5 Layout Fundamentals

Good layout for DC-DC converters can be implemented by following a few simple design guidelines:

1. Place the power components (catch diode, inductor, and filter capacitors) close together. Make the traces between them short.
2. Use wide traces between the power components and for power connections to the DC-DC converter circuit.
3. Connect the ground pins of the input and output filter capacitors and catch diode as close as possible using generous component-side copper fill as a pseudo-ground plane. Then, connect this to the ground-plane with several vias.
4. Arrange the power components so that the switching current loops curl in the same direction.
5. Route high-frequency power and ground return as direct continuous parallel paths.
6. Separate noise sensitive traces, such as the voltage feedback path, from noisy traces associated with the power components.
7. Ensure a good low-impedance ground for the converter IC.
8. Place the supporting components for the converter IC, such as compensation, frequency selection and charge-pump components, as close to the converter IC as possible but away from noisy traces and the power components. Make their connections to the converter IC and its pseudo-ground plane as short as possible.
9. Place noise sensitive circuitry, such as radio-modem IF blocks, away from the DC-DC converter, CMOS digital blocks, and other noisy circuitry.

Figure 5-1 LM3477 Evaluation Board PCB Layout (Top Side)

Figure 5-2 LM3477 Evaluation Board PCB Layout (Bottom Side)