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EVM Assembly Drawings and Layout Guidelines

Figure 8. Bottom Side Placement

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 TPS2378.

• 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 TPS2378 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)

SLVU682–March 2012 TPS2378 EVM User’s Guide

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