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PMP20878: Ripple current of C28

6192337
Prodigy 95 points
Part Number: PMP20878
Other Parts Discussed in Thread: TPS2373, TPS2372

Hello,

Can you provide me with some information on the expected ripple current across C28? Ripple voltage expected of C103? And would U2 be able to utilize the PoE VDD voltage for startup?

Thank you
Tim

  • 0
    TI__Intellectual 2350 points

    Hi, Tim,

    The test report shows the ripple voltage across C68, which is in parallel with C28, to be 15.3mV peak to peak.  C28 has a maximum ESR of 170 milliohms, so the maximum ripple current is 90mA peak to peak.

    I don't have a measurement of the ripple voltage across C103, but it is designed to be 100mV peak to peak or less.  This large value is usually not required, but was in this case to provide hold-p time to complete soft-stop at turn-off.

    Although using the start-up regulator in the TPS2373 is preferred, U2 can be powered from PoE VDD/RTN by connecting VDD to pin 18.  VC_IN (TP24) will connect to U2 pins 13/14.  VC_IN and VC_OUT on the TPS2373 should be left open.  If the start up regulator in the TPS2373 is not used, VC_IN needs to have a bias to operate properly.  A 12V SOD-123 zener diode connected to VC_IN (U1-15)/RTN with a 12k/1206 resistor pull-up to VDD will be needed.

    Thanks,

    David

  • 0 in reply to David Strasser
    Prodigy 95 points

    Hello,

    Thank you for the response! It answered my questions but I have another question.

    "If the start up regulator in the TPS2373 is not used, VC_IN needs to have a bias to operate properly.  A 12V SOD-123 zener diode connected to VC_IN (U1-15)/RTN with a 12k/1206 resistor pull-up to VDD will be needed."

    So to clarify on this. If the regulator on the TPS2373 is not used, a zener and pullup would be needed for VC_IN of U1. If a TPS2372 was used in its place this would not be needed?

  • 0 in reply to 6192337
    TI__Intellectual 2350 points

    Yes, that is correct.  Note that the TPS2372 does not have the APD function for the primary side adapter priority.  If this is needed you will need to stay with the TPS2373 and add the pull-up.

    Thanks,

    David