8.2.1.2.4 Inductor Selection

The inductor is selected to limit the inductor ripple current to a value between 20 and 40 percent of the maximum load current. The minimum value of the inductor required in this application can be calculated from Equation 13:

Equation 13.

Based on Equation 13 , the minimum value of the inductor is calculated to be 85 µH for V_IN = 80-V (max) and inductor current ripple will be 40 percent of the maximum load current. Allowing some margin for inductance variation and inductor saturation, a higher standard value of L1 (L) = 100 µH is selected for this design.

The peak inductor current at maximum load must be smaller than the minimum current limit threshold of the high side FET as given inElectrical Characteristics table. The inductor current ripple at any input voltage is given by:

Equation 14.

The peak-to-peak inductor current ripple is calculated to be 81 mA and 341 mA at the minimum and maximum input voltages respectively. The maximum peak inductor current in the buck FET is given by Equation 15:

Equation 15.

In this design with maximum output current of 1-A, the maximum peak inductor current is calculated to be approximately 1.17 A at V_IN,max = 80 V, which is less than the minimum high-side FET current limit threshold.

The saturation current of the inductor must also be carefully considered. The peak value of the inductor current will be bound by the high side FET current limit during overload or short circuit conditions. Based on the high side FET current limit specification in theElectrical Characteristics, an inductor with saturation current rating above 1.9 A (max) should be selected.