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TPS23881: how many times of handshaking and power or not when demoted?

xiang tao
Prodigy 100 points
Part Number: TPS23881
Other Parts Discussed in Thread: TPS2372,

Tool/software:

1、how many detection and classing periods are there before power start up?

below is the waveform captured on the dual signature PD VDD, the PSE is TPS23881, PD is TPS2372, why alt-A has 3 detection and classing periods and so does alt-B?

on page 22 of TPS23881 datasheet, only 2 detection periods and 1 classing period shown on both alt-A and alt-B

2、why does alt-B always power before alt-A on my case? alt-A is the pri(odd) channel of TPS23881 and alt-B is the aux.(even) channel.

3、does PSE power start up when demoted?

e.g. PSE set to 4-pair 30W, dual calss-5 signature PD connected to the PSE, and primary channel is demoted to class4, so in this case,  primary channel power on or off?

I think it still powers on when demoted, but in my design, I see the PSE doesn't power on.

Thanks in advance!

  • 0
    Prodigy 100 points

    I want to declare that in question 3, although the PD class resistor is set to class 5(45W) but the actual power consumption is less than 2W.

  • 0 in reply to xiang tao
    TI__Mastermind 37061 points

    Hi Xiang,

    Thanks for reaching out!

    1. TPS23881 is supposed to only have one full detection + classification. In your waveform, it may indicates the classification failed (not within the below ranges or overcurrent). You can check the DISCOVERY Registers to see what is the faults in classification. Are you testing with a real PD or a PD emulator?

    2. Theoretically either Alt-A or Alt-B could start first if you enable the detection & classification, then connect the PD - PSE. If your PSE-PD is pre-connected, the Alt-A/B sequence could be fixed by startup sequence. 

    3. Power demotion mode allows power on but PSE will have a lower power limit. In your case the PSE should be on after valid detection & classification but with 30W power limit. The power policing should be with 4-pairs dual signature 

    Best regards,

    Diang