ZHCSLC6B May 2020 – January 2022 TPS92200
PRODUCTION DATA
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 also increases.
1. Calculate the total dynamic resistance of the LED string (RLED) using the LED manufacturer's data sheet.
2. Calculate the required impedance of the output capacitor (ZOUT) given the acceptable peak-to-peak ripple current through the LED string, ILED(ripple) × IL(ripple), is the peak-to-peak inductor ripple current as calculated previously in inductor selection.
3. Calculate the minimum effective output capacitance required.
4. Increase the output capacitance appropriately due to the derating effect of applied dc voltage.
See Equation 6, Equation 7, and Equation 8.
After the output capacitor is chosen, Equation 9 can be used to estimate the peak-to-peak ripple current through the LED string.
OSRAM SFH4715A IR LED is used here. The dynamic resistance of this LED is 0.29 ohm at 1.5-A forward current. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR and small temperature coefficients. In this design, a 10-µF, 35-V X7R ceramic capacitor is chosen, the part number is GRM32ER7YA106KA12L. The calculated ripple current of the LED is about 23.8 mA.