The parameters for selecting the inductor are the inductance and saturation current. The inductance is based on the desired peak-to-peak ripple current and is normally chosen to be in the range of 20% to 40% of the maximum output current capability of the device (example 3-A for LMQ66430). Note that when selecting the ripple current use the maximum device current. Equation 7 can be used to determine the value of inductance. The constant K is the ratio of peak-to-peak inductor current ripple to the maximum device current. For this example, choose K = 0.3 and find an inductance of L = 8.1 µH. Select the standard value of 10 µH.

Ideally, the saturation current rating of the inductor is at least as large as the high-side switch current limit, I_PEAKMAX (see the Electrical Characteristics). This makes sure that the inductor does not saturate, even during a short circuit on the output. When the inductor core material saturates, the inductance falls to a very low value, causing the inductor current to rise very rapidly. Although the valley current limit, I_VALMAX, is designed to reduce the risk of current runaway, a saturated inductor can cause the current to rise to high values very rapidly. This can lead to component damage. Do not allow the inductor to saturate. Inductors with a ferrite core material have very hard saturation characteristics, but usually have lower core losses than powdered iron cores. Powered iron cores exhibit a soft saturation, allowing some relaxation in the current rating of the inductor. However, they have more core losses at frequencies above about 1 MHz. In any case, the inductor saturation current must not be less than the maximum peak inductor current at full load.

The maximum inductance is limited by the minimum current ripple for the current mode control to perform correctly. As a rule-of-thumb, the minimum inductor ripple current must be no less than about 10% of the device maximum rated current under nominal conditions.