Use Equation 10 and Equation 11 to calculate the inductor ripple current (assuming CCM operation) and peak inductor current, respectively.

Equation 10.

Equation 11.

For most applications, choose an inductance such that the inductor ripple current, ΔI_L, is between 30% and 50% of the rated load current at nominal input voltage. Use Equation 12 to calculate the inductance.

Equation 12.

For applications in which the device must support input transients exceeding 72 V, select the inductor to be at least 22 μH, which ensures that excessive current rise does not occur in the power stage due to the potential large inductor current slew that can occur in an output short-circuit condition.

Choosing a 22-μH inductor in this design results in 1.36-A peak-to-peak ripple current at a nominal input voltage of 48 V, equivalent to 39% of the 3.5-A rated load current. For designs which must operate up to the maximum input voltage at the full-rated load current of 3.5 A, the inductance needs to increase to ensure current limit (I_PEAK current limit) is not hit.

Check the inductor data sheet to make sure the saturation current of the inductor is well above the current limit setting of the LM5013. TI recommends a saturation current greater than 7 A. Ferrite-core inductors have relatively lower core losses and are preferred at high switching frequencies, but exhibit a hard saturation characteristic – the inductance collapses abruptly when the saturation current is exceeded. This collapse results in an abrupt increase in inductor ripple current, higher output voltage ripple, and reduced efficiency, and in turn compromising reliability. Note that inductor saturation current levels generally decrease as the core temperature increases.