Few months ago I had a session here related to the LT4294IMS#PBF PD issue, that was found to be related to the hot swap N-FET SOA characteristics, that were not good enough.
I have replaced the transistor(to CSD19537Q3) with better characteristics, that allow more than 3A continuous current through it, based on the SOA graph.
Unfortunately we still facing nearly the same problem and need your help.
LT4294 requests 62W, but board in fact, consumes no more than 30W.
The LT4294 schematics implemented according to the data sheet and the evaluation board schematics(DC2911A). We are facing same behavior on all boards we assembled. We use IEEE802.3bt certified POE supply(POE60U-1BT). After successful classification/negotiation board starts and continues working for few hours, then LT4294 and N-FET became extremally hot, PWRGD start oscillating like a clock - rising and failing cycles, the N-FET output shows rising and failing behavior like a saw. The input is stable and equal ~56V. LT4294 is intended to pass power to the Ethernet switch, IP cams, radar, CPU, USB HUB and some on-board electronics. The working mode is not changing along all hours(in some case minutes) till the failure. We have no any problem working several days on AUX power|(12VDC) with the same board, indicating there is no problem in onboard electronics, but once we wish to work from POE problem appear after few hours, usually within first couple hours(it can be few minutes sometime).
Please see attached schematics. I can provide layout if need...I have no clue what is wrong here as we are already several month in the same session and it seems that everting was tested and checked.. Please help.
[edited by: ALirov586 at 8:11 AM (GMT -4) on 19 Jul 2021]