9.2.2.3 Inductor Selection

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. Experience shows that the best value for inductor ripple current is 30% of the maximum load current. Note that when selecting the ripple current for applications with much smaller maximum load than the maximum available from the device, use the the maximum device current. Equation 3 can be used to determine the value of inductance. The constant K is the percentage of inductor current ripple. For this example K = 0.4 was chosen with an inductance 5.1 µH; the standard value of 7.8 µH was selected.

Equation 3.

Ideally, the saturation current rating of the inductor is at least as large as the high-side switch current limit, I_SC. This ensures 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_LIMIT, is designed to reduce the risk of current runaway, a saturated inductor can cause the current to rise to high values very rapidly. This may 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 device low-side current limit, I_LIMIT. In order to avoid sub-harmonic oscillation, the inductance value must not be less than that given in Equation 4:

Equation 4.