Rapid discharging circuit upon detection of abnormality
Provided is a protection circuit that is connected between a power supply terminal and an output terminal, and turns off an output transistor when an abnormality occurs in a system, the output transistor outputting a current to a load connected to the output terminal, the protection circuit including: a first discharge unit that is connected between a gate electrode of the output transistor and the power supply terminal, and discharges an electric charge of the gate electrode until a potential of the gate electrode becomes equal to a power supply potential, when an abnormality occurs in the system, and a second discharge unit that is connected between the gate electrode and a source electrode of the output transistor, and discharges the electric charge of the gate electrode until the potential of the gate electrode becomes equal to an output potential, when an abnormality occurs in the system.
1. A protection circuit that is connected between a power supply terminal and an output terminal, and turns off an output transistor when an abnormality occurs in a system, the output transistor outputting a current to a load connected to the output terminal, the protection circuit comprising:
a first discharge unit that is connected between a gate electrode of the output transistor and the power supply terminal, and discharges an electric charge of the gate electrode until a potential of the gate electrode becomes equal to a power supply potential, when an abnormality occurs in the system;
a second discharge unit that is connected between the gate electrode and a source electrode of the output transistor, and discharges the electric charge of the gate electrode until the potential of the gate electrode becomes equal to an output potential, when an abnormality occurs in the system; and
a normal turn-off circuit that is connected between the gate electrode and the source electrode of the output transistor, and includes a depletion-type transistor that turns off the output transistor when the system is in a normal state.
2. The protection circuit according to claim 1 , wherein the second discharge unit comprises a transistor connected between the gate electrode and the source electrode.
3. The protection circuit according to claim 1 , wherein:
the first discharge unit comprises:
a first depletion-type transistor having a drain connected to the gate electrode of the output transistor, and a gate and a source that are short-circuited; and
a diode connected between the gate electrode of the output transistor and the source of the first transistor, and
a parasitic bipolar transistor of the diode causes the discharge.
4. The protection circuit according to claim 1 , wherein:
the first discharge unit comprises:
a first depletion-type transistor having a drain connected to the gate electrode of the output transistor, and a gate and a source that are short-circuited;
a second enhancement-type transistor having a drain connected to the gate electrode of the output transistor, and a gate and a source that are short-circuited; and
a third enhancement-type transistor connected between the second transistor and the source electrode of the output transistor,
a parasitic bipolar transistor of the second transistor discharges an electric charge of the gate electrode until the potential of the gate electrode becomes equal to the power supply voltage, and
a parasitic bipolar transistor of the third transistor discharges an electric charge of the gate electrode until the potential of the gate electrode becomes equal to the output potential.
5. The protection circuit according to claim 3 , wherein the output transistor comprises an N-type transistor formed in a P-well formed in an N-type semiconductor substrate.
6. The protection circuit according to claim 4 , wherein:
the first transistor and the second transistor are N-type transistors, and
the third transistor comprises a P-type transistor.
7. The protection circuit according to claim 6 , wherein the parasitic bipolar transistor comprises a parasitic bipolar transistor of the diode, and discharges an electric charge of the gate electrode of the output transistor to the power supply terminal by using a current from the first transistor as a base current.
8. The protection circuit according to claim 6 , wherein:
the parasitic bipolar transistor comprises first to third parasitic bipolar transistors serving as parasitic bipolar transistors of the second transistor, and a fourth parasitic bipolar transistor serving as a parasitic bipolar transistor of the third transistor,
the first parasitic bipolar transistor discharges an electric charge of the gate electrode of the output transistor to the power supply terminal by using a current from the first transistor as a base current,
the second parasitic bipolar transistor supplies a current to the third and fourth parasitic bipolar transistors by using the current from the first transistor as the base current,
the third parasitic bipolar transistor discharges the electric charge of the gate electrode of the output transistor to the power supply terminal with a current from the second parasitic bipolar transistor, by using the current from the first transistor as the base current, and
the fourth parasitic bipolar transistor discharges the electric charge of the gate electrode of the output transistor to the output terminal with the current from the second parasitic bipolar transistor.
9. A high-side output circuit that switches a current flowing through a load, comprising:
an output transistor that is connected between a power supply terminal and an output terminal, and outputs a current to the load connected to the output terminal;
a normal turn-off circuit that is connected between a gate electrode and a source electrode of the output transistor, and includes a depletion-type transistor that turns off the output transistor when a system is in a normal state;
an abnormality detection circuit that detects an abnormality in the system; and
a protection circuit that turns off the output transistor when the abnormality detection circuit detects an abnormality in the system,
wherein the protection circuit comprises:
a first discharge unit that is connected between a gate electrode of the output transistor and the power supply terminal, and discharges an electric charge of the gate electrode until a potential of the gate electrode becomes equal to a power supply potential, when an abnormality occurs in the system; and
a second discharge unit that is connected between the gate electrode and a source electrode of the output transistor, and discharges the electric charge of the gate electrode until the potential of the gate electrode becomes equal to an output potential, when an abnormality occurs in the system.
10. The high-side output circuit according to claim 9 , wherein the abnormality detection unit comprises at least one of:
a current sensor that detects an abnormality of a load current flowing through the load;
a temperature sensor that detects a temperature of the output transistor; and
an output voltage sensor that detects a voltage of the output terminal.
11. A method of turning off an output transistor when an abnormality occurs in a system, the output transistor being connected between a power supply terminal and an output terminal and outputting a current to a load connected to the output terminal, the method comprising:
discharging, upon detection of an abnormality in the system, an electric charge of the gate electrode of the output transistor until a potential of the gate electrode becomes equal to a power supply potential;
discharging, upon detection of an abnormality in the system, the electric charge of the gate electrode of the output transistor until the potential of the gate electrode becomes equal to an output potential; and
discharging, upon detection of a normality in the system, the electric charge of the gate electrode of the output transistor via a depletion-type transistor that couples between the gate electrode of the output transistor and the output terminal.