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TPS2373: 802.3af support for 37V input

Part Number: TPS2373

Hi Team,

The UVLO spec in the TPS2373-3 datasheet has a UVLO rising threshold of 38.1V and to support 802.3af we'd need to support down to 37V. Is there a way to get the TPS2373-3 to support 37V so the system can be 802.3af compliant as well as 802.3bt?

Thank you,


  • Hi Trenton, 

    The TPS2373 is IEEE 802.3bt compliant which is backwards compatible with IEEE 802.3at and IEEE 802.3af. 

    I think the confusion is around the power-on. 

    During detection, class and power-up of the PD, there is a very small voltage drop across the 100m cable so the voltage at the PD will be around the same voltage as at the PSE, which at a minimum is 44V worst case. This is well above the 38.1V rising UVLO and so the PD will turn on. As the PD then turns on to full load, the voltage at the PD could potentially drop below 44V. Taking into account full load and 100m of cable the worst case voltage is 37V. However, the falling UVLO threshold is around 32V and so the power would not be disconnected until <32V is seen at the PD. 

    Let me know if you have any more questions about this. Thanks! 

    Best Regards,

    Brett Colteaux

  • They were using direct inject PoE injectors that don't do POE negotiation to test the 37V 802.3af point so that's why our part was triggering UVLO and thought our part can't support 802.3af.