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DP83848C: DP83848C PHY input ratings

Rob Strange
Prodigy 140 points
Part Number: DP83848C

Hi,

We have a DP83848C PHY which is being destroyed by Power-over-Ethernet transients.

I am adding TVS protection to the PHY side of the magnetics.  For now I'm adding 2 back-back 3V diodes as per some recommendations I've found on the web, but I'm not convinced this is the correct solution.

I'm puzzled by the datasheet absolute maximum ratings for this chip, as they claim to be -0.5V to Vcc+0.5V - fair enough for the logic signals, but the magnetics interface is differential about Vcc, so I don't understand the Vcc+0.5 limit and wonder if the ratings for the magnetics interface are different.  (e.g. I found an SMSC PHY with a 6V rating).

I'd appreciate any assistance in clarifying what the limits are and whether there are any recommended TVS diode configurations to prevent damage.

Thanks,

Rob

  • 0
    TI__Mastermind 18425 points
    Hi Rob,

    We have a reference design for DP83848 POE: www.ti.com/.../snoa476c.pdf It is for a different variant of DP83848 but it will still be applicable. Regarding the Abs Max in the datasheet, they are the DC limit for the pins. Can you share the TVS protection diode part number that you are planning to use?

    -Regards,
    Aniruddha
  • 0 in reply to Aniruddha Khadye
    Prodigy 140 points

    Thanks for the reply.

    During out investigation of PHY failure, one of the things that puzzled us was that the reference design you pointed to does not have TVS diodes in it to protect the PHY.  It was through following designs such as this that we ended up with our failing design.

    (The one big difference between that reference design and ours is that our customer made the PoE PSU non-isolating, which I think may be making things worse.)

    We were thinking of using uClamp3301D to start with, per this microchip app note:

    http://ww1.microchip.com/downloads/en/AppNotes/en562768.pdf

    We were hoping Texas Instruments might have a similar recommendation.

    Best regards,

    Rob