The output capacitor is responsible for filtering the inductor current, and supplying load current during transients. Capacitor selection depends on application conditions as well as ripple and transient requirements. Best performance is achieved by using ceramic capacitors or combinations of ceramic and other types of capacitors. For high output voltage conditions, such as 12 V and above, finding ceramic capacitors that are rated for an appropriate voltage becomes challenging. In such cases choose a low-ESR capacitor. It is a good idea to use a low-value ceramic capacitor in parallel with other capacitors, to bypass high frequency noise between ground and V_OUT.

For a given input and output requirement, Equation 28 gives an approximation for a minimum output capacitor required.

Equation 28.

where

• r = Ripple ratio of the inductor ripple current (I_Lripple / 6 A )
• ΔV_OUT = Target output voltage undershoot, for example, 5% to 10% of V_OUT
• D’ = 1 – duty cycle
• f_SW = switching frequency
• I_OUT = load current

Along with Equation 28, for the same requirement calculate the maximum ESR with Equation 29.

Equation 29.

The output capacitor is also the dominating factor in the loop response of a peak-current mode controlled buck converter. A simplified estimation of the control loop crossover frequency can be found by Equation 18.

Select C_OUT so that the f_X is no higher than 1/6 of the switching frequency. Typically, f_X / f_SW = 1/10 to 1/8 provides a good combination of stability and performance.

For this design, one 0.47-µF, 50-V X7R and four 22-µF, 16-V, X7R ceramic capacitors are used in parallel based on a target output voltage overshoot value of 10%.