JPH04125685A - Lcd driving power source circuit - Google Patents

Abstract

PURPOSE:To reduce the time lag of a detection, and to protect a liquid crystal display element (LCD) displaying panel by attaining a sequence by the detection of a voltage falling point without depending on the detection of the voltage. CONSTITUTION:A first transistor 11 is connected with a positive power source side 7 for a microprocessor unit (CPU), and a second transistor 12 is connected with negative power source sides 9 and 10 for LCD. Then, the voltage of a power source line for driving an IC for driving the LCD is simultaneously set according to the turning-on and off of a power source switch. Therefore, the voltage of the power source 7 of the CPU falls according to the power source turning-off, and the voltage -VLCD2 of the terminal 10 of the negative power source is turned to an earth potential by the turning-on of the second transistor 12 at the time of the stop of the operation of the CPU. Thus, the time lag can be sharply reduced.

Description

Detailed Description of the Invention (a) Field of Industrial Application The present invention relates to a power supply circuit for driving a liquid crystal display element (abbreviated as "LCD"), and particularly relates to a power supply circuit for driving a liquid crystal display element (abbreviated as "LCD"). This invention relates to a driving power supply circuit.

(Example) Conventional technology Generally, to drive an LCD, a driving IC is used to supply power to the power supply and a CPU as a central information processing unit that performs various controls. Performs AC drive.

At this time, in recent years, in LCD display panels, the drive circuit, that is, the drive IC is built into the unit.
The power supply for such an LCD display panel is required to protect the built-in IC and to avoid applying DC voltage to the LCD.

Therefore, as an example, in the circuit shown in Fig. 4, the CPU and +V
When the LCD voltage is below a certain reference voltage, -VLC2 is turned off.As shown in Figure 5, its characteristics are that the consumption IE sources such as the CPU are small and the capacity of the power supply for them is large. If so, there is no problem as it will drop slowly even after the power is turned off, but if the current as a load such as a CPU is large and the output capacity of the power supply is small, the drop will be rapid when the power is turned off, and - A slight timing delay in turning off the power supply (vLcl) may cause the above-mentioned power supply sequence to be inconsistent. A known example is JP-A-2-33190, which shows an example of using two voltages.

(c) Problems to be Solved by the Invention As mentioned above, when a driving IC is built into an LCD display panel, if the sequence of voltages applied to the built-in IC is inappropriate, the power supply sequence is In an unsuitable case, the circuit as shown in Figure 4 is used, and as shown in the waveform shown in Figure 5, the negative power supply to the LCD panel is maintained even when the power is turned off and the CPU power supply voltage drops and the CPU stops operating. remains. In such a case, there is a risk of latch-up of the IC or deterioration of optical characteristics due to an electrochemical reaction of the LCD due to the DC applied voltage.

The present invention provides a novel LCD driving power supply circuit that eliminates the above drawbacks.

(d) Means for Solving the Problems The present invention connects a first transistor to the positive power supply side for a microprocessor unit (CPU) and a second transistor to the negative power supply side for driving the LCD. IC of
The voltage of the driving tR line is set to rise simultaneously or simultaneously with turning on and turning off the power switch, and to fall in plain text.

*) Function According to the present invention, the LCD display panel can be protected by a sequence based on detecting a voltage drop point without detecting a voltage, without the time delay of detection as in the conventional method.

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In the drawing, which shows a block diagram of the CD drive power supply circuit, Fig. 2 is a circuit diagram of the same circuit, and Fig. 3 is a characteristic diagram of the same circuit.
(1) is a switching power supply stage, (2) and (3) are each CP
1 to U etc. is the source and LCD driving power supply, (4) is the CPU
, (5) is the power supply control stage, (6) is the LCD display panel unit, (7) is the first power supply terminal for the CPU, etc., and (8> is the L
The positive voltage terminals of the CD drive power supply, (9) and (10> are each L)
Negative voltage terminal of CD drive power supply and negative power supply terminal of LCD,
(11) is the first transistor, (12) is the second transistor, (13) (14>(15) is the voltage dividing resistor, (1
6>(17) indicates a resistor and a capacitor, each of which constitutes a time constant circuit (18).

Next, the operation of the LCD driving power supply circuit of the present invention will be explained.
When the voltage -vLcDl of the power supply terminal (9), which is the source of
It is turned on by the base current flowing through the terminal (6), and its collector current flows through the resistors (13) and (14) to the terminal (
9) Flow to the side.

Here, from the voltage relationship, it is set so that the second transistor (12
) is turned off.

Therefore, -VLCD* is obtained through the resistor (15) having a resistance value (R4) smaller than -VLCDI to the extent that it does not adversely affect the drive of the LCD panel unit (6), and the capacitor <17> has +V Lc+)- V mm (
Vs* is the base-emitter voltage of the first transistor (11) applied thereto.

When the power switch is turned off, the time constant (CR,) determined by the resistor (16) and capacitor (17) is set larger than the rate of fall of +VLCD, so the base of the first transistor (11) - The collector current of the first transistor (11) decreases in the direction of reverse bias of the emitter, and when it becomes smaller than VLCDI/R3, the second transistor (12) turns on.

Along with this - ■. , is +V,. At the time of descent, it quickly becomes the first ground and is set to 0 volts. Note that the second transistor (12) is protected from overcurrent by a resistor (15).

This pattern is shown in Fig. 3. In the figure (a), the voltage (Vcpt+) of the power supply (7) of the CPU etc. drops as the power is turned off, and the negative power supply (-Vcpt+) drops at the point where the CPU stops operating.
LCDI> terminal (10) voltage -V LCDI is the second
When the zero transistor (12) is turned on, the voltage becomes the ground potential as shown in Fig. 3, and the above-mentioned operation is immediately performed due to the voltage drop, and the time delay can be minimized.

(G) Effects of the Invention According to the LCD driving power supply circuit of the present invention, novel effects that cannot be obtained by conventional sequences based on voltage detection can be obtained by using a sequence based on voltage drop point detection. There is no detection time delay, and therefore, it is possible to prevent latch-up of the built-in IC for driving the LCD panel or deterioration of optical characteristics due to electrochemical reaction of the LCD due to application of DC voltage.

[Brief explanation of the drawing]

Figure 1 is a principle block diagram of the LCD driving power supply circuit of the present invention, Figure 2 is a circuit diagram of the same, Figure 3 is a characteristic diagram of the circuit, and Figure 4 is a diagram of the circuit.
5 and 5 respectively show a circuit diagram and a characteristic diagram of the conventional circuit. (2)...Power supply for CPU, etc., (3)...Power supply for LCD drive, 4)...CPU, (5)...Power control stage, (6)...LCD display panel unit ,
(7)...Power terminal for CPU, etc., (8)...LC
Positive voltage terminal for D drive, (9) (10)...LCD
Negative voltage terminal for driving, (11)...first transistor, (12)...second (7) transistor, (
13) (14) (15)...Voltage dividing resistance, (18)
...Time constant circuit.

Claims (1)

[Claims] (1) Positive power supply terminal and L for microprocessor unit
A collector-emitter path of a first transistor and a plurality of resistors for voltage division are connected between a negative power supply terminal for driving a CD, and a parallel circuit consisting of a resistor and a capacitor is connected between the base of the first transistor and ground. . A power supply circuit for driving an LCD, characterized in that a collector-emitter path of a second transistor whose base is connected to a common connection point of the plurality of resistors is connected to a negative power supply terminal for driving an LCD.