line between the liquid crystal display panel 19 and power source, and the ground. Namely, when the output of a liquid crystal power source control circuit 10 falls to 'low', 1st and 2nd transistors 121 and 122 turn off and a positive voltage to a voltage divider circuit 18 and the power supply to a DC-DC converter 15 are cut off. At the same time, a 3rd transistor 14 turns ON to supply a base current to a 4th transistor 16, and then the impedance between the collector and emitter of the 4th transistor 16 nearly becomes '0', so that charges accumulated in a large-capacity capacitor 17 are discharged with a short-circuit current flowing to the 4th transistor 16.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit for driving a liquid crystal display panel.

0002

2. Description of the Related Art Conventionally, for example, a power supply circuit for driving a liquid crystal display panel in an electronic device using a battery as a power source has been constructed as shown in FIG. That is, when driving power is supplied from the power supply 1 to the liquid crystal display panel 2, the output of the liquid crystal power supply control circuit 3 is "high", and the first and second transistors 41 and 42 are turned on. Therefore, a negative voltage is generated on the output side of the DC-DC converter 5. The voltage is normally supplied to the liquid crystal driver 7 via a voltage dividing circuit 6. Usually, a fairly large capacitor 8 is connected to the output side of the DC-DC converter 5 in order to remove ripples in the output voltage.

0003

[Problems to be Solved by the Invention] In such a conventional liquid crystal display panel power supply circuit, as shown in FIG. 5(a), when the output of the liquid crystal power supply control circuit 3 becomes "low", the liquid crystal When looking at the process in which the power supply to the driver 7 is cut off, the electric charge accumulated in the large-capacity capacitor 8 is discharged, and as shown in FIG. 5(b), the output side of the DC-DC converter 5 is A certain amount of time will elapse before the voltage completely reaches 0 volts (V). During this time, the operation of the liquid crystal driver 7 becomes unstable, and as shown in FIG.
As shown in c), there was a problem in that the display screen of the liquid crystal display panel 11 was disturbed.

Furthermore, in many cases, the operation of the AC generating circuit built in the liquid crystal driver 7 becomes abnormal until the voltage on the output side of the DC-DC converter 5 completely reaches 0 volts. Therefore, a DC voltage is applied to the liquid crystal, which causes a problem of rapidly deteriorating the life and characteristics of the liquid crystal.

The present invention has been made in view of the above circumstances, and minimizes the disturbance of the display screen of the liquid crystal display panel and the application of DC voltage to the liquid crystal until the power supply line reaches 0 volts. It is an object of the present invention to provide a power supply circuit for driving a liquid crystal display panel that can suppress the problem.

[0006]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a method for connecting the power supply line between the liquid crystal display panel and the power supply and the ground when the power supply to the liquid crystal display panel is stopped. The device is characterized in that it is provided with short-circuiting means for short-circuiting.

[0007]

[Operation] When the power supply to the liquid crystal display panel is stopped, the power supply line between the liquid crystal panel and the power supply is short-circuited to the ground by the short-circuit means, so that the power supply line is not connected to the power supply line. The charge stored in the large capacitor connected to the power supply line is rapidly discharged, and the power supply line is
The time elapsed to reach the bolt is significantly reduced,
Disturbances of the liquid crystal display screen and application of DC voltage to the liquid crystal that occur during the elapsed time are suppressed to a minimum.

[0008]

Embodiments Hereinafter, embodiments of the present invention will be explained based on FIGS. 1 to 3.

FIG. 1 is a block diagram of a power supply circuit for driving a liquid crystal display panel according to an embodiment of the present invention.
10 is a liquid crystal power supply control circuit that supplies power to a liquid crystal driver (described later) when its output is "high"; 11 is a battery that is a power source; and 121 and 122 are circuits that turn on power supply to a liquid crystal display panel (described later). /first and second transistors which are switching elements for switching off.

131, 132, 133, 134 are the first
, second transistors 121 and 122, and resistors that limit the base currents of third and fourth transistors 14 and 16, which will be described later. 14 is turned on when the output of the liquid crystal power supply control circuit 10 is low. This is the third transistor.

15 is a DC-DC converter that generates a "negative" DC voltage from a "positive" DC voltage; 16 is a connection between the output side of the DC-DC converter 15 and the ground when the third transistor 14 is turned on; a fourth transistor (shorting means) that short-circuits the DC-DC converter 1;
This is a large capacity capacitor to stabilize the output of 5.

Reference numeral 18 supplies a step potential for alternating current to the liquid crystal driver 19 through the first and second transistors 121 and 12.
The positive voltage that is the output of 2 and the DC-DC converter 15
19 is a liquid crystal display panel; 20 is a liquid crystal driver for driving this liquid crystal display panel 19;

Next, the operation of the power supply circuit for driving a liquid crystal display panel having the above structure will be explained.

First, as shown in FIG. 2(a), when the output of the liquid crystal power supply control circuit 10 becomes "low", the first and second transistors 121 and 122 are turned off, and the voltage dividing circuit 18
The positive voltage to the DC-DC converter 15 and the power supply to the DC-DC converter 15 are cut off.

At the same time, the third transistor 14 is turned on and the base current is supplied to the fourth transistor 16, so that the impedance between the collector and emitter of the fourth transistor 16 becomes almost "0". Therefore, the electric charge accumulated in the large capacitor 17 is as shown in FIG.
As shown in (d), the short circuit current flowing through the fourth transistor 16 causes rapid discharge.

Along with this, as shown in FIG. 2(b), the DC-
The output voltage of the DC converter 15 shifts to 0 volts in a short time, and as shown in FIG. It is also possible to prevent adverse effects on the lifespan and characteristics due to voltage application.

Next, another embodiment of the present invention will be explained based on FIG.

In this embodiment, the same parts as those in the embodiment shown in FIGS. 1 and 2 described above will be described with the same reference numerals in the drawings.

In the embodiment shown in FIGS. 1 and 2, the charge accumulated in the capacitor 18 is discharged by the fourth transistor 1.
6, whereas this embodiment uses a relay.

In FIG. 3, 21 is a relay, 22 is a relay 21
A third transistor 23 is used to control the current supply to the drive coil of the third transistor 22, and a third transistor 23 is used to supply a pulsed base current to the third transistor 22 at the moment the output of the liquid crystal power supply control circuit 10 becomes "low". A second capacitor 24 forming the differential circuit is a resistor that limits the emitter current of the third transistor 22.

Then, the output of the liquid crystal power supply control circuit 10 is "
When the voltage becomes low, a differential pulse-like current flows through the third transistor 22 by the second capacitor 23, and the relay 21 is turned on for a short time, discharging the charge accumulated in the first capacitor 17 almost instantaneously.

When the output of the liquid crystal power supply control circuit 10 becomes "high" again, the charges accumulated in the second capacitor 23 are discharged through the respective resistors 131 to 133 and 23.

In this embodiment, since the relay 21 is used, it is disadvantageous for making the shorting means compact, but the transition time of the voltage on the output side of the DC-DC converter 15 is as shown in FIG. It has the advantage that it can be made shorter than the embodiment shown in FIG.

Furthermore, in each of the above embodiments, an example has been described in which only the negative voltage supply line supplied to the voltage dividing circuit 18 is short-circuited, but the positive voltage supply line, that is, the first
By adding a function of shorting between the collector sides of the second transistors 121 and 122 and the ground, it is possible to more effectively prevent disturbances in the display screen of the liquid crystal display panel.

[0025]

[Effects of the Invention] As explained above, according to the present invention, the charge accumulated in the large capacity capacitor connected to the power supply line is rapidly discharged until the power supply line reaches 0 volts. As a result, the following effects (1) and (2) are achieved.

(2) Disturbances in the display screen of the liquid crystal display panel that occur during the time it takes for the power supply line to reach 0 volts can be prevented from being noticed by the user.

(2) By causing the liquid crystal driver to operate unstablely within the time it takes for the power supply line to reach 0 volts, it is possible to prevent an adverse effect on the life and characteristics of the liquid crystal due to DC voltage being applied to the liquid crystal.

[Brief explanation of the drawing]

FIG. 1 is a block configuration diagram of a power supply circuit for driving a liquid crystal display panel according to an embodiment of the present invention.

FIG. 2 is a time chart showing the operation of the circuit.

FIG. 3 is a diagram similar to FIG. 1 according to another embodiment of the present invention.

FIG. 4 is a block diagram of a conventional power supply circuit for driving a liquid crystal display panel.

FIG. 5 is a time chart showing the operation of the circuit.

[Explanation of symbols]

16 Fourth transistor (shorting means) 19 Liquid crystal display panel

Claims (1)

[Claims] 1. A liquid crystal display comprising short-circuiting means for short-circuiting a power supply line between the liquid crystal display panel and a power source and ground when power supply to the liquid crystal display panel is stopped. Power supply circuit for driving the display panel.