SNVS953G December 2012 – May 2021 LM25018
The minimum inductance is selected to limit the output ripple to 30 to 40 percent of the maximum load current. In addition, the peak inductor current at maximum load must be smaller than the minimum current limit threshold as given in Equation 13. The inductor current ripple is calculated using Equation 13.
The maximum ripple is observed at maximum input voltage. To achieve the required output current of 300 mA without exceeding the peak current limit threshold, lower ripple current is required. Substituting VIN = 48 V and ΔIL = 30 percent × IOUT (max) results in L1 = 200 μH. The higher standard value of 220 μH is chosen. With this inductor value, the peak-to-peak minimum and maximum inductor current ripple are 21 mA and 82 mA at minimum and maximum input voltages, respectively. The peak inductor and switch current is shown in Equation 14.
The calculated peak current of 341 mA is smaller than the minimum current limit threshold, which is 390 mA. In addition, the selected inductor must be able to operate at the maximum current limit threshold of 750 mA during startup and overload conditions without saturating.