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TPS2372: TPH & TPL not producing expected results.

Part Number: TPS2372
Other Parts Discussed in Thread: TPS23880EVM-008, USB2ANY, BOOST-PSEMTHR-007


I'm having a bit of an issue with the TPS2372 TPH and TPL outputs. Hopefully
you can tell me what I'm doing wrong.

We think that their output should be a direct function of the number
of class cycles produced by the PSE per table 2 in the datasheet.
Unfortunately, the observed outputs are different.

By changing the requested class, we can observe a change in number of
class cycles:

Requested Observed Expected Observed
Class     Class    TPH TPL  TPH TPL
          Cycles
3         1        1   1    1   1
4         3        1   0    0   0
5         4        0   1    0   0
6         4        0   1    0   0

As you can see, TPH and TPL are either both high or both low.
We never observe them having different values. This has been confirmed with
multiple boards. We also checked that they are not shorted together.

Attached is cut/paste of the relevant schematic sections as well as
a scope shot for the power on sequence when we request class 4 operation.
The scope is observing VDD relative to VSS as seen by the TPS2372-4.

Any ideas?



  • Hedley,

    You have reached out to us over a holiday break for Texas Instruments' employees based in the US. Everyone is off until next Monday (4/22). We will get to your request ASAP next week.

    Thank you for your understanding,
  • Hi Hedley,

    From the waveform you showed, it looks like you connected a PoE++ PSE NOT a IEEE802.3bt standard Type 3 or Type 4 PSE as the PSE doesn't present first long finger classification. Can you please order TPS23880EVM kit (including TPS23880EVM-008, BOOST-PSEMTHR-007 and USB2ANY) and test with your PD board and see if you can get correct TPH and TPL signals? Thanks.

    Best regards,

    Penny

  • Hi Penny,

    Thanks for the response.

    I'm gonna get asked about this.  The TPS2372 datasheet says "Supports PoE++ PSE".  Does that support not extend to TPL/TPH indication of allocated power?

    Just out of curiosity, do you think the length of the first pulse is the issue?

    Thanks,

    Hedley

  • Hi Hedley,

    POE++ doesn't have the same power negotiation methodology and it doesn't have TPL , TPH and BT to indicate type of PSE. The TPH and TPL are based on number of class fingers that PSE sends. PoE++ PSE (class 4 to class 8) only sends 3 finger classifications to PD. There's no way for the PD to know the power capability of the PSE.  BT type 3 and type 4 PSE sends 3,4,5 finger to PDs based on the PSE's allocated power, and PD can differentiate the PSE's power capability based on number of fingers received. The first long finger is the only indication to PD that the PSE is type 3 or type 4.  Type 3 or Type 4 PDs look for this long finger classification to apply new BT features(such as new MPS signal). Please see more details in the attached slides and we also have videos available in PoE portal(http://www.ti.com/power-management/power-over-ethernet-poe/support-training.html). Thanks.

    Best regards,

    Penny

    UpgradingIEEE802.3atandNon-StandardHighPowerPSESystemtoIEEE802.3btCompliantSystem_Full.pdf