JPS63124721A - Overvoltage protecting circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an overvoltage protection circuit for protecting an element such as an LSI from an overvoltage applied to a signal input terminal of the element.

[Conventional technology]

2. Description of the Related Art Conventionally, an overvoltage protection circuit is provided to suppress overvoltage in order to prevent elements from being destroyed due to overvoltage applied to an input terminal. For example, FIG. 2 is an example of this, in which an overvoltage protection circuit is provided between the signal input terminal 2 and the input terminal 1 of the element 3 to be protected.

This protection circuit connects a protection resistor R1 and a diode D to a signal input terminal 2.

At the other end is a transistor Q and an operational amplifier TCI.

It has a circuit configuration in which an overvoltage absorbing circuit 4 made up of resistors R2, R and a reference voltage V r a f is connected.

[Problem that the invention seeks to solve]

In the conventional overvoltage protection circuit described above, the current flowing through the diode D changes depending on the magnitude of the voltage applied to the input terminal 1, so the voltage drop across the diode D changes. Furthermore, since the voltage drop across the diode D1 varies depending on the temperature, no matter how stable the overvoltage absorption circuit 40 connected to the cathode side of the diode is, the input voltage of the element 3 to be protected will change.

For this reason, if the element to be protected does not have much margin against overvoltage, such as an LSr, fluctuations in the voltage drop of this diode cannot be ignored.

SUMMARY OF THE INVENTION An object of the present invention is to provide an overvoltage protection circuit that can effectively protect against overvoltage even when the margin for overvoltage of an element to be protected is small by eliminating the influence of fluctuations in voltage drop of a diode.

[Means for solving problems]

In the overvoltage protection circuit of the present invention, a second diode having the same characteristics as the diode connected to the signal input terminal is inserted in series between the diode and the overvoltage absorption circuit, and the voltage across the second diode is reduced to the overvoltage. The configuration is such that it is subtracted from the reference voltage of the absorption circuit.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention. A protection resistor R5 is connected between the signal input terminal 2 and the input terminal l of the LSI 3, which is an element to be protected, and a diode D1 is connected to the signal input terminal 2. A second diode D2 is connected to this diode D, and further a subtraction circuit 5 and an overvoltage absorption circuit 4 are connected.

That is, the diode D1 has its anode connected to the signal input terminal 2 and its cathode connected to the anode of a second diode D2 having the same characteristics.

Further, the collector of a transistor Q is connected to the cathode of this second diode D2, and the emitter of this transistor Q is connected to ground.

The transistor Q is connected to a bias resistor R3 and to an operational amplifier IC1 through a resistor R2, thereby controlling its base current. Further, both ends of the second diode D2 are connected to the operational amplifier IC2, and furthermore, the output of the operational amplifier IC2 is connected to one terminal of the operational amplifier ICI through the reference voltage 1,1. On the other hand, the child terminal of the operational amplifier IC1 is connected to the cathode of the diode D.

Next, the operation of the overvoltage protection circuit having the above configuration will be explained.

When an excessive voltage is applied to input terminal 1, the excessive voltage becomes the reference voltage V,. , the transistor Q turns on and the resistor R1, diode D1゜second diode D
2. Current flows through transistor Q to ground.

At this time, the voltage drop across the diode D+ and the voltage across the second diode D2 are approximately equal, so the operational amplifier IC
Since I compares the signal input terminal 2 with the reference voltage V r a f and controls the current of the transistor Q, the voltage of the signal input terminal 2 is maintained at a voltage substantially equal to the reference voltage V, . . . , t.

At this time, even if the voltage drop of the diode D1 changes depending on the temperature or flowing current, the voltage across the second diode D2 also changes in the same way as the voltage drop of the diode D, so the voltage at the No. 43 input terminal 2 does not become higher than the reference voltage V r a r regardless of the temperature or the magnitude of the voltage applied to the input terminal.

Therefore, the voltage drop in the diode D is canceled by the subtraction circuit 5, and the set voltage value for overvoltage detection in the overvoltage protection circuit can be stably maintained regardless of the magnitude or temperature of the overvoltage.
Even elements with a small margin against overvoltage can be effectively protected.

〔Effect of the invention〕

As explained above, the present invention inserts a second diode having the same characteristics as the diode connected to the signal input terminal in series between the diode and the overvoltage absorption circuit.
Since the voltage across the diode is subtracted from the reference voltage of the overvoltage absorption circuit, the voltage drop across the diodes that make up the overvoltage protection circuit can be canceled, and the set voltage value in the overvoltage protection circuit can be changed depending on the magnitude of the overvoltage and temperature. can be suppressed.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is a conventional circuit diagram. 1... Input terminal, 2... Signal input terminal, 3... Element to be protected (LSI), 4... Overvoltage absorption circuit, 5
... Subtraction circuit, D, ... Diode, D2 ... Second diode, IC1, IC2 ... Operational amplifier, Q
...Transistor, R+, Rz, R3...Resistance, ■,...Reference voltage. Figure 1

Claims (2)

[Claims] (1) In an overvoltage protection circuit comprising a diode connected to a signal input terminal and an overvoltage absorption circuit, a second diode having the same characteristics as the diode is inserted in series between the diode and the overvoltage absorption circuit; An overvoltage protection circuit comprising a circuit that subtracts the voltage across the second diode from the reference voltage of the overvoltage absorption circuit. (2) The overvoltage protection circuit according to claim 1, wherein the subtraction circuit is configured to compare the voltages across the second diode and subtract the reference voltage of the overvoltage absorption circuit based on this comparison value. .