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 of the device. A reasonable value of K_IND must be 20% to 60% of maximum I_OUT supported by converter. During an instantaneous overcurrent operation event, the RMS and peak inductor current can be high. The inductor saturation current must be higher than peak current limit level.

Equation 8.

Equation 9.

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 inductor core loss since the current ripple is larger. Larger inductor current ripple also implies larger output voltage ripple with the same output capacitors. With peak current mode control, it is recommended to have adequate amount of inductor ripple current. A larger inductor ripple current improves the comparator signal-to-noise ratio.

For this design example, choose K_IND = 0.4. The minimum inductor value is calculated to be 15.36 µH. Choose the nearest standard 15-µH ferrite inductor with a capability of 1.5-A RMS current and 2.5-A saturation current.