The output capacitance depends on the output ripple voltage requirement, output ripple current, as well as any output voltage deviation requirement during a load transient.

The output ripple voltage is a function of both the output capacitance and capacitor ESR. The worst-case output ripple is described in Equation 25.

Equation 25.

where

• C_O is the output capacitance.
• ESR is the equivalent series resistance of the output capacitance.

The output ripple voltage is typically between 90% and 95% due to the ESR component.

The output capacitance requirement typically increases in the presence of a load transient requirement. During a step load, the output capacitance must provide energy to the load (light to heavy load step) or absorb excess inductor energy (heavy to light load step) while maintaining the output voltage within acceptable limits. The amount of capacitance depends on the magnitude of the load step, the speed of the loop, and the size of the inductor.

Stepping the load from a heavy load to a light load results in an output overshoot. Excess energy stored in the inductor must be absorbed by the output capacitance. The energy stored in the inductor is described in Equation 26.

Equation 26.

where

Equation 27.

• I_OH is the output current under heavy load conditions.
• I_OL is the output current under light load conditions.

Energy in the capacitor is described in Equation 28.

Equation 28.

where

Equation 29.

where

• V_f is the final peak capacitor voltage.
• V_i is the initial capacitor voltage.

Substituting Equation 27 into Equation 26, then substituting Equation 29 into Equation 28, then setting Equation 28 equal to Equation 26, and then solving for C_O yields the capacitance described in Equation 30.

Equation 30.