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TPS23881: Autonomous mode

bifang jiang
Prodigy 20 points

Part Number: TPS23881

Hi All,

A device uses TPS23881 without MCU. The AUTO pin resistor of TPS23881 is 7.68kΩ (4-pair 90W). We found that TPS23881 cannot supply power to a PD when it is connected with a PD which only supports IEEE802.3af. If the connected ethernet cable （4-pair） is replaced with 2 pairs of wires,TPS23881 can supply power to the PD. If a PD supports IEEE802.3at/IEEE802.3bt, TPS23881 can supply power to the PD with 2-pair/ 4-pair wire. What reason caused it? How to fix it?

I look forward to getting your reply.

Thanks .

Best regards,

  • 0
    TI__Mastermind 21320 points

    Hello,

    can you send a schematic of the PD and PSE?

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 20 points

    Hello Michael,

    Thank you for  your information.

    We got the following results after trying to replace sense resistor.

    Rsense(mΩ) Power on PD（only 802.3af & 1.8W） Max. loading power(W)
    200 Yes 15
    100 Yes 30
    50 Yes 60
    40 Yes 66
    34 No 90
    20 No

    127

    Why can't TPS23881 supply power to low-power PD when the sense resistor is less than 34mΩ? How to fix it.

    Thanks.

    Best regards,

  • 0 in reply to bifang jiang
    TI__Genius 15685 points

    Hi Bifang,

    For standard application, you are not supposed to change the sense resistor. Changing the sense resistor value means fooling the PSE of current measurement. The classification current will be shifted. Can you share your full schematic including the RJ45 connection?

    What are you trying to achieve? Can you please list your use cases with PSE's setting and PD settings? Once the PSE is configured as 90W, you should be able to power up class 8 PD with 90W being sourced without changing the sense resistor. The sense resistor should always be 200mohm. 

  • 0 in reply to Penny Xu
    Prodigy 20 points

    Hi Penny,

    The device is just a 10G media converter that supports IEEE 802.3bt (90-watt) PSE. TPS23881 acts as PSE without MCU and SRAM. We set 7.68K for the AUTO pin resistor. The low-power PD only supports IEEE802.3af. The following is the schematic of RJ45 connection. The RJ45 connector includes a transformer.

    Thank you for your support.

    Best Regards,

    Bifang

  • 0 in reply to bifang jiang
    TI__Genius 15685 points

    Bifang,

    Can you please clarify the issue and help me understand the issue?

    1. When the sense resistor is 200mohm, PSE is configured to 90W, 4 pair, is the max power send to IEEE802.3af PD 15W? If that's the case, it is expected as the max alloed power PSE can send to IEEE802.3af PD is 15W. 

  • 0 in reply to Penny Xu
    Prodigy 20 points

    Hi Penny,

    The maximum output power refers to the test results connected to 802.3bt PD.

    After changing the sense resistor, we did two tests. One was to check whether the TPS23881 PSE can supply power to PD devices that only support IEEE802.3af. One was to test with PD supporting IEEE802.3bt to test the maximum power that PSE can output.

    Thanks,

  • +1 in reply to bifang jiang
    TI__Genius 15685 points

    Hi Bifang,

    TPS23881 can support 90W to a IEEE802.3bt compliant class 8 PD without changing sense resistor. Let's focus on this scenario first to configure out why you are having issues with that. 

    Can you please capture port voltage and current waveforms when you connect a class 8 PD to PSE with 200mohm sense resistor? If possible please also capture all PSE registers' status? Thanks.