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TPS23753 24VAC Aux Application Note

Joe Rule
Intellectual 330 points
Other Parts Discussed in Thread: TPS23753

Hello,

     In the application note SLUA477 "PoE Powered Device for 24VAC Building Power Applications", the 24VAC input is OR'ed with the PoE input by way of a S2DA-13 rectifying diode.  In the datasheet of this component, I see a max I(r) of 350uA. Is it possible for this amount of reverse current (when only connecting PoE to the circuit), could build a charge on C2, ultimately increasing the voltage going in to APD over time to 1.5V and unintentionally hot swapping VSS/RTN?

http://www.ti.com/lit/an/slua477/slua477.pdf

Thanks,
Joe

  • 0
    TI__Mastermind 41760 points
    Hi Joe,
    I have not seen this type of issue before in the past. I would expect some of the other circuits in the adapter side would help in pulling the voltage down at that node. Is this what you are seeing in your application?
  • 0 in reply to Darwin Fernandez
    Intellectual 330 points
    We had another circuit that we saw issues with but differs too much to assume direct comparison with the SLUA77 app note ref design. So we are adapting it to be similar to the ref design. But what came to our attention is the high max leakage current at high temp of the rectifying diode, D5 (S2DA-13) according to the Diodes Inc. datasheet...350uA. The only places for this to drain in the circuit are the D2, C2, R3/R7, C12 and the APD input on TPS23753. I've looked up some of these. D2 max of 100uA, the APD is 1uA, C2 is 3uA...I'm assuming C12 to be negligible. If even 100uA is left going through the R3/R7, it would suggest the potential at the C2+ node is 1M*100uA = 100V...leaving APD at ~5.9V. So I have to be missing something. Either the S2DA-13 (D5) can be nowhere close to ever having a reverse current of 350uA, or there is another part of the circuit drawing much of that current that I have not accounted for.

    As for the S2DA-13, the datasheet shows a 25C reverse current of 5uA and 125C reverse current of 350uA...and the chart shows less that 100uA at 125C, so I've got a call in to Diodes Inc. as well to clear this up. It makes sense that 5uA wouldn't be a problem of course. My concern is if there really are cases where the current could be over 100uA to where there really is a potential building on C2 that will effect APD level.