SLUSEJ1 December 2021 TPSM8D6C24
The target maximum output-voltage ripple is 20 mV. Under this requirement, the minimum output capacitance for ripple is 110 µF. Depending on the duty cycle and the number of phases, there can also be some inductor ripple current cancellation. This reduces the amount of ripple current the capacitors need to absorb, reducing the output voltage ripple. This capacitance value is smaller than the output capacitance required for the transient response, so select the output capacitance value based on the transient requirement. Considering the variation and derating of capacitance, in this design, four 470-µF low-ESR tantalum polymer bulk capacitors and twenty-six 47-µF ceramic capacitors were selected to meet the transient specification with sufficient margin. The selected nominal COUT is equal to 3102 µF. The 470-µF capacitors selected have an ESR of 10 mΩ.
With the output capacitance value selected, the ESR must be considered because this example uses mixed output capacitor types. First, use Equation 16 to calculate the maximum allowable impedance for the output capacitor bank at the switching frequency to meet the output voltage ripple specification. Equation 16 indicates the output capacitor bank impedance should be less than 2.1 mΩ. The impedance of the ceramic capacitors alone is calculated with Equation 17 to be 0.2 mΩ. This is much less than the calculated maximum, so the ESR of tantalum polymer capacitors does not need to be considered for the output ripple specification.