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Ethernet Magnetics BoB smith PoE termination

Devendhiran K
Prodigy 60 points
Other Parts Discussed in Thread: DP83620

Hi, I am using DP83620SQ/NOPB PHY for our 10/100Mbps Copper Media Ethernet Interface. We had connected through  the Magnetics part "749013011A" which is not PoE supported. But we need to use this for powering external device without following the Standard PoE process (simply appending power with the data signals). 

For PoE interface, the legacy (general) bob smith termination would not be suit. Hence 10nF capacitor is added along with the 75 ohm termination resistor. To increase the power handling capacity through the bob smith termination during surge condition, we are purposely added one more 10nF+75 ohm termination resistor combination along with the existing each 10nF+75 ohm termination combination.

Will this scheme affect the cable characteristic impedance matching scheme?

I had given the corresponding magnetics interface sheet for reference.  Since we are closer to the LAYOUT release,  expecting reply at very earliest.

Thanks,

Devendhiran 

Magnetics-RJ45 Interface.pdf

  • 0
    TI__Mastermind 23675 points

    Hi Devendhiran,

    The 749013011A datasheet states that it is PoE compliant. Above you said it is not though. Did you have a typo? What you are describing is PoE Alternative A and not Alternative B.

    Please note: In your schematic pin #2 and #7 do not show connection to 3.3V supply rail. This is required for the DP83620 PHY. I see no net label, which makes me believe that there is no 3.3V supply there.

    Yes, adding 10nF before the 75 ohm resistors is ok. 

    It is common practice to include Bob Smith termination to minimize emissions, however there are conflicting reports in the necessity of including the termination or omitting it. I would keep the termination as you show, but when doing emissions tests try with and without because you might find that the termination is not even needed, which will save you on your BOM cost.

    Kind regards,

    Ross

  • 0 in reply to Ross Pimentel
    Prodigy 60 points
    Thanks ross for your reply.

    That center tap supply have to be provided as you said.

    My very big concern is whether it is advisable to keep two no.of 75 ohm+10nF combination in parallel to each other for a single center tap pin?.

    Can you give a comment over it?

    Thanks

    Devendhiran
  • 0 in reply to Devendhiran K
    TI__Mastermind 23675 points

    Hi Devendhiran,

    I have not seen the type of proposed termination you have currently. What is typically used are three series components. A 22nf cap with a 75 ohm resistor with a 1nf cap.

    Kind regards,

    Ross