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TPS23754: PD does not start when probes are attached to VDD_VSS

Sina Vosooghi
Prodigy 60 points
Part Number: TPS23754

I used the TPS23754 reference design for a 12V , 2A output. But the POE circuit can only provide about 15W when I use a Trendnet POE+ switch. As soon as I increase the load, the PSE cuts off the power. When I use a simple POE+ injector, I can draw the whole 25W. So I'm assuming the issue is with class detection. I am trying to probe the VDD-VSS to see the startup wave from but the PSE doesn't bring the rails up when I have my Tek P5050 probe connected to VDD-VSS. As soon as I remove the Probe, the rails come up. 

1. I have the 63.4 resistor so why isn't my device classifying as type 4?

2. Do I need a specific probe and probing technique for analyzing the VDD-VSS?

  • 0
    TI__Genius 9230 points
    Sina,

    I have notified our PD expert & we will get back to you shortly.

    Thanks,
    Thomas A.
  • 0 in reply to Thomas Amlee
    TI__Genius 9230 points
    Sina,

    Apologies for the delay.

    As part of the .at standard, classification can be done on the physical layer, or on the data link layer (DLL). The midspan is only able to do hardware classification, while the switch can do both hardware & DLL.

    Since the midspan is providing full class 4 power (25W) and you have the correct 63.4 ohm resistor in place, you are passing hardware classification.

    Take a look at page 17 of the TPS23754 datasheet: www.ti.com/.../tps23756.pdf

    #6 at the bottom of the page says "[PD] must not draw more than 13W if it has not.... received permission through DLL." With that, I would check that the switch is configured to allow type 2 level power.

    To answer your 2 questions: Resistance is correct. Your probing technique is fine (using the test points, I'm assuming).

    Thanks,
    Thomas A.
  • 0 in reply to Thomas Amlee
    TI__Genius 9230 points
    Hello Sina,

    I am closing the thread now. If your issue is resolved there is no need to respond. If you need any clarity on this issue, just reply. If you have another question, please make another post.

    Thanks,
    Thomas A.