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ZHCSOE3E August 2021 – September 2023 LMR38020

PRODUCTION DATA

9.2.2.4 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 maximum device current. Equation 10 can be used to determine the value of inductance. The constant K is the percentage of inductor current ripple. For this example, choose K = 0.4 and find an inductance of L = 14 µH. Select the next standard value of L = 15 µH.

Equation 10.

L = \frac{{(V}_{I N} {- V}_{O U T})}{f_{S W} \times K \times I_{{O U T}_{m a x}}} \times \frac{V_{O U T}}{V_{I N}}

Ideally, the saturation current rating of the inductor is at least as large as the high-side switch current limit, I_SC. This makes sure 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 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. 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 approximately 1 MHz. In any case, the inductor saturation current must not be less than the maximum peak inductor current at full load.

To avoid subharmonic oscillation, the inductance value must not be less than that given in Equation 11:

Equation 11.

L_{M I N} \geq M \times \frac{V_{O U T}}{f_{S W}}

where

L_MIN = minimum inductance (H)
M = 0.25 for a 2-A device
ƒ_SW = switching frequency (Hz)

The maximum inductance is limited by the minimum current ripple 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.