USB short circuit protection

1. A power circuit, comprising:

a first transistor having a first source, a first drain, and a first gate, the first source coupled to an input voltage terminal;

a second transistor having a second source, a second drain, and a second gate, the second drain coupled to the first drain, and the second source coupled to an output voltage terminal;

a third transistor having a third source, a third drain, and a third gate, the third source coupled to the first gate;

a current sensing circuit coupled between the first source and the first drain, the current sensing circuit providing a current sense output signal;

a shutoff circuit having a shutoff input and a shutoff output, the shutoff input coupled to the current sense output, and the shutoff output coupled to the third gate and the first gate;

a fourth transistor having a fourth source, a fourth drain, and a fourth gate, the fourth drain coupled to the second gate, and the fourth source coupled to the second source; and

a current limiting circuit coupled between the third drain and the fourth gate, wherein the fourth transistor turns on responsive to current flowing through the third transistor.

2. The power circuit of claim 1 ,

wherein the first transistor is an n-channel transistor; and

wherein the second transistor is an n-channel transistor.

3. The power circuit of claim 1 , wherein the first transistor is configured to turn on responsive to a signal at the first gate, and the second transistor is configured to turn off responsive to the first transistor turning on.

4. The power circuit of claim 1 , the current limiting circuit comprising:

a resistor having a first resistor terminal and a second resistor terminal, the first resistor terminal coupled to the third drain; and

a diode having an anode and a cathode, the anode coupled to the second resistor terminal and the cathode coupled to the fourth gate.

5. The power circuit of claim 4 , wherein the third transistor is a p-channel transistor and the resistor is a first resistor, the power circuit further comprising:

a second resistor connected between the first gate and the third gate; and

a switch coupled between the third gate and a reference voltage node.

6. The power circuit of claim 5 , further comprising a circuit configured to produce a pulse to instruct the switch to couple the reference voltage node to the third gate.

7. The power circuit of claim 5 , wherein the fourth transistor is an n-channel transistor, the anode is a first anode, and the cathode is a first cathode, the power circuit further comprising:

a third resistor coupled between the fourth source and the fourth gate; and

a Zener diode having a second anode and a second cathode, the second anode coupled to the fourth source and the second cathode coupled to the fourth gate.

8. The power circuit of claim 7 ,

wherein the first transistor is an n-channel transistor; and

wherein the second transistor is an n-channel transistor.

9. The power circuit of claim 4 , wherein the resistor is a first resistor, the anode is a first anode, the cathode is a first cathode, and the fourth transistor is an n-channel transistor, the power circuit further comprising:

a second resistor coupled between the fourth source and the fourth gate; and

a Zener diode having a second anode and a second cathode, the second anode coupled to the fourth source and the second cathode coupled to the fourth gate.

10. The power circuit of claim 9 ,

wherein the first transistor is an n-channel transistor; and

wherein the second transistor is an n-channel transistor.

11. The power circuit of claim 4 , wherein the third transistor is a p-channel transistor and the resistor is a first resistor, the power circuit further comprising:

a second resistor coupled between the third source and the third gate; and

a switch having a current path between the third gate and a reference voltage node.

12. The power circuit of claim 11 , further comprising a circuit configured to produce a pulse to instruct the switch to couple the reference voltage node to the third gate.

13. The power circuit of claim 11 , wherein the fourth transistor is an n-channel transistor, the anode is a first anode, and the cathode is a first cathode, the power circuit further comprising:

a third resistor coupled between the fourth source and the fourth gate; and

a Zener having a second anode and a second cathode, the second anode coupled to the fourth source and the second cathode coupled to the fourth gate.

14. The power circuit of claim 4 , wherein the fourth transistor is an n-channel transistor the anode is a first anode, and the cathode is a first cathode, the power circuit further comprising:

a second resistor coupled between the fourth source and the fourth drain; and

a Zener diode having a second anode and a second cathode, the second anode coupled to the fourth source and the second cathode coupled to the fourth gate.

15. A circuit, comprising:

a first transistor having a first source, a first drain, and a first gate, the first source coupled to an input;

a second transistor having a second source, a second drain, and a second gate, the second drain coupled to the first drain, and the second source coupled to an output;

a third transistor having a third source, a third drain, and a third gate, the third source coupled to the first gate;

a current sensing circuit coupled between the first source and the first drain, the current sensing circuit providing a current sense output signal;

a shutoff circuit having a shutoff input and a shutoff output, the shutoff input coupled to the current sense output, and the shutoff output coupled to the third gate and the first gate;

a fourth transistor having a fourth source, a fourth drain, and a fourth gate, the fourth drain coupled to the second gate and the fourth source coupled to the output; and

a current limiting circuit coupled between the third drain and the fourth gate, wherein the fourth transistor turn on responsive to current flowing through the third transistor.

16. The circuit of claim 15 , wherein the current limiting circuit comprises a resistor and a diode coupled in series, with the resistor coupled to the third drain.

17. The circuit of claim 16 ,

wherein the resistor has a first resistor terminal and a second resistor terminal, the first resistor terminal coupled to the third drain; and

wherein the diode has an anode and a cathode, the anode coupled to the second resistor terminal and the cathode coupled to the fourth gate.

18. The circuit of claim 15 , wherein the third transistor is a p-channel transistor, the power circuit further comprising:

a resistor coupled between the third source and the third gate; and

a switch having a current path coupled between the third gate and a reference voltage node.

19. The circuit of claim 15 ,

wherein the first transistor is an n-channel transistor; and

wherein the second transistor is an n-channel transistor.

20. A method, comprising:

conducting a first current from a gate of a first transistor through a first resistor in response to detection of a short circuit condition at an output;

conducting, by a second transistor, in response to a voltage across the first resistor;

while the second transistor is conducting, conducting a second current from the gate through the second transistor, through a second resistor, and through a third resistor; and

in response to a voltage across the third resistor:

conducting, by a third transistor, to cause a fourth transistor not to conduct; and

disconnecting the output from the first transistor.

21. The method of claim 20 , wherein conducting the first current from the gate through the first resistor comprises:

conducting, by a switch, in response to detection of the short circuit condition; and

while the switch is conducting, conducting the first current from the gate through the switch and the first resistor.