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POE with LTC4267-1

Hi all

I have a problem with LTC4267-1. I try to use it for an application but when I connect it to the switch the output voltage instead of being 12V as foreseen by the diagram oscillates between 12V and 0V

I attach the schematics


Thanks for help!


  • Hi Andrea,

    Can you try switching out the MOC211M to a part with a specified minimum and maximum CTR? MOC207M would be a better choice with a CTR between 100% and 200%. You might want to try bumping up C314 to 22nF or 33nF. You may also want to confirm the PVCC voltage is steady and above 7V but less than 11V.

    Best Regards,


  • Hi Eric 

    Thanks for your reply.


    Unfortunately i can't find the MOC211M ...
    During this time I redesigned the board and set the output voltage to 5V.
    I have some improvements there but the circuit still doesn't work.
    In input I have the correct voltage coming from the ethernet switch (the switch LED indicates a connected POE device) but PVCC is equal to 0.6V and the Q1 mosfet is short-circuited DRAIN and SOURCE.
    I have already tried to replace LTC4267, Si3440DV and PA1137 but as soon as I connect to the switch the situation reappears.

    PS I find the same behavior if I change the transformer with a PA1138 and if I change the resistors R9 and R10 to have an output voltage of 12V

    thanks for yours help