Layout is a critical portion of good power supply design. See Figure 7-19 for a PCB layout example. Key guidelines to follow for the layout are:
- Make VIN, PGND, and SW traces as wide as possible
to reduce trace impedance and improve heat
dissipation.
- Place a 10nF to 100nF capacitor at the VIN pin to PGND as close as possible to
the device on the same side of the PCB. Place the
remaining ceramic input capacitance next to these
high frequency bypass capacitors. The remaining
input capacitance can be placed on the other side
of the board but use as many vias as possible to
minimize impedance between the capacitors and the
pins of the IC.
- Use multiple vias near the PGND pins and use the layer directly below the
device to connect them together. This action helps
to minimize noise and can help heat
dissipation.
- Use vias near the VIN pin and provide a low impedance connection between them
through an internal layer.
- Place the BP5 capacitor as close as possible to the BP5 and AGND pins.
- Place the bottom resistor in the FB divider as close as possible to the FB and
AGND pins of the IC. Also keep the upper feedback resistor and the feedforward capacitor
near the IC. Connect the FB divider to the output voltage at the desired point of
regulation.
- Use multiple vias in the AGND island to connect back to internal PGND layers.
Do not place these vias between the BP5 capacitor and the AGND pin. These
vias conduct switching currents between the BP5 capacitor and PGND. Placing
the vias near the AGND pin can add noise to the FB divider.
- Return the FSEL and MODE resistors to a quiet AGND island.