Power management with over voltage protection

1. A power management circuit, comprising:

a switch configured to receive a voltage from a host power source, a voltage from an external power source, an indication of host power source validity, and an indication of external power source validity;

wherein the switch is configured to provide a voltage to an internal supply rail using (1) the host power source when the indication of host power source validity indicates a valid host power source, and (2) the external power source when the indication of host power source validity indicates an invalid host power source and the indication of external power source validity indicates a valid external power source; and

an over voltage protection circuit, including:

an over voltage detection circuit configured to detect an over voltage condition on the external power source; and

a clamp circuit configured to (1) clamp the external power source for an initial time period when the indication of external power source validity indicates a valid external power source, and (2) clamp the external power source when the over voltage detection circuit detects the over voltage condition on the external power source.

2. The power management circuit of claim 1 , including:

a host power source detection circuit configured to validate the host power source and to provide the indication of host power source validity; and

an external power source detection circuit configured to validate the external power source and to provide the indication of external power source validity;

wherein the host power source includes a battery, and wherein the external power source includes an external bus voltage.

3. The power management circuit of claim 2 , wherein the clamp circuit includes a clamp switch configured to selectively short the clamp circuit for an after the initial time period when the indication of external power source validity indicates a valid external power source and the over voltage detection circuit detects that no over voltage condition exists on the external power source.

4. The power management circuit of claim 2 , including a bandgap reference circuit configured to provide a bandgap voltage reference;

wherein the host power source detection circuit includes a comparator configured to validate the host power source using the value voltage of the host power source and the bandgap voltage reference; and

wherein the external power source detection circuit includes a comparator configured to validate the external power source using the value voltage of the external power source and the bandgap voltage reference.

5. The power management circuit of claim 1 , including a voltage regulator configured to provide a regulated external power source; and

wherein the switch is configured to provide the voltage to the internal supply rail using the regulated external power source when the indication of host power source validity indicates an invalid host power source and the indication of external power source validity indicates a valid external power source.

6. The power management circuit of claim 1 , wherein the switch includes:

a first input configured to receive the voltage from the host power source;

a second input configured to receive the voltage from the external power source;

a first control input configured to receive the indication of the host power source validity;

a second control input configured to receive the indication of the external power source validity; and

an output configured to provide an the voltage to the internal supply rail.

7. The power management circuit of claim 1 , wherein the clamp circuit includes a clamp switch configured to selectively short the clamp circuit when the over voltage detection circuit does not detect the over voltage condition on the external power source.

8. The power management circuit of claim 7 , wherein the clamp switch includes a transistor;

wherein the clamp circuit includes a clamp control circuit including an oscillator, a counter, and a charge pump; and

wherein the clamp control circuit is configured to receive an indication of a detected over voltage condition on the external power source and to control the clamp switch using the indication of over voltage detection circuit detects the detected over voltage condition on the external power source.

9. A power management method, comprising:

receiving indications of host and external power source validities;

providing a voltage to an internal supply rail using a host power source when the indication of host power source validity indicates a valid host power source;

providing an a voltage to the internal supply rail using an external power source when the indication of host power source validity indicates an invalid host power source and the indication of external power source validity indicates a valid external power source;

detecting an over voltage condition on the external power source; and

clamping the external power source using a clamp circuit when the over voltage condition is detected on the external power source; and

clamping the external power source for an initial time period using the clamp circuit when the indication of external power source validity indicates a valid external power source.

10. The method of claim 9 , including:

validating the host power source and providing the indication of host power source validity; and

validating the external power source and providing the indication of external power source validity.

11. The method of claim 10 , including selectively shorting the clamp circuit for an after the initial time period when the indication of external power source validity indicates a valid external power source and no over voltage condition exists on the external power source.

12. The method of claim 10 , including:

providing a bandgap voltage reference;

validating the host power source using the value voltage of the host power source and the bandgap voltage reference; and

validating the external power source using the value voltage of the external power source and the bandgap voltage reference.

13. The method of claim 9 , including providing a regulated external power source; and

wherein the step of providing the voltage to the internal supply rail using the external power source includes using the regulated external power source when the indication of host power source validity indicates an invalid host power source and the indication of external power source validity indicates a valid external power source.

14. The method of claim 9 , including selectively shorting a the clamp circuit when the over voltage detection circuit does not detect an no over voltage condition exists on the external power source.

15. The method of claim 14 , including:

receiving an indication of a detected over voltage condition on the external power source; and

controlling the clamp circuit using the indication of over voltage detection detects the detected over voltage condition on the external power source.

16. A power management system, comprising:

an electronic device, including a host power source and a universal serial bus (USB) port, the USB port configured to receive a voltage from an external power source;

a switch configured to receive a voltage from the host power source, the voltage from the external power source, an indication of host power source validity, and an indication of external power source validity;

wherein the switch is configured to provide a voltage to an internal supply rail using (1) the host power source when the indication of host power source validity indicates a valid host power source, and (2) the external power source when the indication of host power source validity indicates an invalid host power source and the indication of external power source validity indicates a valid external power source; and

an over voltage protection circuit, including:

an over voltage detection circuit configured to detect an over voltage condition on the external power source; and

a clamp circuit configured to (1) clamp the external power source for an initial time period when the indication of external power source validity indicates a valid external power source, and (2) clamp the external power source when the over voltage detection circuit detects the over voltage condition on the external power source.

17. The power management system of claim 16 , including:

a host power source detection circuit configured to validate the host power source and to provide the indication of host power source validity; and

an external power source detection circuit configured to validate the external power source and to provide the indication of external power source validity.

18. The power management system of claim 17 , wherein the clamp circuit includes a clamp switch configured to selectively short the clamp circuit for an after the initial time period when the indication of external power source validity indicates a valid external power source and the over voltage detection circuit detects that no over voltage condition on the external power source exists.

19. The power management system of claim 17 , including a bandgap reference circuit configured to provide a bandgap voltage reference;

wherein the host power source detection circuit includes a comparator configured to validate the host power source using the value voltage of the host power source and the bandgap voltage reference; and

wherein the external power source detection circuit includes a comparator configured to validate the external power source using the value voltage of the external power source and the bandgap voltage reference.

20. The power management circuit system of claim 16 , wherein the clamp circuit includes a clamp switch configured to selectively short the clamp circuit when the over voltage detection circuit does not detect the over voltage condition on the external power source.