The most critical parameters for the inductor are the inductance, saturation current, and the rated current. The inductance is based on the desired peak-to-peak ripple current, Δi_L. Since the ripple current increases with the input voltage, the maximum input voltage is always used to calculate the minimum inductance L_MIN. Use Equation 12 to calculate the minimum value of the output inductor. K_IND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current of the device. A reasonable value of K_IND should be 20% to 40%. Note that when selecting the ripple current for applications with much smaller maximum load than the maximum available from the device, the maximum device current must still be used. During an instantaneous short or overcurrent operation event, the RMS and peak inductor current can be high. The inductor current rating must be higher than the current limit of the device.

Equation 11.

Equation 12.

In general, it is preferable to choose lower inductance in switching power supplies because it usually corresponds to faster transient response, smaller DCR, and reduced size for more compact designs. Too low of an inductance can generate too large of an inductor current ripple such that overcurrent protection at the full load can be falsely triggered. It also generates more conduction loss and inductor core loss. Larger inductor current ripple also implies larger output voltage ripple with the same output capacitors. With peak current mode control, it is not recommended to have too small of an inductor current ripple. A larger peak current ripple improves the comparator signal to noise ratio.

For this design example, choose K_IND = 0.3, and find an inductance of approximately 3.58 µH. Select the next standard value of 3.3 μH.