AnsweredAssumed Answered

Question LT3980 PCB layout for PoE project

Question asked by Mars.Chang on Mar 7, 2018
Latest reply on Mar 13, 2018 by EHorsma

Dear sir,

 

Customer has a question, when designing with Linear Tech’s PoE solutions. First of all, customer would like to know if impedance matching is required for Ethernet signals from transformer to RJ45 connector. If not required, are there other requirements for layout?

 

E.g. spacing between differential pair. I know that it is required for sections from PHY to transformer. But if I apply the same rules to the other side of the transformer, it will be very hard to satisfy the current capacity on each trace.

 

E.g. in our application, the power consumption is 60 W so there will be a maximum of 350 mA on each Ethernet signal lines (4 pairs). An online calculator told me 350 mA needs a 0.3 mm wide trace on our PCB but the PCB manufacturer said 100 ohm differential impedance requires 0.09 mm wide traces. Could you give us some suggestions on this?

 

Sincerely,

Outcomes