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. 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 until up to the maximum on-time, 6 μs. Wide range of frequency foldback allows the device output voltage stay in regulation with a much lower supply voltage VIN. 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 VIN 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.

Figure 7-4 Frequency Foldback at Dropout (V_OUT = 5 V)