Selecting the switching frequency is the first step in the design process. Higher switching frequencies (for example, around 1.8 MHz) yield a smaller total solution size. However, the small size comes at the cost of increased switching losses, decreasing the efficiency of the converter. Higher efficiency is achieved by selecting a relatively lower switching frequency (for example, around 440 kHz) but requires physically larger components. Harmonics of the switching frequency must be considered in designs that have strict EMC requirements. The example application is selected to have a switching frequency of 2.1 MHz.

The LM5155x/LM5156x has a maximum duty cycle limit that is frequency dependent. See the LM5155x/LM5156x data sheet for details on the maximum duty cycle limit.