Minimum on time (t_{ON_MIN}) is the shortest duration of time that the high-side switch can be turned on. t_{ON_MIN} is typically 70 ns for the LMR51420. Minimum off time (t_{OFF_MIN}) is the shortest duration of time that the high-side switch can be off. t_{OFF_MIN} is typically 150 ns. In CCM operation, t_{ON_MIN} and t_{OFF_MIN} limit the voltage conversion range without switching frequency foldback.

The minimum duty cycle without frequency foldback allowed is:

The maximum duty cycle without frequency foldback allowed is:

Given a required output voltage, the maximum V_IN without frequency foldback can be found by:

The minimum V_IN without frequency foldback can be calculated by:

In the LMR51420, a frequency foldback scheme is employed after t_{ON_MIN} or t_{OFF_MIN} is triggered, which can extend the maximum duty cycle or lower the minimum duty cycle.

The on time decreases while V_IN voltage increases. After the on time decreases to t_{ON_MIN}, the switching frequency starts to decrease while V_IN continues to increase, which lowers the duty cycle further to keep V_OUT in regulation according to Equation 4.

The frequency foldback scheme also works after larger duty cycle is needed under low V_IN condition. The frequency decreases after the device reaches t_{OFF_MIN}, which extends the maximum duty cycle according to Equation 5. In such condition, the frequency can be as low as approximately 133 kHz. A wide range of frequency foldback allows for the LMR51420 output voltage to stay in regulation with a much lower supply voltage V_IN, which leads to a lower effective dropout.

With frequency foldback while maintaining a regulated output voltage, V_{IN_MAX} is raised, and V_{IN_MIN} is lowered by decreased f_SW.