Layout is a critical portion of good power supply
design. The following guidelines help users design a PCB with the best power
conversion performance, thermal performance, and minimized generation of unwanted
EMI.
- Place the input bypass capacitor
CIN as close as possible to the VIN and GND pins. Grounding for
both the input and output capacitors must consist of localized top side planes
that connect to the GND pin.
- Minimize trace length to the FB
pin net. Both feedback resistors, RFBT and RFBB, must be
located close to the FB pin. If VOUT accuracy at the load is
important, make sure VOUT sense is made at the load. Route
VOUT sense path away from noisy nodes and preferably through a
layer on the other side of a shielded layer.
- Use ground plane in one of the
middle layers as noise shielding and heat dissipation path if possible.
- Make VIN,
VOUT, and ground bus connections as wide as possible. This action
reduces any voltage drops on the input or output paths of the converter and
maximizes efficiency.
- Provide adequate device
heat-sinking. GND, VIN, and SW pins provide the main heat dissipation path, make
the GND, VIN, and SW plane area as large as possible. Use an array of
heat-sinking vias to connect the top side ground plane to the ground plane on
the bottom PCB layer. If the PCB has multiple copper layers, these thermal vias
can also be connected to inner layer heat-spreading ground planes. Make sure
enough copper area is used for heat-sinking to keep the junction temperature
below 125°C.