The main parameters for selecting the inductor are the inductance and saturation current. The inductance is based on the desired peak-to-peak ripple current. It 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. For systems with a variable input voltage such as the 12-V battery in a car, 25% is commonly used. This example uses VIN = 13.5 V, which is closer to the nominal voltage of a 12-V car battery. 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 for this calculation. Equation 5 can be used to determine the value of the inductance. The constant K is the percentage of peak-to-peak inductor current ripple to rated output current. For this 10-A, 400-kHz, 5-V example, K = 0.25 is chosen and an inductance of approximately 3.15 μH is found. The closest standard value of 3.0 μH was selected.
Ideally, the saturation current rating of the inductor should be at least as large as the high-side switch current limit, ISC. This ensures that the inductor does not saturate, even during a soft-short condition on the output. A hard short causes the LM6x4xx-Q1 to enter hiccup mode (see Section 8.3.13). A soft short can hold the output current at current limit without triggering hiccup. When the inductor core material saturates, the inductance can fall to a very low value, causing the inductor current to rise very rapidly. Although the valley current limit, ILS-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 could lead to component damage, so it is crucial that the inductor does not 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 saturation current rating of the inductor. However, they have more core losses at frequencies typically above 1 MHz. To avoid subharmonic oscillation, the inductance value must not be less than that given in Equation 6. 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.