Input overvoltage protection circuit with soft-start function

1. An overvoltage protection circuit that prevents an overvoltage from flowing into a system and thereby protects the system, comprising:

a first switch to function as a switch disposed between an input circuit and the system, the input circuit being configured to supply an input voltage, and the first switch being constructed of a semiconductor transistor;

a comparator that determines an occurrence of an overvoltage by comparing the input voltage with a predetermined reference voltage, and when no overvoltage is detected, outputs an operation signal;

a soft switching control circuit that starts up by using the operation signal from the comparator as an enable signal, and turns on the first switch gradually by gradually increasing a voltage between a source and a gate of the first switch; and

a second switch provided between the soft switching control circuit and the input circuit, the second switch being configured to cutoff a current path between the input circuit and the soft switching control circuit in response to an overvoltage detection signal output from the comparator.

2. The overvoltage protection circuit according to claim 1 , wherein a delay circuit is provided between the comparator and the soft switching control circuit, the delay circuit being configured to delay the operation signal from the comparator.

3. The overvoltage protection circuit according to claim 2 , wherein the second switch is an NMOS transistor that is coupled between an output node of the soft switching control circuit and a node between a gate of the first switch and the input circuit, a gate of the NMOS transistor is coupled with an output node of the delay circuit.

4. The overvoltage protection circuit according to claim 1 , wherein

the first switch is a first PMOS transistor, and

a pull-up resistor is provided between a source and a gate of the first PMOS transistor.

5. The overvoltage protection circuit according to claim 1 , wherein

the first switch is a first PMOS transistor, and

a first capacitive element is provided between a source and a gate of the first PMOS transistor.

6. The overvoltage protection circuit according to claim 4 , wherein

the comparator outputs the overvoltage detection signal when an overvoltage is detected,

a third switch is provided in parallel with the pull-up resistor, and

the third switch is turned on in response to the overvoltage detection signal from the comparator.

7. The overvoltage protection circuit according to claim 5 , wherein

the comparator outputs the overvoltage detection signal when an overvoltage is detected,

a third switch is provided in parallel with the first capacitive element, and

the third switch is turned on in response to the overvoltage detection signal from the comparator.

8. The overvoltage protection circuit according to claim 1 , wherein

the comparator outputs the overvoltage detection signal when an overvoltage is detected,

a fourth switch is provided in a branch line, the branch line being branched from a node between the first switch and the system and connected to a ground potential, and

the fourth switch is turned on in response to the overvoltage detection signal from the comparator.

9. The overvoltage protection circuit according to claim 1 , wherein the soft switching control circuit comprises:

a plurality of transistors arranged in parallel to serve as a current mirror with respect to a current source.

10. The overvoltage protection circuit according to claim 1 , wherein the soft switching control circuit further comprises:

a first enabling switch connected to a first one of the plurality of transistors; and

a second enabling switch connected a second one of the plurality of transistors, wherein the first and the second enabling switches are controlled to turn on with a time interval between each other.

11. The overvoltage protection circuit according to claim 1 , further comprising:

an input resistor, one end of the input resistor being coupled with the input voltage;

a clamp circuit coupling with another end of the input resistor and generating a constant voltage at a node between the input resistor and the clamp circuit,

wherein the clamp circuit, the comparator circuit, the soft switching control circuit and the delay circuit are formed in a LSI (large-Scale Integrated) circuit device, the constant voltage is applied to both the comparator circuit, the soft switching control circuit.

12. The overvoltage protection circuit according to claim 11 , further comprising:

a delay circuit provided between the comparator and the soft switching control circuit, the delay circuit being configured to delay the operation signal from the comparator,

wherein the constant voltage is further applied to the delay circuit.

13. The overvoltage protection circuit according to claim 11 , further comprises first to third resistors coupled in series in this order,

wherein the constant voltage is applied to one end of the first resistor, a first reference voltage generated at a node between the first and second resistors is applied to a first non-inverted input node of the comparator circuit, a second reference voltage generated at a node between the second and third resistors is applied to a second non-inverted input node of the comparator circuit, a monitoring voltage based on the input voltage is applied to an inverted input node of the comparator circuit.