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TPS23880: Incorrect PCut for Class 1 & 2

Mike Turner
Prodigy 50 points
Part Number: TPS23880
Other Parts Discussed in Thread: TPS2375EVM

We are using the TPS23880 in automatic mode and although it correctly classifies various PD loads we are getting the ports turning off before it reaches the appropriate limits, specifically for Class 1 and 2. This is tested using the TPS23880 on our own product, a TPS2375EVM as the load and with an electronic load on the output. Power was taken from the TPS23880 GUI. For Class 1 our eLoad reached 2.5W, with GUI displaying 3.7W at which point it cut off with a PCUT exceeded error. For Class 2 the eLoad was 5.5W and the GUI display was 6.83W. Error again was PCUT exceeded. For other classes the output power seemed to be just about ok, although cut off was very close to the limits in most cases.

I'm using latest available firmware and would have expected the results to err on the high side rather than the low side.

  • 0
    TI__Mastermind 37121 points

    Hi Mike,

    Thanks for reaching out!

    Our PSE expert Brandon is traveling now. Please expect a delayed reply. Thanks for your patience.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Genius 15275 points

    Hi Mike,

    Can you confirm the PCUT thresholds using the GUI? Your expectation would be correct, the accuracy error is a positive error gaurenteeing the lower threshold. What SRAM version are you using? 

    Regards,

    Brandon

  • 0 in reply to Brandon Beader
    Prodigy 50 points

    Hi, Attached a couple of screenshots from the GUI. This is at the point where the power draw is about to exceed the limit (Class 1 and Class 2 here). The device in this case is programmed by the GUI but I assume it'll be using the latest version of firmware. Let me know if there's anything else I can provide. The PD is a TPS2375EVM, output is connected to an Electronic load.

  • 0 in reply to Mike Turner
    TI__Genius 15275 points

    Hi Mike,

    The latest SRAM version for this device is SRAM v2. Can you load SRAM v2?

    SRAM v2 can be found in mysecure resources and you will load by clicking the firmware tap at the top of the GUI.

    Regards,

    Brandon

  • 0 in reply to Brandon Beader
    Prodigy 50 points

    Hi Brandon,

    Tried that but still same results - although only checked Class 1 & 2 since these are the ones that seem most problematic. 

    Mike

  • 0 in reply to Mike Turner
    TI__Genius 15275 points

    Hi Mike,

    Let me see if I can reproduce this from my end. I ordered a new EVM to test on my side. Please give me a couple days to recieve new EVM and reproduce. 

    Can you show me the GUI screen after the load is drawn above the threshold? Do it not shutoff the port? What is the behavior? 

    Also, your operating voltage is slightly above 57V, can you reduce your power supply to 52V and see if issue persists. 57V is the maximum operating voltage within specification for PoE.

    Regards,

    Brandon

  • 0 in reply to Brandon Beader
    Prodigy 50 points

    Hi Brandon,

    OK, reducing the input voltage was something I meant to try anyway to see if the issue persisted across the range. I'm out of the office for a few days but I'll do that, and get further screen grabs as soon as I can.

    Thanks

    Mike

  • 0 in reply to Mike Turner
    TI__Expert 3190 points

    Hello Mike, 

    Please do provide screen grabs once they are available. 
    Thank You. 

    Regards,
    Deepak 

  • 0 in reply to Deepak Rozario
    Prodigy 50 points

    Hi, So at 52V input the results are more in-line with my expectations. I've attached a couple of images from the GUI, just before it drops out. However also of note is that this was done with a bench supply and when I set the bench supply to 57V the results were also similarly acceptable. I suspect the stability of the PSU may be the main factor here. The PSU we normally use is a 'lump-in-cord' style with 100W capability. It may be that at low loads the voltage isn't particularly stable. I'll do a bit more probing on the unit before we close this off, but looks like that's potentially the cause.

    Thanks

    Mike 

  • 0 in reply to Mike Turner
    TI__Mastermind 37121 points

    Hi Mike,

    Thanks for your update. Brandon is on leave today. Please expect a delay on reply. 

    Best regards,

    Diang

  • +1 in reply to Diang Xing
    TI__Genius 15275 points

    Hi Mike,

    I have seen similar issues with an instable supply. When the supply voltage is stable under low load conditions the PCUT behavior is normal.

    Please let me know your conclusion and if we can close out this item. Thanks!

    Regards,

    Brandon

  • 0 in reply to Brandon Beader
    Prodigy 50 points

    Hi Brandon, Yes, further testing indicated that the PSU itself wasn't particularly stable at low loads. In a real-life installation we'd either have a higher load overall, which would stabilise the PSU, or be using a more appropriate PSU. Thanks for your help.