9.2.1.2.1 Inductor Selection

Use Equation 3 to calculate the minimum value of the output inductor (L_MIN).

Equation 3.

where

• K_IND is a coefficient that represents the amount of inductor ripple current relative to the maximum LED current.
• I_LED is the maximum LED current.
• V_OUT is the sum of the voltage across LED load and the voltage across the sense resistor.

In general, the suggested value of K_IND is between 0.2 and 0.4. For an application that can tolerate higher LED current ripple or use larger output capacitors, one can choose 0.4 for K_IND. Otherwise, a smaller K_IND like 0.2 can be chosen to get low-enough LED current ripple.

With the chosen inductor value the user can calculate the actual inductor current ripple using Equation 4.

Equation 4.

The inductor rms-current and saturation-current ratings must be greater than the rms current and saturation current seen in the application. This ensures that the inductor does not overheat or saturate. During power up, transient conditions, or fault conditions, the inductor current can exceed its normal operating current. For this reason, the most conservative approach is to specify an inductor with a saturation current rating equal to or greater than the converter current limit. This is not always possible due to application size limitations. The peak-inductor-current and rms-current equations are shown in Equation 5 and Equation 6.

Equation 5.

Equation 6.

In this design, choose K_IND = 0.3. According to the LED manufacturer’s data sheet, the IR LED has 1.75-V forward voltage at 1.5-A current, so V_OUT = 1.75 V × 3 + 0.2 V = 5.45 V and the calculated inductance is 11.9 µH. A 10-µH inductor (part number is 744066100 from Wurth) is chosen. With this inductor, the ripple, peak, and rms currents of the inductor are 0.53 A, 1.77 A, and 1.51 A, respectively. The chosen inductor has ample margin.