There are three considerations for selecting the value of the output capacitor:

1. Stability
2. Steady state output voltage ripple
3. Regulator transient response to a change load current

First, the minimum output capacitance should be calculated based on these three requirements. Equation 17 calculates the minimum capacitance to keep the LC double pole below 1/30th the f_SW in order to meet stability requirements. This requirement helps keep the LC double pole close to the internal zero. Equation 18 calculates the minimum capacitance to meet the steady state output voltage ripple requirement of 10 mV. This calculation is for CCM operation and does not include the portion of the output voltage ripple caused by the ESR or ESL of the output capacitors.

Equation 19 and Equation 20 calculate the minimum capacitance to meet the transient response requirement of 50mV with a 10-A step. These equations calculate the necessary output capacitance to hold the output voltage steady while the inductor current ramps up or ramps down after a load step.

The output capacitance needed to meet the overshoot requirement is the highest value so this sets the required minimum output capacitance for this example. Stability requirements can also limit the maximum output capacitance and Equation 21 calculates the recommended maximum output capacitance. This calculation keeps the LC double pole above 1/100th the f_SW. It is possible to use more output capacitance but the stability must be checked through a bode plot or transient response measurement. The selected output capacitance is 4 × 100-µF 10-V ceramic capacitors. When using ceramic capacitors, the capacitance must be derated due to DC and AC bias effects. The selected capacitors derate to 85% their nominal value giving an effective total capacitance of 340μF. This effective capacitance meets the minimum and maximum requirements.

This application uses all ceramic capacitors so the effects of ESR on the ripple and transient were ignored. If using non-ceramic capacitors, as a starting point, the ESR should be below the values calculated in Equation 22 to meet the ripple requirement and Equation 23 to meet the transient requirement. For more accurate calculations or if using mixed output capacitors, the impedance of the output capacitors should be used to determine if the ripple and transient requirements can be met.