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 for systems with a fixed input voltage and 25% for systems with a variable input voltage such as the 12 volt battery in a car. Note that when selecting the ripple current for applications with much smaller maximum load than the maximum available from the device, the maximum device current must still be used. Equation 8 can be used to determine the value of inductance. The constant K is the percentage of inductor current ripple. For this example, K = 0.25 was chosen and an inductance of approximately 2 µH was found. The next standard value of 1.5 μH was selected.

Equation 8.

The saturation current rating of the inductor must be at least as large as the high-side switch current limit, I_L-HS (see Section 8.5). 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_L-LS, is designed to reduce the risk of current run-away, 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. Powdered iron cores exhibit a soft saturation, allowing some relaxation in the current rating of the inductor. However, they have more core losses at frequencies typically above 1 MHz. In any case, the inductor saturation current must not be less than the device high-side current limit, I_L-HS (see Section 8.5). To avoid subharmonic oscillation, the inductance value must not be less than that given in Equation 9. The maximum inductance is limited by the minimum current ripple required 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.

Equation 9.

Equation 9 assumes that this design must operate with input voltage near or in dropout. If minimum operating voltage for this design is high enough to limit duty factor to below 40%, Equation 8 can be used in place of Equation 9.

Note that choosing an inductor that is larger than the minimum inductance calculated using Equation 8 and Equation 9 results in less output capacitance being needed to limit output ripple but more output capacitance being needed to manage large load transients. See Section 10.2.2.4.