Minimum on time, t_{ON_MIN}, is the smallest duration of time that the high-side switch can be on. t_{ON_MIN} is typically 70 ns in the TPS563300. Minimum off time, t_{OFF_MIN}, is the smallest duration that the high-side switch can be off. t_{OFF_MIN} is typically 140 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:

Equation 5.

The maximum duty cycle without frequency foldback allowed is:

Equation 6.

Given a required output voltage, the maximum V_IN without frequency foldback is:

Equation 7.

The minimum V_IN without frequency foldback is:

Equation 8.

In TPS563300, a frequency foldback scheme is employed once 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. Once 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 once larger duty cycle is needed under low V_IN condition. The frequency decreases once the device hits its t_{OFF_MIN}, which extends the maximum duty cycle according to Equation 6. Wide range of frequency foldback allows the TPS563300 output voltage to stay in regulation with a much lower supply voltage V_IN, which allows a lower effective dropout.

With frequency foldback, V_{IN_MAX} is raised, and V_{IN_MIN} is lowered by decreased f_SW.