The output capacitor reduces the high-frequency ripple current through the LED string. Various guidelines disclose how much high-frequency ripple current is acceptable in the LED string. Excessive ripple current in the LED string increases the rms current in the LED string, and therefore the LED temperature increases.
Once the output capacitor is chosen, Equation 12 can be used to estimate the peak-to-peak ripple current through the LED string.
An OSRAM IR LED, SFH4715A, is used here. The dynamic resistance of this LED is 0.25 Ω at 1.5-A forward current. In this design, a 10-µF, 35-V X7R ceramic capacitor is chosen, the part number is GRM32ER7YA106KA12L, from muRata. The calculated ripple current of the LED is about 20 mA.