SLUSCC7C July 2016 – June 2018 TPS546C23
The output-voltage ripple is the second criterion for output capacitor selection. Use Equation 23 to calculate the minimum output capacitance required to meet the output-voltage ripple specification.
In this case, the target maximum output-voltage ripple is 12 mV. Under this requirement, the minimum output capacitance for ripple is 330 µF. Because this capacitance value is smaller than the output capacitance required for the transient response, select the output capacitance value based on the transient requirement. Considering the variation and derating of capacitance, in this design, two 470-µF low-ESR polymer bulk capacitors and four 47-µF ceramic capacitors were selected to meet the transient specification with sufficient margin. Therefore COUT is equal to 1128 µF.
With the target output capacitance value selected, use Equation 24 to calculate the maximum ESR that the output-capacitor bank allows to meet the output-voltage ripple specification. Equation 24 indicates the ESR should be less than 1.3 mΩ. Each 470-µF ceramic capacitor contributes approximately 1.3 mΩ, making the effective ESR of the output capacitor bank approximately 0.65 mΩ which is within the specification with sufficient margin.