ZHCSRQ4 august 2023 TPSM8S6C24
PRODUCTION DATA
The target maximum output-voltage ripple is 10 mV. Under this requirement, the minimum output capacitance for ripple is 2 x 148 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 must 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, 4 x 330-μF low-ESR tantalum polymer bulk capacitors and 18 x 100-μF ceramic capacitors were selected to meet the transient specification with sufficient margin. The selected nominal COUT is equal to 3120 μF. The 330-μF capacitors selected have an ESR of 6 mΩ.
With the output capacitance value selected, the ESR must be considered because this example 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 16indicates the output capacitor bank impedance must be less than 0.65 mΩ. The impedance of the ceramic capacitors alone is calculated with Equation 16 to be 0.125 mΩ. This is much less than the calculated maximum, so the ESR of tantalum polymer capacitors does not must be considered for the output ripple specification.