LTC4266 circuit powered via external PoE

You are connecting to a non-compliant IEEE device since power is forced on to the port without proper detection.  This type of device can cause damage to other connected devices not capable of PoE.   

it is a Cisco SG300 series switch , I guess it is a IEEE compliant device. 

But do you have any suggestion to protect the LTC4266 circuit for external power over PoE ?

Are you connecting PSE output port to PSE output port?  The detection signature "should be" corrupted in this configuration and full voltage and power should not be applied on the cable.  Either power is forced on or somehow a valid signature is detected and power is turned on by the second PSE.

Please clarify if this setup is different.

This type of request does not typically come up.  But to protect from the scenario above, a DC blocking component such as a diode (which is lossy) or ideal diode (such as the LTC4357) could be used to block any backfeed supply.

yes , it is PSE output connectect to another PSE output.
The othe PSE device detecs a class 0 device and delivers about 20W to the electronics behind the LCT4266.
This is all I can check and verify in the Cisco SG300 management interface.
Fet Bad event is detected by the LTC4266 but of course this is because current is flowing in reverse direction.

and what would be a good place to add the diode ?
before or after the mosfet ?
also take note that in fact the diode over the mosfet is responsible that external power can be fed in the circuit.

and what would be a good place to add the diode ?
before or after the mosfet ?
also take note that in fact the diode over the mosfet is responsible that external power can be fed in the circuit.