The parameters for selecting the inductor are the inductance and saturation current. The inductance is based on the desired peak-to-peak ripple current, which 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 12V automotive battery, 25% is commonly used.

When selecting the ripple current for applications with lower maximum load than the maximum available from the device, the maximum device current must still be used. For the 4A device, use Equation 6 to determine the value of inductance. The constant K is the percentage of peak-to-peak inductor current ripple to rated output current. Choose K = 0.3 for this 5V, 4A, 2.1MHz example, resulting in an inductance of approximately 1.2µH.

Equation 6.

For the 6A device, use Equation 10 to determine the value of inductance. The constant K is the percentage of peak-to-peak inductor current ripple to rated output current. Choose K = 0.3 for this 5V, 6A, 2.1MHz example, resulting in an inductance of approximately 0.8µH.

Equation 7.

The saturation current rating of the inductor must be higher than the high-side switch current limit, I_L-HS (see the Electrical Characteristics). These requirements prevent inductor saturation during an overload condition on the output. While an output short-circuit condition causes the LM64440-Q1 or LM64460-Q1 to enter hiccup mode, an overload condition can hold the output current at current limit without triggering hiccup. When the inductor core material saturates, the inductance can fall to a low value, causing the inductor current to rise rapidly. Although the valley current limit, I_L-LS, reduces the risk of current runaway, a saturated inductor causes the instantaneous current to increase to a high value. This can lead to component damage, avoiding inductor saturation is crucial.

Inductors with a ferrite core material have 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, powdered iron cores typically have higher core losses at frequencies above 1MHz.

To avoid subharmonic oscillation, the inductance value must not be less than that given by Equation 8. The maximum inductance is limited by the minimum current ripple required for 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 converter maximum rated current under nominal conditions.

Equation 8.

Equation 8 assumes that this design must operate with the input voltage near or in dropout. Use Equation 9 instead if the minimum input voltage for a given design is high enough to limit the duty cycle to less than 40%.

Equation 9.