The output capacitor is typically selected by the output load transient-response requirement and by allowable output voltage ripple.

• The output capacitor must supply the load with the required current when it is not immediately provided by the regulator. When the output capacitor supplies load current, the impedance of the capacitor affects the magnitude of voltage deviation during the transient.
• The ripple voltage developed across the output capacitor is due to the ripple current in the capacitor and in turn the ripple current is usually due to either the ESR or the value of the capacitor. The ESR of aluminum electrolytics and most tantalums are too high to allow for effective ripple reduction. Often, a combination of several electrolytic capacitors have to be paralleled to obtain large enough value of capacitance with low enough ESR in addition to several ceramic capacitors that offer much lower ESR but at the price of lower capacitance value.

Equation 23 calculates the minimum output capacitance needed to satisfy overvoltage and undervoltage requirements during the load step. In practical design to account for de-rating and variation it is strongly recommended to multiply the calculated capacitance value by a factor between 1.5 and 5 based on the tests in the actual circuit. In this example, two 330-µF polymer capacitors with ESR of 15 mΩ were chosen as well as three 100-µF ceramic capacitor with ESR of 3 mΩ. The total equivalent capacitance C_OUT is 1004 µF. The additional 0.1-µF capacitor (C₃₈ and C₃₉) is used for filtering of high frequency noise.

Equation 23.

where

• I_TRAN(min) is the load transient step
• ∆V_OUT is the output voltage deviation during load transient
• C_OUT(min⁡) is the minimum required capacitance

Using the known target output capacitance value, Equation 24 calculates the maximum ESR allowed to meet the output voltage ripple specification.

Equation 24.