The designer needs to account for three primary considerations when selecting the value of the output capacitor. The output capacitor determines the modulator pole, the output voltage ripple, and how the regulator responds to a large change in load current. The output capacitance must be selected based on the most stringent of these three criteria.

The first criterion is the desired response to a large change in the load current. The output capacitor needs to supply the load with current when the regulator cannot. This situation can occur if there are desired hold-up times for the regulator where the output capacitor must hold the output voltage above a certain level for a specified amount of time after the input power is removed. The regulator is also temporarily not able to supply sufficient output current if there is a large, fast increase in the current needs of the load such as a transition from no load to full load. The regulator usually needs two or more clock cycles for the control loop to see the change in load current and output voltage and adjust the duty cycle to react to the change. The output capacitor must be sized to supply the extra current to the load until the control loop responds to the load change. The output capacitance must be large enough to supply the difference in current for two clock cycles while only allowing a tolerable amount of droop in the output voltage. Equation 10 shows the minimum output capacitance necessary to accomplish this.

Equation 10.

where

• ΔI_out is the change in output current.
• ƒ_sw is the regulator's switching frequency.
• ΔV_out is the allowable change in the output voltage.

Equation 11 calculates the minimum output capacitance needed to meet the output voltage ripple specification.

Equation 11.

where

• ƒ_sw is the switching frequency.
• V_oripple is the maximum allowable output voltage ripple.
• I_oripple is the inductor ripple current.

Equation 12 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple specification.

Equation 12.

Additional capacitance deratings for aging, temperature, and DC bias must be factored in, which increase this minimum value. Capacitors generally have limits to the amount of ripple current they can handle without failing or producing excess heat. An output capacitor that can support the inductor ripple current must be specified. Some capacitor data sheets specify the root mean square (RMS) value of the maximum ripple current. Equation 13 can be used to calculate the RMS ripple current the output capacitor needs to support.

Equation 13.