The minimum on time, T_{ON_MIN}, is the smallest duration of time that the high-side MOSFET can be on. T_{ON_MIN} is typically 45 ns in the device. The minimum off time, T_{OFF_MIN}, is the smallest duration that the high-side MOSFET can be off. T_{OFF_MIN} is typically 105 ns in the device. In CCM operation, T_{ON_MIN} and T_{OFF_MIN} limit the voltage conversion range given a fixed switching frequency.

The minimum duty cycle allowed is:

Equation 7.

The maximum duty cycle allowed is:

Equation 8.

In the device, a frequency foldback scheme is employed to extend the maximum duty cycle when T_{OFF_MIN} is reached. The switching frequency decreases once longer duty cycle is needed under low VIN conditions. With the duty increased, the on time is extended up to the maximum on time of 6 μs. A wide range of frequency foldback allows the device output voltage to stay in regulation with a much lower supply voltage V_IN. This leads to a lower effective dropout voltage.

Given an output voltage, the maximum operation supply voltage can be found by:

Equation 9.

At lower supply voltage, the switching frequency decreases once T_{OFF_MIN} is triggered. The minimum V_IN without frequency foldback can be approximated by:

Equation 10.

Taking considerations of power losses in the system with heavy load operation, V_{IN_MAX} is higher than the result calculated in Equation 9. With frequency foldback, V_{IN_VIN} is lowered by decreased f_SW, as shown in Figure 7-4.