The most critical parameters for the inductor are the inductance, saturation current, and the RMS 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 9 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. A reasonable value of K_IND must be 20%-40%. 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 current limit.

Equation 9.

Equation 10.

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 over current protection at the full load can be falsely triggered. It also generates more conduction loss since the RMS current is slightly higher. Larger inductor current ripple also implies larger output voltage ripple with 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 = 0.4, the minimum inductor value is calculated to be 7.64 µH, and a nearest standard value is chosen: 8.2 µH. A standard 8.2-μH ferrite inductor with a capability of 3-A RMS current and 6-A saturation current can be used.