SNVSB97A December 2018 – September 2021 LM34936-Q1
The inductor selection is based on consideration of both buck and boost modes of operation. For Buck mode, inductor selection is based on limiting the peak-to-peak current ripple ΔIL to approximately 40% of the maximum inductor current at the maximum input voltage. The target inductance for Buck mode is:
For Boost mode, the inductor selection is based on limiting the peak-to-peak current ripple ΔIL to approximately 30% of the maximum inductor current at the minimum input voltage. The target inductance for Boost mode is:
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 VIN of 30 V, 24 V, and 6 V, respectively.
The maximum average inductor current occurs at the minimum input voltage and maximum load current:
The peak inductor current occurs at minimum input voltage and is given by:
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 RSENSE (refer to the subsection on selecting RSENSE). The peak inductor current in current limit when in Boost mode is given by:
The peak inductor current in current limit when in Buck mode happens at high input voltage and is given by:
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.