SNVSB79B November 2018 – May 2020 LM25180
Choose a turns ratio based on an approximate 60% max duty cycle at minimum input voltage using Equation 14, rounding up or down as needed.
Select a magnetizing inductance based on the minimum off-time constraint using Equation 15. Choose a value of 30 µH to allow some margin for this application. Specify a saturation current of 2 A, above the maximum switch current specification of the LM25180.
Note that a higher magnetizing inductance provides a larger operating range for BCM and FFM, but the leakage inductance may increase based on a higher number of primary turns, NP. The primary and secondary winding RMS currents are given by Equation 16 and Equation 17, respectively.
Find the maximum output current for a given turns ratio using Equation 18, where the typical value for ISW-PEAK is the 1.5-A switch current peak threshold. Iterate by increasing the turns ratio if the output current capability is too low at minimum input voltage.