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 75ns for the LMR514x5 . 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 135ns. 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 LMR514x5, a frequency foldback scheme is employed after the 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 go up, which lowers the duty cycle further to keep V_OUT in regulation according to Equation 5.

The frequency foldback scheme also works after larger duty cycle is needed under low V_IN condition. The frequency decreases after the device hits the t_{OFF_MIN}, which extends the maximum duty cycle according to Equation 6. In such condition, the frequency can be as low as approximately 200kHz. Wide range of frequency foldback allows for the LMR514x5 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.