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LT4294 application issue

Hi ADIexpert

I have two questions for the LT4294 application

1. May I know how long does it usually take for the handshaking from detection to power up between a PSE and PD?

If I connect a adaptor (use the Aux pin function) and a PSE to a LT4294 PD device at the same time, 

according to the datasheet, the signature res is off and power should be 100%  from adaptor

when I remove the adaptor, since I don't want to reset the system, I would need to know the time for handshaking and add it as the holdup time for my DC/DC converter

2. I have a system as the attachement

When the adaptor and Sifos test equip (as a PSE) are both connect to the PD, Sifos failed to detect the signature res of LT4294, but when the adaptor was removed, everything was fine

since the adaptor power  is blocked by the Diode Oring and should not have any effect  on LT4294, I have  no idea what caused the failure of detection,

May I have your suggestion what I should check more for the issue?

  • Hi ADIexpert

    For question1, I have checked the spec and got the answer.

    if you have any suggestion on question2, it would be appreciated.

    thanks

    Ken

  • Hello,

    The time it takes for a PSE to power on a PD depends heavily on the specific PSE and its Type. IEEE 802.3bt defines maximum timing for Detection, Connection Check, Classification, and Power On. Adding all these together, the time between connecting a PD and the PSE providing power could be over 1 second.

    Looking at your block diagram, there could be a leakage path that could corrupt the detection signature. Please email me directly and I can assist with reviewing your schematic.

     

    Best Regards,

    Eric