10.1.1.5 Capacitor Choice

Be aware of the voltage coefficient of the decoupling capacitors. Physically undersized ceramic capacitors (with respect to the capacitance value to physical size ratio) experiences a large reduction in capacitance. Depending on the VIN voltage chosen and the voltage rating of the capacitors, physically undersized capacitors can experience up to a 90% reduction in capacitance, leading to insufficient decoupling.

Choose decoupling capacitors (C17 and C119) with a value of 0.1 µF and a size of 0402. Place the decoupling capacitors as close as possible to the power block VIN pins. Even an increased distance as small as 1 mm in compared to the reference design can cause a large increase in voltage ringing amplitude. Because the parasitic inductance of the route combined with the effective inductance of the capacitor affects the switching node, select capacitors with a higher SRF rating. Place the VIN decoupling capacitors on the top layer very close to the power blocks to minimize parasitic inductance. Place the 10-µF decoupling capacitors (C18, C114, C115, C116, C117, and C118) near the power blocks, They do not need to be as close as the 0.1-µF capacitors.