7.4.1.8 PD Hotswap Operation

IEEE 802.3at has taken a new approach to PSE output limiting. A type 2 PSE must meet an output current versus time template with specified minimum and maximum sourcing boundaries. The peak output current may be as high as 50 A for 10 μs or 1.75 A for 75 ms. This makes robust protection of the PD device even more important than it was in IEEE 802.3-2008.

The internal hotswap MOSFET is protected against output faults and input voltage steps with a current limit and deglitched (time-delay filtered) foldback. An overload on the pass MOSFET engages the current limit, with V_RTN-V_VSS rising as a result. If V_RTN rises above approximately 12 V for longer than approximately 800 μs, the current limit reverts to the inrush value, and turns the converter off. The 800 μs deglitch feature prevents momentary transients from causing a PD reset, provided that recovery lies within the bounds of the hotswap and PSE protection. Figure 27 shows an example of recovery from a 16 V PSE rising voltage step. The hotswap MOSFET goes into current limit, overshooting to a relatively low current, recovers to approximately 1000 mA full current limit and charges the input capacitor while the converter continues to run. The MOSFET did not go into foldback because V_RTN-V_VSS was below 12 V after the 800 μs deglitch.

Figure 27. Response to Output Short Circuit

The PD control has a thermal sensor that protects the internal hotswap MOSFET. Conditions like startup or operation into a V_DD-to-RTN short cause high power dissipation in the MOSFET. An over-temperature shutdown (OTSD) turns off the hotswap MOSFET and class regulator, which are restarted after the device cools. The hotswap MOSFET is re-enabled with the inrush current limit when exiting from an over-temperature event.

Pulling DEN to V_SS during powered operation causes the internal hotswap MOSFET to turn off. The hotswap switch is forced off under the following conditions:

1. V_APD above V_APDEN (approximately 1.5 V)
2. V_{(DEN –VSS)}< V_{PD_DIS} when V_VDD– V_VSS is in the operational range,
3. PD OTSD is active, or
4. V_{(DEN –VSS)}< PoE UVLO falling threshold (approximately 32 V).