The switching frequency can be chosen to optimize efficiency (1 MHz) or ripple/noise (2.2 MHz). Using the 2.2-MHz setting increases the gain of the feedback loop and can result in lower output noise. However, additional considerations for minimum on-time and duty cycle must also be considered. First, calculate the duty cycle using Equation 3. Higher efficiency results in a shorter on-time, so a conservative approach is to use a higher efficiency than expected in the application.

Equation 3.

where

• η is the estimated efficiency (use the value from the efficiency curves or 0.9 as an conservative assumption)

Then, calculate the on-time with both 1 MHz and 2.2 MHz using Equation 4. The on-time must always remain above the minimum on-time of 70 nsec. Use the maximum input voltage and maximum efficiency to determine the minimum duty cycle, D_min. Use the maximum switching frequency for f_SW.

Equation 4.

then

• If t_{ON_min} min < 70 ns with 2.2 MHz, use 1 MHz.

• If t_{ON_min} min < 70 ns with 1 MHz, reduce the maximum input voltage.

• If t_{ON_min} min ≥ 70 ns for both cases, use 1 MHz for highest efficiency, or 2.2 MHz for lowest noise and ripple.