8.2.3.1 Inductor Selection for the DCDC Converters

The three converters operate with 2.2 µH output inductors. Larger or smaller inductor values can be used to optimize performance of the device for specific conditions. The selected inductor has to be rated for its DC resistance and saturation current. The DC resistance of the inductor influences directly the efficiency of the converter. Therefore, an inductor with the lowest DC resistance should be selected for the highest efficiency.

For a fast transient response, a 2.2 μH inductor in combination with a 22 μF output capacitor is recommended. For an output voltage above 2.8 V, an inductor value of 3.3 μH minimum is required. Lower values result in an increased output voltage ripple in PFM mode. The minimum inductor value is 1.5 μH, but an output capacitor of 22 μF minimum is needed in this case.

Equation 4 calculates the maximum inductor current under static load conditions. The saturation current of the inductor should be rated higher than the maximum inductor current as calculated with Equation 4. This is recommended because during heavy load transient the inductor current rises above the calculated value.

Equation 4.

where

• f = Switching frequency (2.25 MHz typical)
• L = Inductor value
• ΔI_L = Peak-to-peak inductor ripple current
• I_Lmax = Maximum inductor current

The highest inductor current occurs at maximum Vin.

Open core inductors have a soft saturation characteristic and they can usually handle higher inductor currents versus a comparable shielded inductor.

A more conservative approach is to select the inductor current rating just for the maximum switch current of the corresponding converter. Consideration must be given to the difference in the core material from inductor to inductor which has an impact on efficiency especially at high switching frequencies. See Table 4 and the typical applications for possible inductors.