JPH0714253B2 - Power failure detection circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power failure detection circuit that detects a decrease in input voltage of a power supply circuit, and more particularly to a circuit that prevents malfunction of this circuit.

[Conventional technology]

Conventionally, as this type of circuit, a circuit shown in FIG. 3 has been known.

That is, in the figure, when the input voltage V _IN is higher than the predetermined reference voltage V _IS , the input voltage detection circuit 1 indicates “L”.
The "low" signal, and if low, the "H" signal is supplied to the base of the switching transistor 2. Therefore, the output terminal 3 connected to the collector of the switching transistor 2, when the reference voltage V _IS> input voltage V _IN in the case of "H", the reference voltage V _IS <input voltage V _IN is "L "
Signal is output. The collector of the switching transistor 2 is suspended by the power supply voltage V _CC via the resistor 4.

Further, as shown in FIG. 4, the input voltage V _{IN of the} power supply circuit shown in (a) gradually decreases, and the input voltage V _IN <reference voltage
When it becomes V _IS , the input voltage detection signal which is the output signal of the input voltage detection circuit 1 shown in (b) changes from “L” to “H” and is output from the output terminal 3 shown in (d). The signal level changes from a high level, which is approximately the same potential as the power supply voltage, to a low level, which is approximately the same potential as the ground potential.

[Problems to be Solved by the Invention]

However, as shown in (c) of the figure, when the power supply voltage V _CC drops, the power supply of the input voltage detection circuit 1 is insufficient and the output "H" signal cannot be held, and once becomes "H". The input detection signal returns to the "L" signal again regardless of the input voltage _IN <reference voltage V _IS . If the power supply voltage V _CC is not 0 at the moment of returning to the “L” signal, (d)
As shown in (1), an erroneous output signal is output from the output terminal 3. Note that this output signal has substantially the same potential as the power supply voltage V _CC . As described above, the conventional power failure detection circuit has a problem that it malfunctions when the power supply voltage of the input voltage detection circuit is insufficient.

[Means for Solving the Problems]

The present invention has been made to solve such a problem, and an input voltage detection circuit that outputs a signal when the input voltage of a power supply circuit drops and a signal output from this input voltage detection circuit as a base are input. The NPN transistor whose emitter is grounded and whose collector is the output terminal, the shunt regulator that is turned off when the power supply voltage drops and turned on when the power supply voltage increases, and the cathode of this shunt regulator are connected to the base and the emitter is connected to the power supply voltage. It is composed of a PNP transistor which is lifted and whose collector is connected to the collector of the NPN transistor.

[Action]

The drop in the input voltage is detected by the shunt regulator, the PNP transistor is turned off, and the supply voltage from the power supply terminal to the output terminal is cut.

〔Example〕

Next, the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the present invention.

In the figure, the output of the input voltage detection circuit 11 is connected to the base of the switching transistor 12, which is an NPN transistor, and the collector of the switching transistor 12 is connected to the output terminal 13. A resistor 14 is connected between the base and the emitter, and the emitter is grounded.
Further, the collector of the switching transistor 12 is connected via a resistor 15 to the collector of a switching transistor 16 which is a PNP transistor, and the emitter of the switching transistor 16 is connected to a power supply terminal 17 and supplied with a power supply voltage V _CC. There is.

The power supply voltage V _CC via the power supply terminal 17 is divided by the resistors 18 and 19 and input to the reference of the shunt regulator 20, and the cathode of the shunt regulator 20 is supplied via the resistors 21 and 22 to the power supply terminal 17. It is connected to the. The base of the switching transistor 16 is connected to the connection point of these resistors 21 and 22.

In such a configuration, the operation will be described with reference to the waveform chart of FIG.

In the figure, (a) is an input voltage V _IN input to the power supply circuit and a preset reference voltage V _IS , (b) is an input voltage detection signal output from the input voltage detection circuit 11, and (c) is The power supply voltage V _CC supplied to the power supply terminal 17 and a preset predetermined reference voltage V _CS , (d) is the collector voltage V _CC ′ of the switching transistor 12, and (e) is output from the output terminal 13. Represents an output signal.

When the input voltage V _IN > the reference voltage V _IS and the input voltage detection signal is “L”, the switching transistor 12
Turns off. If the power supply voltage V _CC is higher than the reference voltage V _{CS, the} shunt regulator 20 is turned on by the detection resistors 18 and 19, and a current flows through the resistors 21 and 22 and the shunt regulator 20 to cause the resistance 22 A voltage is generated at both ends, and the switching transistor 16 is turned on.
Therefore, the output terminal 13 outputs an "H" signal having a voltage substantially the same as the power supply voltage V _CC . However, after the input voltage detection signal changes from “L” to “H”, input voltage V _IN <reference voltage V
_When it becomes _IS , the power supply of the input voltage detection circuit 11 is lowered and the input voltage detection signal changes from "H" to "L" again. However, in this case, the power supply voltage V _CC also drops because the input voltage V _IN drops, and the shunt regulator 20 immediately turns off at the moment when the power supply voltage V _CC becomes a voltage lower than the reference voltage V _CS . Therefore, the voltage across the resistor 22 disappears, the switching transistor 16 also turns off, and the supply voltage from the power supply terminal 17 to the output terminal 13 is cut off. Therefore, the collector voltage of the switching transistor 12 continues to maintain the "L" state, which is substantially the same potential as the ground potential, and the erroneous "H" signal is not output to the output terminal 13 as in the conventional case.

〔The invention's effect〕

As described above, according to the present invention, the input voltage detection circuit that outputs a signal when the input voltage of the power supply circuit drops, and the signal output from the input voltage detection circuit are input to the base, the emitter is grounded, and the collector is the output terminal. , And a shunt regulator that turns off when the power supply voltage drops and turns on when the power supply voltage increases, the cathode of this shunt regulator is connected to the base, the emitter is lifted to the power supply voltage, and the collector is the NPN transistor described above. With the PNP transistor connected to the collector, the drop in the input voltage is detected by the shunt regulator, the PNP transistor is turned off, and the supply voltage from the power supply terminal to the output terminal is cut.
Therefore, there is an effect that the problem of malfunctioning when the power supply voltage of the input voltage detection circuit is insufficient like the conventional circuit is solved.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a waveform diagram showing waveforms of respective parts of this circuit diagram, FIG. 3 is a circuit diagram showing a conventional configuration, and FIG. It is a wave form diagram which shows the wave form of each part of a circuit diagram. 11 …… Input voltage detection circuit, 12,16 …… Switching transistor, 13 …… Output terminal, 14,15,18,19,21,22 …… Resistance, 17 …… Power supply terminal, 20 …… Shunt regulator.

Claims (1)

[Claims] 1. An input voltage detection circuit for comparing an input voltage of a power supply circuit with a predetermined first reference voltage and outputting a signal when the input voltage is lower than the first reference voltage, and the input voltage detection circuit. An NPN transistor that inputs the signal output from the circuit to the base, has an emitter grounded, and has a collector as an output terminal, and compares the power supply voltage with a predetermined second reference voltage so that the power supply voltage is lower than the second reference voltage. A shunt regulator that turns off when it drops and turns on when the power supply voltage rises above this second reference voltage, the cathode of this shunt regulator is connected to the base through a resistor, the emitter is lifted to the power supply voltage, and the collector pulls up the resistance. An electric disconnection detection circuit including a PNP transistor connected to the collector of the NPN transistor via the NPN transistor.