Power transistor detection with self-protection

1. A method for establishing a powered link over a transmission line comprising:

enabling a current source to provide a constant, predetermined current to a terminal, no current flowing between a drain of a power transistor coupled to the terminal and a source terminal of the power transistor while the current source is enabled, and the power transistor only conducting current in a subthreshold region of transistor operation while the current source is enabled;

sensing a voltage on the terminal to generate a sensed voltage level;

comparing the sensed voltage level on the terminal to a predetermined voltage level; and

disabling the current source in response to the sensed voltage level equaling or exceeding the predetermined voltage level.

2. The method as recited in claim 1 further comprising configuring a power sourcing equipment based on an amount of time between the enabling and the disabling.

3. The method as recited in claim 1 wherein the voltage does not enter a voltage range of possible threshold voltages for a power transistor coupled to the terminal.

4. The method as recited in claim 1 further comprising generating a digital time signal indicative of a time between the enabling and the disabling.

5. The method as recited in claim 4 wherein the digital time signal is used as a proxy for a gate-to-source capacitance of a power transistor coupled to the terminal.

6. The method as recited in claim 4 wherein generating the digital time signal comprises:

polling a flag signal; and

incrementing the digital time signal in response to the flag signal having a first value.

7. The method as recited in claim 4 further comprising generating an indication of an error based on the digital time signal being less than a predetermined minimum value of the digital time signal.

8. The method as recited in claim 4 further comprising generating an indication of an error based on the digital time signal exceeding a predetermined maximum value of the digital time signal.

9. The method as recited in claim 4 further comprising configuring a driver coupled to the terminal based on a value of the digital time signal within a predetermined range of values of the digital time signal.

10. A system for delivering power over a transmission line, the system comprising:

a device having a terminal, a current source configured to provide a constant predetermined current to the terminal, a sensing circuit configured to sense a voltage on the terminal to generate a sensed voltage level, and a logic circuit configured to enable the current source and configured to disable the current source in response to the sensed voltage level on the terminal being greater than or equal to a predetermined voltage level, the current source being disabled prior to a gate-to-source voltage of a power transistor coupled to the terminal reaching a threshold voltage of the power transistor.

11. The system as recited in claim 10 further comprising the power transistor coupled to the terminal, the power transistor conducting in a subthreshold region of transistor operation while the current source is enabled.

12. The system as recited in claim 11 , further comprising the transmission line coupled to the power transistor, the transmission line being an Ethernet cable.

13. The system as recited in claim 10 wherein the logic circuit comprises a time-to-digital converter configured to generate a digital time signal indicative of an amount of time the current source is enabled.

14. The system as recited in claim 13 wherein the digital time signal is used as a proxy for a gate-to-source capacitance of a power transistor coupled to the terminal.

15. The system as recited in claim 13 further comprising a driver coupled to the terminal, the logic circuit being configured to adjust the driver based on a value of the digital time signal.

16. The system as recited in claim 15 wherein the logic circuit comprises:

a processor; and

software stored in a storage element coupled to the processor, the software being executable by the processor to, after disabling the current source, configure the driver based on the value of the digital time signal within a predetermined range of values of the digital time signal.

17. The system as recited in claim 10 wherein the device is a power sourcing equipment and the transmission line is an Ethernet cable.

18. A method for establishing a powered link over a transmission line comprising:

powering up a power sourcing equipment integrated circuit;

prior to enabling a normal mode of operation of the power sourcing equipment integrated circuit, driving a current through a gate terminal of the power sourcing equipment integrated circuit during an initialization sequence of the power sourcing equipment integrated circuit, the current being disabled prior to a gate-to-source voltage of a power transistor reaching a threshold voltage of the power transistor; and

configuring the power sourcing equipment integrated circuit based on a time for a terminal of the power sourcing equipment to achieve a predetermined voltage level.

19. The method as recited in claim 18 wherein the normal mode of operation includes detection and classification of a powered device by the power sourcing equipment integrated circuit, and prior to the normal mode of operation, the current causing the power transistor to conduct in a subthreshold region of transistor operation.