EVM Assembly Drawings and Layout Guidelines
6.2
Layout Guidelines
The layout of the PoE front end should follow power and EMI/ESD best practice guidelines. A basic set of
recommendations include:
•
Parts placement must be driven by power flow in a point-to-point manner; RJ-45, Ethernet transformer,
diode bridges, TVS and 0.1-µF capacitor, and TPS2379.
•
All leads should be as short as possible with wide power traces and paired signal and return.
•
There should not be any crossovers of signals from one part of the flow to another.
•
Spacing consistent with safety standards like IEC60950 must be observed between the 48-V input
voltage rails and between the input and an isolated converter output.
•
The TPS2379 should be located over split, local ground planes referenced to VSS for the PoE input
and to RTN for the switched output.
•
Large copper fills and traces should be used on SMT power-dissipating devices, and wide traces or
overlay copper fills should be used in the power path.
6.3
EMI Containment
•
Use compact loops for dv/dt and di/dt circuit paths (power loops and gate drives)
•
Use minimal, yet thermally adequate, copper areas for heat sinking of components tied to switching
nodes (minimize exposed radiating surface).
•
Use copper ground planes (possible stitching) and top layer copper floods (surround circuitry with
ground floods)
•
Use 4 layer PCB if economically feasible (for better grounding)
•
Minimize the amount of copper area associated with input traces (to minimize radiated pickup)
•
Use Bob Smith terminations, Bob Smith EFT capacitor, and Bob Smith plane
•
Use Bob Smith plane as ground shield on input side of PCB (creating a phantom or literal earth
ground)
•
Use of ferrite beads on input (allow for possible use of beads or 0 ohm resistors)
•
Maintain physical separation between input-related circuitry and power circuitry (use ferrite beads as
boundary line)
•
Possible use of common-mode inductors
•
Possible use of integrated RJ-45 jacks (shielded with internal transformer and Bob Smith terminations)
•
End-product enclosure considerations (shielding)
10
TPS2379 EVM User’s Guide
SLVU687 – March 2012
Copyright © 2012, Texas Instruments Incorporated
