9.2.1.2.3 Output Capacitor Selection

The output capacitor reduces the high-frequency current ripple through the LED string. Excessive current ripple

increases the RMS current in the LED string, therefore increasing the LED temperature.

1. Calculate the total dynamic resistance of the LED string (RLED) using the LED manufacturer's datasheet.

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 with

the selected inductor.

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.

4.'& =

¿8(

¿+(

× # KB .'&O

(6)

<%176 =

(4.'& + 45'05' ) × +.'&(NELLHA )

+.(NELLHA ) F +.'&(NELLHA )

(7)

%176 =

1

2è × B59 × <%176

(8)

Once the output capacitor is chosen, Equation 9 can be used to estimate the peak-to-peak ripple current through

the LED string.

+.'&(NELLHA ) =

<%176 × +.(NELLHA )

<%176 + 4.'& + 45'05'

(9)

CREE WLED is used here. The dynamic resistance of the LED is 0.29 ohm at 3-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 calculated ripple

current of the LED is about 23.8mA.

9.2.1.2.3.1 Sense Resistor Selection

The maximum LED current is 3 A at 100% PWM duty and the corresponding VREF is 200 mV. By using Equation

9, the sense resistance is calculated as 67 mΩ.

(10)

Note that the power consumption of the sense resistor is 603 mW, requiring enough margin of the resistor's

power rating in selection.

9.2.1.2.3.1.1 Other External Components Selection

In this design, a 0.1-µF, 50-V X7R ceramic capacitor is chosen for CBOOT. Additionally, a 1-µF, 50-V X7R

ceramic capacitor is chosen for CSENSE. For loop stability, it is recommended to select a 1-nF, 10-V X7R ceramic

capacitor for CCOMP and a 1-kΩ resistor for RCOMP.

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