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TPS2384: TPS2384 ; PD Detection Function Test Result with Device for or not for PoE

Kazuhito Ogawa
Intellectual 255 points
Part Number: TPS2384
Other Parts Discussed in Thread: TPS23861

I have designed and developed the TPS2384PAP PCB.

And I have tested the PD Detection Function of TPS2383 with PD Device for or not for PoE.

As a result, I have got the performed time waveform such as attached file graph.

In the case of PD Device for PoE, the test result time waveform is valid data

as same as TPS2384 datasheet.

But, in the case of Device not for PoE, the result is different waveform to the datasheet.

It is maybe invalid at Device not for PoE,

Question1)

Why does it have occured such a funny invalid test result in the case of load Device not for PoE?

Question2)

Please teach me the reason of invalid test result happened.

Question3)

Please teach me the solution how to improve my designed schematics in terms of invalid test result.TPS2384PAP - PD Detection Function Test Result with Device for or not for PoE.pdf

  • 0 in reply to Kazuhito Ogawa
    TI__Genius 9230 points
    Hello Kazuhito-san,

    Our PSE expert has been notified.

    Thanks!
    Thomas A.
  • 0
    TI__Genius 15655 points

    Kazuhito,

    I took a look at your waveform. The result is expected. For standard PDs, IEEE standard defines the detection resistor should be within 19K to 26.5K, and the parallel capacitance should be less than 0.15uF. The TPS2384 uses a four-point measurement technique using two low level probe signals (typically 4.4 V and 8.8 V) during the discovery process to determine whether a valid PD is present. I don't think your non PoE device meet PD requirement. Even you add 25k in front of your device and treat PSE to pass the detection, the device can't pass classification. During classification, standard PD will remove the detection resistor from the port and present a classification resistor. measure the class voltage from PSE, and then turn on internal voltage regulator across the class resistor and then PSE measures the class current to determine how much power the PD needs. For more PoE knowledge, please go to PoE FAQ page. e2e.ti.com/.../design_notes

    Thanks,

    Penny

  • 0 in reply to Penny Xu
    Intellectual 255 points
    Hello, Penny-san
    I have resolved these issue by getting japan ti technical center' advice.
    As a result, the reason of these issue occurrence is arising from the fact what TPS2384 performs classification operation(applying voltage 17.8V) in the case of Load device impedance is open(high).
    The same phenomena has been occurred in the datasheet of TPS23861/P18.
    Then either both PoE PD or Non-PoE PD has to conform the IEEE 802.3af Standard if getting normal valid operation of discovery 1 and discovery 2.
    So the PD load device may be broken by classification operation applying voltage 17.8V, in the case of the PD load device not conformed IEEE 802.3af Standard.
    Thanks for your advice.
    Best Regards, K.Ogawa