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LM5071: POE feedback optoisolator fails or disconnected, POE output will output 8V triangular wave.

Tuan Tran83
Prodigy 30 points
Part Number: LM5071
Other Parts Discussed in Thread: TPS23755, TPS23753A, TPS23753AEVM-004

Hi

I have a FPGA device that is directly powered from the POE 3.3V output. We are currently using the LM5071 chip. When the feedback optoisolator fails or disconnected, the POE output would

be a 8volt triangular volt. This is above the max absolute of the FPGA and will completely destroy it. Is the any changes to the LM5071 evaluation circuit possible that would output 0v if the feed optoisolator fails or the feedback circuit fail?

Thanks

Tony

  • 0
    TI__Expert 5615 points
    Tony, I am going to assign this thread to our PoE Application Engineer - Darwin Fernandez. He will look into it and advise what can be done.

    From a product portfolio perspective, I want to encourage you to consider moving away from the LM5071 on your next design. This is an older device - which was inherited during TI's acquisition of National - and does not have the same level of technical support (and reference design choices) that our other "TPS" solutions enjoy. I would propose that you look into the TPS23753A or TPS23755 for your next design instead. Both deliver better value (price/performance options) and support.

    Please let me know if having any questions about the product portfolio and I will let Darwin advise on your current technical challenge.

    Thank you for considering PoE solutions from Texas Instruments,
  • 0 in reply to Thomas Lewis
    TI__Mastermind 41790 points

    Hi Tony,

    Can you help clarify how the opto is not operating correctly? Is it failing by a certain operating condition or in normal operation? By disconnect do you mean it's being physically removed?

    For overshoots, it'll typically occur during startup in which you can increase the softstart on the secondary side. This is the circuit near the opto but related to the reference on the secondary.

    It can also occur during a transient (line or load); this is where the opto can affect the stability of the converter and how quick the loop can react to the transient. If this is the case, you may need to recompensate your loop.

    In the end however, I recommend what Thomas has pointed out; we much newer TI designs in our TPS products than our LM parts. For example, we have the TPS23753AEVM-004 which can be orderable to evaluate at the TI store.

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    Prodigy 30 points

    Hi Darwin

    In this situation, the optoisolator is non functional or has a bad solder joint. Is there any way to force the POE output to 0  volts  in such cases when the optoisolator LED/transistor is not functional.

    Thanks

    Tuan

  • 0 in reply to Tuan Tran83
    TI__Mastermind 41790 points
    Hi Tony,

    Unfortunately I don't see a way as is. The best thing you can try to do is add a TVS at the output to clamp the voltage. If it damages short you may be able to protect the downstream load but this failure mode is not guaranteed. Unfortunately, it might be where you will need to have the boards screened to find the ones with poor connections.