SNVSBW0A October 2022 – October 2023 LM64460-Q1
PRODUCTION DATA
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 12-V 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. Use Equation 9 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 5-V, 6-A, 2.1-MHz example, resulting in an inductance of approximately 0.8 µH.
The saturation current rating of the inductor must be higher than the high-side switch current limit, IL-HS (see the Electrical Characteristics). This requirements prevents inductor saturation during an overload condition on the output. While an output short-circuit condition causes the 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, IL-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 1 MHz.
To avoid subharmonic oscillation, the inductance value must not be less than that given by Equation 7. 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 7 assumes that this design must operate with the input voltage near or in dropout. Use Equation 8 instead if the minimum input voltage for a given design is high enough to limit the duty cycle to less than 40%.