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TPS2378 POE, PSE drops Vdd after 350ms

steve sagerian
Prodigy 20 points
Other Parts Discussed in Thread: TPS2378

I am using a TPS2378 in a new design. I have a down stream switch mode being controlled by the CBD signal from the 2378. The design is not working, but I see the PD transition from detection, to operation with 50V being delivered for about 350ms but then it drops..

If I by pass the POE and just tap 50V to the switch mode, it works (IE I don't think the PS is shorted) . What other things can I try?

Thanks in advance

  • 0
    TI__Mastermind 42795 points

    The PD is required to pull > 10mA after turn on or the PSE can remove power (after ~300-400ms). Once your dc/dc converter is working the loading should keep the PSE powered. For now, you can test this by putting a resistor (R < VVDD / 0.01) between VDD and RTN of the TPS2378 and the PSE should stay on.

  • 0 in reply to Eric Wright
    Prodigy 20 points

    I tried adding a 3.9K across the Vdd to Vss (RTN)that worked on one board but did not on 4 others..

    On the one board that worked, I also added a pull up on CBD and connected to the ENABLE on the PS (originally it was tied directly VDD).

    The other boards (those that did not work with the suggested fix) may have been hacked up enough that they had other issues, I am not sure. Any other thoughts?

    Thanks again.