LT4275B not recognized by POE injector


I need help in understanding the problem in my design with LT4275BIMS#PBF.(schematics attached). At the time of negotiation I'm getting an indication of fail on the PSE injector and therefore no power applied to PD. AUX input is not used and it is float. PWRGD connected to EN pin on the power supply with integrated PU. T2P is not used.

The PSE injector used is POE90U-1BT (formally supporting IEE802.3bt standard). As I know IEE802.3bt is back compatible with IEE802.3af and  IEE802.3at so I assume there is no problem with PSE as I'm requiring only 25W during negotiation.

I use only pin #4 and 5 as positive supply and #7, 8(on RJ45 connector) as negative directly connected to the bridge. I'm not using other pairs, so it is only one bridge. The link is 100Mbt.

Is using another pairs mandatory? What can be wrong?

Thank you,



  • whom it can be interested.

    I solved the problem by removing the LT4320.

    In fact this bridge is not sufficient for this application(and it is sad Analog/Linear does not mentioned about it directly in component data sheet).

    When PSE negotiating with PD, PSE looks for 25Kom signature measuring the dV/dI. PSE applies 8.2~10.1V to PD and measure the current.

    The problem with LT4320IDD#PBF that it consumes much more current, therefore "braking" the 25Kom signature. You can use LT4321 dedicated bridge or FDMQ8205 that integrates gate driver and FET in one package. 

  • 0
    •  Analog Employees 
    on Jun 22, 2020 8:30 AM 1 month ago in reply to ALirov586

    Hi Andrey,

    The LTC4320 datasheet does mention “Power-over-Ethernet Powered Device with a Secondary Input” application, but nothing that show it as the PoE input bridge. This refers to a non-PoE auxiliary application. Also, in the product webpage of LTC4320, you can see available demo boards and if you've looked at DC2047A-A, it has the LT4321 for Poe and the LT4320 for an auxiliary input which can be AC or DC.

  • On my opinion such description cause misunderstanding.

    The POE is mentioned, however I would expect a note that the bridge does not supports active POE(only passive) and does not support any of POE standards.

  • 0
    •  Analog Employees 
    on Jul 7, 2020 5:47 AM 28 days ago in reply to ALirov586

    Advantages of LT4321 over the competition:
    - Gate drive is not limited 70V but rated up to 100V. Will not require additional transistors/zeners to protect gate drive pins.
    - As a result of having external FETs, ADI solution will have lower FET rdson and dissipate less power at all PoE power levels.
    - ADI solution is true ideal diode controller and will regulate voltage drop across the top FETs, which provides protection against input shorts and hot-plug tolerance.

  • 0
    •  Analog Employees 
    on Jul 8, 2020 3:34 PM 26 days ago in reply to ALirov586

    Hi Audrey,

    The LT4320 does mention a "PoE PD with a secondary input", I can see how that caused confusion. I've included a figure below showing that particular application. You can test this circuit on our DC2047A-A demonstration board.

    Best Regards,