To support higher duty cycle operation when V_IN close to V_OUT, the TPS563211 includes an On-Time Extension function. This function operates by increasing the HSFET On-Time beyond that during normal operation, thus lowering the operating frequency and allowing large duty cycles to be maintained.

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:

The maximum duty cycle allowed is:

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:

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

Taking considerations of power losses in the system with heavy load operation, V_{IN_MAX} is higher than the result calculated in Equation 4.

If decrease this V_{IN_MIN} voltage, The device would allow periodic switching after trigger min off time. And the on-time is extended until up to the maximum on-time, 6 μs typical, with the duty increased.

Based on 6 us maximum on-time and 105 ns minimum off-time, we can get a real maximum duty, 98%, thanks to this frequency foldback scheme.