To calculate the value of the output inductor, use Equation 23. K_L is a coefficient that represents the amount of inductor ripple current relative to the maximum output current, I_O as shown in Equation 14. The inductor ripple current is filtered by the output capacitor, therefore, choosing high inductor ripple currents impact the selection of the output capacitor since the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. In general, the inductor ripple value is at the discretion of the designer depending on specific system needs. Typical values for K_L range from 0.1 to 0.5. For low output currents, the value of K_L could be increased to reduce the value of the output inductor.

Equation 23.

For this design example, use K_L = 0.1 and the inductor value is calculated to be 0.9 µH for nominal VIN = 5 V.

For the output filter inductor, it is important that the RMS current and saturation current ratings not be exceeded. The RMS and peak inductor current can be found from Equation 25 and Equation 26.

Equation 24.

Equation 25.

Equation 26.

For this design, the RMS inductor current is 18 A and the peak inductor current is 18.9 A. To satisfy this requirement, two Coilcraft SER1360 inductors are used in parallel. These inductors have a saturation current rating of 17 A and a RMS current rating of 9.5 A.

The current flowing through the inductor is the inductor ripple current plus the output current. During power up, faults or transient load conditions, the inductor current can increase above the calculated peak inductor current level calculated previously. In transient conditions, the inductor current can increase up to the switch current limit of the device. For this reason, the most conservative approach is to specify an inductor with a saturation current rating equal to or greater than the switch current limit rather than the peak inductor current.