ZHCSRU8 February 2024
PRODUCTION DATA
The output-voltage ripple is the second criterion for output capacitor selection. Use Equation 14 to calculate the minimum output capacitance required to meet the output-voltage ripple specification.
In this case, the target maximum output-voltage ripple is 10mV. Under this requirement, the minimum output capacitance for ripple is 165µF. 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, 2 × 330µF low-ESR tantalum polymer bulk capacitors and 9 × 100µF ceramic capacitors were selected to meet the transient specification with sufficient margin. Therefore the selected nominal COUT is equal to 1560µF.
With the output capacitance value selected, the ESR must be considered. This consideration is important in this example because this consideration uses mixed output capacitor types. First use Equation 15 to calculate the maximum allowable impedance for the output capacitor bank at the switching frequency to meet the output voltage ripple specification. Equation 15 indicates the output capacitor bank impedance must be less than 1.3mΩ. The impedance of the ceramic capacitors is calculated with Equation 16 and the impedance of the bulk capacitor is calculated with Equation 17. The result from Equation 17 shows the impedance of the bulk capacitor at the switching frequency is dominated by the ESR. Equation 18 calculates the total output impedance of the output capacitor bank at the switching frequency to be 0.25mΩ which meets the 1.3mΩ requirement.