JP2970126B2 - Liquid crystal display voltage supply circuit - Google Patents

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 voltage supply circuit for use in a display device using a liquid crystal display and for preventing an inrush current.

[0002]

2. Description of the Related Art FIG. 4 shows a configuration of a conventional liquid crystal display voltage supply circuit.

In FIG. 4, a liquid crystal display voltage supply circuit 1 comprises an emitter follower circuit of transistors Tr1 and Tr2, and supplies a voltage to the liquid crystal display 2. The liquid crystal display 2 has a control circuit 2a and a driver circuit 2c for supplying a voltage to the liquid crystal unit 2b.

The variable resistor VR1 together with the resistor R2 controls the base voltage of the transistor Tr1 to change the output voltage of the transistor Tr1. The transistor Tr2 is connected to the resistor R2 to make the transistor Tr1 conductive (ON, ON) and nonconductive (OFF, OFF). The collector of the transistor Tr1 (c) and Tran
The collector of the transistor Tr2 is connected to the power supply terminal of the liquid crystal display 2 minus (−) 34V.

Next, the operation in this configuration will be described. The control circuit 2a, the driver circuit 2c, and the liquid crystal unit 2b of the liquid crystal display 2 require separate power supplies. Then, a driver circuit 2c for preventing destruction due to a malfunction is provided.
Is turned on after the control circuit 2a is completely operated. After the voltage plus (+) 5 V serving as a power supply for this control is applied and all the signals including the control signal Sc are supplied to the liquid crystal display 2, the display signal ( LCD enable signal ) Si is activated to activate the transistor. Tr2
Turn on. When the transistor Tr2 is turned on, the voltage VB divided by the resistor R1, the variable resistor VR1, and the resistor R2 from the base voltage of + 5V of the transistor Tr1.
1 to turn on the transistor Tr1. A current flows through the emitter (E) of the transistor Tr1 and the liquid crystal 2b
Is supplied with the voltage VB1.

[0006]

However, in the conventional liquid crystal display voltage supply circuit, the transistor T
The base of r1 is changed from a voltage of + 5V to an instantaneous voltage of about -20V.
The voltage changes to -30V, and the emitter voltage also drops sharply. In this case, the liquid crystal display 2 has a capacitor capacity of several + microfarads inside, and the transistor T
A large current instantaneously flows through the emitter (E) of r1.

FIG. 5 shows the voltage / current characteristics of the emitter (E) of the transistor Tr1. Simultaneously with the turning on of the transistor Tr2, the base (B) voltage of the transistor Tr1 drops sharply, and a large current 2.5A flows to the emitter (E). In this case, there is a disadvantage that the transistor Tr1 operates outside the SOA (safe operation area) and is sometimes destroyed.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems. An excellent liquid crystal display voltage supply capable of effectively preventing an inrush current and enabling a transistor to be used safely in an SOA (safe operation area). It is intended to provide a circuit.

[0009]

Means for Solving the Problems In order to achieve this object, a liquid crystal display voltage supply circuit of the present invention, first
Between the transistor and its base and ground
The connected capacitor and liquid crystal display signal are supplied,
A second transistor for driving a base of the first transistor;
A transistor and a base voltage of the first transistor
And a resistor for adjusting the
As a result, the base voltage of the first transistor changes suddenly.
From the liquid crystal display, and
Limit the inrush current flowing to the transistor emitter.
It is characterized by the following.

Also, the base of the first transistor and the ground
Instead of the while, in which the capacitor is connected between the base and the power supply circuit.

[0011]

Therefore, according to the liquid crystal display voltage supply circuit of the present invention, the first transistor in the emitter follower circuit is provided .
The capacitor connected between the base of the transistor and the ground or the power supply circuit prevents the rapid change of the base voltage of the first transistor, gradually changes the emitter current, and applies the voltage to the liquid crystal display with the capacitor capacity. Since the supply is performed, the rush current can be effectively prevented, and the used transistor can be safely operated in the SOA (safe operation area).

[0012]

EXAMPLES The following will be described in detail with reference to examples of the liquid crystal display voltage supply circuit of the present invention with reference to the drawings.

FIG. 1 shows the configuration of the first embodiment.
The same components as those shown in FIG. 3 are denoted by the same reference numerals.

In FIG . 1, a liquid crystal display voltage supply circuit 1 supplies a voltage to a liquid crystal display 2. The output of the variable resistor VR1 changes by controlling the voltage of the base (B) of the transistor Tr1 together with the resistors R1 and R2. The transistor Tr2 is a transistor Tr
1 is connected to a resistor R2 for turning on and off. The collector of the transistor Tr2 and the collector (c) of the transistor Tr1 supply terminal minus the liquid crystal display 2 - is connected to the 34V (). The capacitor C1 is connected between the ground GND and the base (B) of the transistor Tr1.

Next, the operation of the configuration of the first embodiment will be described. After a control voltage plus (+) 5 V is applied to the liquid crystal display 2 and a signal including a control signal Sc to be supplied to the liquid crystal display 2 is supplied and the liquid crystal display 2 operates,
The display signal ( LCD enable signal ) Si is activated to turn on the transistor Tr2. Transistor Tr2
Turns on the base (B) of the transistor Tr1 from the voltage + 5V to the resistor R1, the variable resistor VR1, and the resistor R2.
, The voltage VB1 changes from -20V to -30V. . Here, the capacitor C1 connected to the base (B) is charged with a voltage of +5 V, and this voltage gradually decreases due to the time constant of the capacitor C1, the variable resistor VR1, and the resistor R2. Therefore, the emitter (E) voltage of the transistor Tr1 also gradually decreases to the voltage VB1. Here, the current supplied to the liquid crystal display 2 is output as a differential waveform of the voltage change of the transistor Tr1 due to the capacitance of the liquid crystal display 2. If the voltage changes gradually, the current becomes a small current and the rush current is prevented. Is done. Since the voltage of the capacitor C1 changes from + 5V to -20V to -30V, the capacitor C1 must be bipolar when connected to the ground GND.

FIG. 2 shows the voltage / current characteristics of the emitter (E) of the transistor Tr1. When the transistor Tr2 turns on, the voltage of the emitter gradually decreases.
In this case, the emitter current is 400 mA or less, indicating that no rush current has occurred.

As described above, according to the first embodiment, the rush current is prevented only by connecting the bipolar capacitor C1 between the base (B) of the transistor Tr1 and the ground GND, and the transistor Tr1 is connected in the SOA. And the destruction of the transistor Tr1 can be prevented.

FIG. 3 shows the configuration of the second embodiment. In the second embodiment, the capacitor C1 shown in FIG.
Is connected to the base (B) of the transistor Tr1 and the ground GN.
From D, the voltage is changed to the base (B) and the voltage + 5V end.

Next, the operation of the configuration of the second embodiment will be described. The operation of the second embodiment is the same as that of the first embodiment, except that the other of the capacitors C1 has the voltage
Even if the base (B) of the transistor Tr1 changes from a voltage of +5 V to a voltage of −20 V to −30 V, the voltage of the capacitor C1 changes from 0 V to −2 V.
5V to -35V. In this case, both unipolar and bipolar capacitors can be used.

The voltage / current characteristics of the emitter (E) of the transistor Tr1 in operation are the same as those shown in FIG.

As described above, in the second embodiment, a low-cost monopolar capacitor C1 can be used. As the unipolar capacitor C1, a monopolar electrolytic capacitor can be used. This monopolar electrolytic capacitor is of a surface mount type, and is not found in a bipolar electrolytic capacitor. Therefore, the dip type electrolytic capacitor is not mixed during the recent surface mounting, and the processing can be performed only by the surface mounting, so that there is no need to replace the automatic mounting machine and the workability is improved. In addition, a unipolar capacitor is inexpensive, and low cost can be realized.

[0022]

As is apparent from the above description, the liquid crystal display voltage supply circuit according to the present invention includes the capacitor connected between the base of the second transistor in the emitter follower circuit and the ground or the power supply circuit. 1
Since the emitter voltage is gradually changed by preventing the rapid change of the base voltage of the transistor, and the voltage is supplied to the liquid crystal display with the capacitor capacity ,
Inrush current can be effectively blocked, and the transistor used is SOA
This has the effect that it can operate safely within the (safe operation area).

[Brief description of the drawings]

FIG. 1 shows a first embodiment of a liquid crystal display voltage supply circuit according to the present invention.
Configuration diagram showing an embodiment of

FIG. 2 is a diagram illustrating an emitter voltage / current characteristic of a transistor, which is used for explaining the operation of the embodiment.

FIG. 3 is a configuration diagram in a second embodiment.

FIG. 4 is a configuration diagram showing a configuration of a conventional liquid crystal display voltage supply circuit.

FIG. 5 is a diagram illustrating an emitter voltage / current characteristic of a transistor, which is provided for explaining the operation of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Liquid crystal display voltage supply circuit 2 Liquid crystal display C1 Capacitor R1, R2 Resistor Tr1, Tr2 Transistor VR1 Variable resistor

Claims (2)

(57) [Claims] 1. A liquid crystal display connected to a driver circuit.
A first transistor for supplying a voltage, a capacitor connected between the base of the transistor and ground, a resistor connected to the base of the first transistor for adjusting a base voltage, and a display signal, And a second transistor for driving a base of the first transistor. 2. A liquid crystal display connected to a driver circuit.
A first transistor for supplying a voltage, a capacitor connected between the base of the transistor and a power supply, a resistor connected to the base of the first transistor for adjusting a base voltage, and a display signal; And a second transistor for driving a base of the first transistor.