The value of the output capacitor in a buck regulator influences the steady state voltage ripple as well as the output voltage response to a load transient. Given the peak-to-peak inductor current ripple (ΔI_L), the output voltage ripple can be approximated by:

Equation 7.

where

• ΔV_OUT (V) is the amount of peak-to-peak voltage ripple at the power supply output.
• R_ESR (Ω) is the series resistance of the output capacitor.
• f_SW (Hz) is the switching frequency.
• C_OUT (F) is the output capacitance used in the design and is the sum of C_o1 through C_o4.

For the evaluation board, four 100-µF 6.3-V X5R ceramic capacitors were selected for the output capacitance to provide adequate transient and DC bias performance in a relatively small package. From the technical specifications of this capacitor, the ESR is approximately 3 mΩ and the effective in-circuit capacitance is approximately 60 µF (reduced from 100 µF due to the 1.5-V DC bias and worst case tolerance). With these values, the peak-to-peak voltage ripple when operating from a VIN of 12 V is:

Equation 8.