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TPS23881: PCUT threshold or Pac Margin values when set to auto class auto adjustment enabled

Charan Kumar Reddy K R
Prodigy 90 points
Part Number: TPS23881

Hi,

we are TPS23881 in one of our design. we are using it in both 2 pair and 4 pair modes. 

While powering on the channel in auto/semi auto mode, we wanted to use auto class auto adjustment mode enabled as per 0x50 register. 

Could you please let us know, in case of auto class auto adjustment mode enabled what are the values set for PCUT threshold or Pac Margin? 

Regards,

Charan.

  • 0
    TI__Genius 15685 points

    Hi Charan,

    As described in TPS23881 datasheet, when AAC bit is set, the port PCUT value will be set to the measured autoclass power in register 0x51 to 0x54. Thanks. 

    Best regards,

    Penny

  • 0 in reply to Penny Xu
    Prodigy 90 points

    Hi Penny,

    Measured autoclass for Class4 shows 30W in 0x51-0x54. What is the allowed power margin in case of AAC bit is set for class 4? 

    If we set 2-Pair Police registers to 34W,  and AAC bit is also set, will it consider the limit set in POLICE register for PCUT.

    Regards,

    Charan.

  • 0 in reply to Charan Kumar Reddy K R
    TI__Genius 15685 points

    Charan,

    The police configuration registers(1Eh-21h, 2Ah and 2Bh) are the ones we set PCUT values. the power margins are only needed to be considered if you want to manually set PCUT value based on autoclass power measurement. If the AAC bits are set, you don't need to worry about the PCUT setting. The TPS23881 will take care of it by itself. Thanks.

    Best regards,

    Penny

  • 0 in reply to Penny Xu
    Prodigy 90 points

    Hi Penny,

    We are trying to load 30W on each channel in Auto mode.

     when AAC bits are set and police registers are not configured, we are seeing tovld fault when loaded with 30W.

    Can you please let us know if we need to do something else to avoid tovld fault.

    Regards,

    charan.

  • 0 in reply to Charan Kumar Reddy K R
    TI__Genius 15685 points

    Hi Charan,

    How long is your ethernet cable? Can you try to reduce the load power and see? I think the total power PSE sees (your load power 30W + cable loss) already exceeds the PCUT.  For an IEEE compliant PD, the maximum power consumption is 25.5W to counter for the cable loss. In addition, the TPS23881 has the below PCUT tolerance spec. Thanks. 

    Best regards,

    Penny