JPH11338558A - Constant-voltage output device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a microcomputer normally resettable by holding a reset pulse output terminal voltage Vp at 0V even if the output of a source voltage detecting circuit has dropped when a source voltage has dropped. SOLUTION: A constant-voltage circuit 1 and the source voltage detecting circuit 2 which detects the source voltage are provided and a transistor(TR) 4 which turns on and off with its output generates a reset pulse. A resistance 3 and a resistance 5 are connected between the base and collector of the TR and a constant-voltage output terminal. When the source voltage VCC drops and the constant voltage output Vc decreases to power to off the microcomputer 7, the reset pulse Vp is lowered to Lo to reset the microcomputer. At this time, even when the source voltage drops until the source voltage detecting circuit does not operate, the TR 4 is held ON by the capacitor 6 and resistance 3, so the reset state can be maintained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant voltage output device comprising an analog integrated circuit and used for television signal processing and the like.

[0002]

2. Description of the Related Art A conventional constant voltage output device will be described below. FIG. 3 shows a configuration of a conventional constant voltage output device. In FIG. 3, the constant voltage circuit 8 outputs a constant voltage Vc from the power supply voltage VCC, and supplies the constant voltage Vc to each unit. The power supply voltage detection circuit 9 outputs Lo when the constant voltage Vc is equal to or higher than a predetermined value, and outputs Hi when the constant voltage Vc is equal to or lower than the predetermined voltage.
The transistor 10 performs an on / off operation by the Hi / Lo output of the power supply voltage detection circuit 9 and outputs a reset pulse generated by turning on / off the transistor to a microcomputer (hereinafter simply referred to as a microcomputer) 13. Also resistance 11
Is connected between the output of the constant voltage circuit 8 and the collector of the transistor 10, and the capacitor 12 is connected between the output of the constant voltage circuit 8 and the ground.

[0003] The operation of the constant voltage output device configured as described above will be described below. FIG. 4 shows a power supply voltage VCC of a conventional constant voltage output device and a constant voltage output circuit 9.
3 is a characteristic diagram of the output voltage Vc and the reset pulse output Vp. First, when the power supply rises and VCC starts to rise, the output Vc of the constant voltage circuit 8 becomes VCC-Vc of the constant voltage circuit 8.
It rises according to the characteristics, and when it reaches a steady state, it stops at the voltage V1 at which the microcomputer 13 can operate sufficiently. This V1 is determined by the constant voltage circuit 8, and the time until VCC becomes V1 is determined by the current supply capability of the constant voltage circuit 8, the capacity of the capacitor 12, and the input impedance from the power supply terminal of the microcomputer 13 to GND.

When VCC starts to rise from 0 V and rises to the voltage V2 at which the power supply voltage detection circuit 9 operates, the output voltage of the power supply voltage detection circuit 9 rises and the transistor 10 turns on. Therefore, Vp becomes 0 V and drops to Lo level, so that Vc, which is the power supply voltage of the microcomputer 13, is reset once before the voltage at which the microcomputer 13 can operate.

Further, as VCC rises, a certain threshold voltage V higher than V2 and lower than V1.
It is assumed that the power supply voltage detection circuit 9 is set so that the transistor 10 is turned off in Step 3. This threshold voltage V3
In the above, the microcomputer 13 needs to be operable. Here, assuming that the resistance value of the resistor 11 is R and the current flowing through the resistor 11 is Ic, Vp is represented by the following equation (1): Vp = Vc−Ic × R (1) The transistor 10 is now in an off state When Ic = 0, Vp = Vc (2) Therefore, when the transistor 10 is turned off, Vp becomes V
The voltage becomes the same as the voltage c, and the microcomputer 13 is released from the reset state when the voltage Vp rises to the Hi level, and returns to the normal state.

Next, the operation when the power supply is turned off will be considered. V
When the CC starts to drop and drops below the threshold voltage V3, the transistor 10 is turned on, and the microcomputer 13 is reset when Vp becomes 0V. As described above, the power supply voltage detection circuit 9 turns on and off the transistor 10 according to the fluctuation of the power supply voltage VCC, thereby sending a reset pulse to the microcomputer 13 to prevent the microcomputer 13 from malfunctioning.

[0007]

However, in the above-mentioned conventional configuration, when the power supply falls, as shown in FIG.
Gently drops below VCC due to the charge charged in the constant voltage output capacitor 12. When VCC falls to the voltage V2 at which the power supply voltage detection circuit 9 does not operate, the output voltage of the power supply voltage detection circuit 9 cannot be held at Hi. Accordingly, the transistor 10 is turned off, and Vp becomes the same voltage as Vc again from 0 V, so that the reset pulse sent to the microcomputer 13 becomes Hi again. As a result, even if the voltage of Vc, which is the power supply of the microcomputer 13, becomes lower than the voltage V3 at which the operation of the microcomputer 13 cannot be performed due to the charge of the capacitor, the microcomputer 13 malfunctions because the reset pulse is Hi in the normal state. There was a problem of getting up.

The present invention solves the above-mentioned conventional problem. When the power supply voltage VCC falls below V3, the reset pulse output terminal voltage Vp is maintained at 0V even if the supply voltage to the power supply voltage detection circuit 9 falls. By doing so, the microcomputer 13
It is an object of the present invention to propose a device capable of preventing malfunction of the device.

[0009]

According to the present invention, there is provided a constant voltage circuit for outputting a constant voltage, and Lo, if the power supply voltage is equal to or higher than a certain value.
A power supply voltage detection circuit that outputs Hi if the voltage is equal to or less than a certain value;
A first resistor connected between an output of the constant voltage circuit and an output of the power supply voltage detection circuit, a transistor that performs on / off operation with a Hi / Lo output of the power supply voltage detection circuit, A second resistor connected between the output of the circuit and the collector of the transistor; and a capacitor connected between the output of the constant voltage circuit and ground. It is characterized by outputting.

With this configuration, even if the power supply voltage falls below the voltage at which the power supply voltage detection circuit does not operate, the reset pulse output can be maintained at 0 V, and the microcomputer can be reset normally.

[0011]

FIG. 1 shows a configuration of a constant voltage output device according to an embodiment of the present invention. In this constant voltage output device, as shown in FIG. 1, a constant voltage circuit 1 and a power supply voltage detection circuit 2 are the same as those of the above-described conventional example, and thus detailed description is omitted. This constant voltage output circuit is a constant voltage circuit 1
, A resistor 4 connected between the output of the power supply voltage detection circuit 2, a transistor 4 for performing on / off operation with the Hi / Lo output of the power supply voltage detection circuit 2, an output of the constant voltage circuit 1, and a transistor 4 connected to the collector of the second
And a capacitor 6 connected between the output of the constant voltage circuit 1 and the ground. A microcomputer (hereinafter, referred to as a microcomputer) 7 uses the output of the constant voltage circuit 1 as a power supply, receives a reset pulse Vp generated by turning on / off the transistor 4, and performs a reset operation at Lo.

Next, the operation of this embodiment will be described with reference to FIG. First, when the power is turned on, the power VC
When C starts to rise, the output Vc of the constant voltage circuit 1 rises in accordance with the VCC-Vc characteristic of the constant voltage circuit 1, and stops at a voltage V1 at which the microcomputer 7 can operate sufficiently in a steady state. This V1 is determined by the constant voltage circuit 1, and the time until VCC becomes V1 is determined by the current supply capability of the constant voltage circuit 1, the capacity of the capacitor 6, and the input impedance from the power supply terminal of the microcomputer 7 to GND.

When VCC starts to rise from 0 V and rises to the voltage V2 at which the power supply voltage detection circuit 2 operates, the output voltage of the power supply voltage detection circuit 2 rises and the transistor 4 turns on. Therefore, Vp becomes 0 V and drops to Lo level, so that Vc is reset once before the voltage at which the microcomputer 7 becomes operable.

Further, as VCC rises, a certain threshold voltage V higher than V2 and lower than V1.
It is assumed that the power supply voltage detection circuit 2 is set so that the transistor 4 is turned off at 3. The microcomputer 7 needs to be operable at the threshold voltage V3 or higher.
Here, the resistance value of the resistor 5 is R, and the current flowing through the resistor 5 is Ic.
Then, Vp becomes as in the following equation (3). Vp = Vc−Ic × R (3) Now, when the transistor 4 is turned off, since Ic = 0, Vp = Vc (4) When the transistor 4 is turned off, Vp becomes Vc
When the voltage Vp rises to the Hi level, the microcomputer 7 is released from the reset state and returns to the normal state.

Next, the operation when the power supply is turned off will be considered. V
When the CC starts to drop and drops below the threshold voltage V3, the output of the voltage detection circuit 2 rises, the transistor 4 is turned on, and the microcomputer 7 is reset when Vp becomes 0V.

Here, Vc is the capacitor 6 of the constant voltage circuit 1.
, And gradually drops down from VCC due to the electric charges charged to VCC. Voltage V2 at which power supply voltage detection circuit 2 stops operating
When VCC drops to this point, the output voltage of the power supply voltage detection circuit 2 drops. However, since the output of the power supply voltage detection circuit 2 is not set to the open state by the resistor 3 but to the same potential as Vc, the transistor 4 is kept on and Vp is set to 0.
V can be held.

As a result, even if the voltage of Vc, which is the power supply of the microcomputer 7, becomes lower than the voltage V3 at which the operation of the microcomputer 7 cannot be performed due to the electric charge of the capacitor, the reset pulse is Lo in the reset state. Does not malfunction.

[0018]

As described above, according to the constant voltage output circuit of the present invention, the reset pulse output can be maintained at 0 V even when the power supply voltage is lower than the voltage at which the power supply voltage detection circuit does not operate. The microcomputer can be normally reset.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a constant voltage output device according to an embodiment of the present invention.

FIG. 2 shows VCC and V of the constant voltage output device of the present embodiment.
It is a figure which shows c and Vp characteristic.

FIG. 3 is a circuit diagram showing an example of a conventional constant voltage output device.

FIG. 4 is a diagram showing VCC, Vc, and Vp characteristics of a conventional constant voltage output device.

[Explanation of symbols]

 1,8 constant voltage circuit 2,9 power supply voltage detection circuit 3,5,11 resistor 4,10 transistor 6,12 capacitor 7,13 microcomputer

Claims (1)

[Claims] 1. A constant voltage circuit for outputting a constant voltage, Lo when the power supply voltage is equal to or higher than a constant voltage, and H when the power supply voltage is equal to or lower than a predetermined voltage.
i, a first resistor connected between the output of the constant voltage circuit and the output of the power supply voltage detection circuit, and an on / off output at the Hi / Lo output of the power supply voltage detection circuit. A transistor performing an off operation; a second resistor connected between an output of the constant voltage circuit and a collector of the transistor; and a capacitor connected between an output of the constant voltage circuit and ground. And outputting a reset pulse generated by the transistor.