8.2.1.2.2 Inductor Material Selection

When selecting an inductor, make sure that it is capable of supporting the peak output current without saturating. Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating correctly. To prevent the inductor from saturating over the entire –40°C to +125°C range, pick an inductor with a saturation current higher than the upper limit of I_CL listed in Electrical Characteristics.

Ferrite core inductors are recommended to reduce AC loss and fringing magnetic flux. The drawback of ferrite core inductors is their quick saturation characteristic. The current limit circuit has a propagation delay and so is oftentimes not fast enough to stop a saturated inductor from going above the current limit. This has the potential to damage the internal switch. To prevent a ferrite core inductor from getting into saturation, the inductor saturation current rating should be higher than the switch current limit I_CL. The LMR12015/20 is quite robust in handling short pulses of current that are a few amps above the current limit. Saturation protection is provided by a second current limit which is 30% higher than the cycle-by-cycle current limit. When the saturation protection is triggered thedevice turns off the output switch and attempt to soft start. (When a compromise has to be made, pick an inductor with a saturation current just above the lower limit of the I_CL.) Be sure to validate the short-circuit protection over the intended temperature range.

An inductor's saturation current is usually lower when hot. Consult the inductor vendor if the saturation current rating is only specified at room temperature.

Soft saturation inductors such as the iron powder types can also be used. Such inductors do not saturate suddenly and therefore are safer when there is a severe overload or even shorted output. Their physical sizes are usually smaller than the Ferrite core inductors. The downside is their fringing flux and higher power dissipation due to relatively high AC loss, especially at high frequencies.