SLVSEW4 April 2019 TPS650002-Q1
The typical value for the converter inductor is 2.2-μH output inductor. Larger or smaller inductor values in the range of 1.5 μH to 3.3 μH can optimize the performance of the device for specific operation conditions. The selected inductor must be rated for its DC resistance and saturation current. The DC resistance of the inductance influences the efficiency of the converter directly. An inductor with lowest DC resistance must be selected for highest efficiency. For more information on inductor selection, refer to the Choosing Inductors and Capacitors for DC/DC Converters application report.
Equation 1 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 2. TI recommends this because during heavy load transient, the inductor current rises above the calculated value.
The highest inductor current occurs at maximum VIN.
Open-core inductors have a soft saturation characteristic and 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. Consider that the core material from inductor to inductor differs and impacts the efficiency especially at high-switching frequencies.
The step down converter has internal loop compensation. TI designed the internal loop compensation to work with a certain output filter corner frequency calculated as in Equation 3:
The selection of external L-C filter must be consistent with Equation 3. The product of L × COUT must be constant while selecting smaller inductor or increasing output capacitor value.