JAJSES8B October 2017 – November 2018 TPS2372
IEEE802.3bt includes new PSE output limiting requirements for Type 3 and 4 operation to cover higher power and 4-pair applications. Type 2, 3 and 4 PSEs must meet an output current vs time template with specified minimum and maximum sourcing boundaries. The peak output current per each 2-pair may be as high as 50 A for 10 μs or 1.75 A for 75 ms, and the total peak current becomes twice these values when power is delivered over 4 pairs. This makes robust protection of the PD device even more important than it was in IEEE 802.3-2012.
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-VSS) rising as a result. If V(RTN-VSS) rises above approximately 14.5 V for longer than approximately 1.65 ms, the current limit reverts to the inrush value and PG output is forced low which turns off the converter, although there is no minimum inrush delay period (81.5-ms) applicable in this case. The 1.65-ms deglitch feature prevents momentary transients from causing a PD reset, provided that recovery lies within the bounds of the hotswap and PSE protection. Figure 21 shows an example of the RTN current profile during VDD to RTN short circuit, using 5-ohm load impedance. The hotswap MOSFET goes into current limit, causing the RTN voltage to increase. Once VRTN exceeds 14.5 V, IRTN, which was clamped to the current limit drops to the level of inrush current limit after 1.65 ms.
The inrush current limit is also reestablished when V(VDD-VSS) drops below UVLO then rises above it.
The PD control has thermal sensors that protect the internal hotswap MOSFET and the MPS pulsed current driver. Conditions like startup or operation into a VDD-to-RTN short cause high power dissipation in the MOSFET. An over-temperature shutdown (OTSD) turns off the hotswap MOSFET, the class regulator, and the MPS driver, which are restarted after the device cools. The hotswap MOSFET will be re-enabled and the TPS2372 will return to inrush phase when exiting from an overtemperature event. Pulling DEN to VSS during powered operation causes the internal hotswap MOSFET to turn off.
The hotswap switch will be forced off under the following conditions: