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. Note that when selecting the ripple current for applications with much smaller maximum load than the maximum available from the device, use the maximum device current. Equation 8 can be used to determine the value of inductance. The constant K is the percentage of inductor current ripple. For this example a 5.6μH inductor is used.

Ideally, the saturation current rating of the inductor is at least as large as the high-side switch current limit, I_HS-LIM (see Section 6.5). This size 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_LS-LIM, is designed to reduce the risk of current runaway, a saturated inductor can cause the current to rise to high values very rapidly. This action 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, powered iron cores have more core losses at frequencies above about 1MHz. In any case, the inductor saturation current must not be less than the maximum peak inductor current at full load.

The minimum inductance value to avoid subharmoic oscillations can be found using Equation 9:

Where:

• M = 0.51