The inductor selection is based on consideration of both buck and boost modes of operation. For the buck mode, inductor selection is based on limiting the peak to peak current ripple ΔI_L to ~40% of the maximum inductor current at the maximum input voltage. The target inductance for the buck mode is:

Equation 13.

For the boost mode, the inductor selection is based on limiting the peak to peak current ripple ΔI_L to ~30% of the maximum inductor current at the minimum input voltage. The target inductance for the boost mode is:

Equation 14.

In this particular application, the buck inductance is larger. Choosing a larger inductance reduces the ripple current but also increases the size of the inductor. A larger inductor also reduces the achievable bandwidth of the converter by moving the right half plane zero to lower frequencies. Therefore a judicious compromise should be made based on the application requirements. For this design a 4.7-µH inductor is selected. With this inductor selection, the inductor current ripple is 5.1 A, 4.3 A, and 2.1 A, at V_IN of 30 V, 24 V, and 6 V respectively.

The maximum average inductor current occurs at the minimum input voltage and maximum load current:

Equation 15.

where a 90% efficiency is assumed. The peak inductor current occurs at minimum input voltage and is given by:

Equation 16.

To ensure sufficient output current, the current limit threshold must be set to allow the maximum load current in boost operation. The inductor peak current during overload depends on the current limit resistor R_SENSE (refer to the sub-section on selecting R_SENSE). The peak inductor current in current limit when in boost mode is given by:

Equation 17.

The peak inductor current in current limit when in buck mode happens at high input voltage and is given by:

Equation 18.

The peak inductor current in current limit is 15 A and 16.5 A in boost mode and buck mode respectively. The inductor should be selected to handle this current.