SNVSB79A November 2018 – July 2019 LM25180
The output capacitor determines the voltage ripple at the converter output, limits the voltage excursion during a load transient, and sets the dominant pole of the converter's small-signal response. For a flyback converter specifically, the output capacitor supplies the load current when the main switch is on, and therefore the output voltage ripple is a function of load current and duty cycle.
Select an output capacitance using Equation 22 to limit the ripple voltage amplitude to less than 1% of the output voltage at minimum input voltage.
Substituting the maximum load current at minimum input voltage from Equation 18, transformer inductance, peak switch current and peak-to-peak ripple voltage specification gives COUT greater than 72 μF.
Mindful of the voltage coefficient of ceramic capacitors, select a 100-µF, 6.3-V capacitor in 1210 case size with X5R or better dielectric. Equation 23 gives the output capacitor RMS ripple current.