The switching frequency can be chosen to optimize efficiency (lower) or ripple noise (higher). Using the higher 1.4 MHz or 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.

where:

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

Then, calculate the on-time with 1 MHz, 1.4 Mhz, and 2.2 MHz using Equation 4. The on-time must always remain above the minimum on-time of 70 ns. Use the maximum input voltage and maximum efficiency to determine the minimum duty cycle, D_min. Use the maximum switching frequency for f_SW.

then

• If t_{ON_min} minimum < 70 ns with 2.2 MHz, use 1.4 MHz.
• If t_{ON_min} minimum < 70 ns with 1.4 MHz, use 1 MHz
• If t_{ON_min} minimum < 70 ns with 1 MHz, reduce the maximum input voltage.
• If t_{ON_min} minimum ≥ 70 ns, use a lower frequency for highest efficiency, or the highest frequency for the lowest noise and ripple.