7.3.5 Minimum ON-time, Minimum OFF-time and Frequency Foldback at Dropout Conditions

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 55ns in the TPS56339. Minimum OFF-time, T_{OFF_MIN}, is the smallest duration that the high-side MOSFET can be off. T_{OFF_MIN} is typically 115 ns in the TPS56339. 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:

And the maximum duty cycle allowed is:

In the TPS56339, 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 V_IN conditions. With the duty increase, the on time will increase, until up to the Maximum ON-time, 5 μs. Wide range of frequency foldback allows the TPS56339 output voltage 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 6.

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 7.

Taking considerations of power losses in the system with heavy load operation, V_{IN_MAX} is higher than the result calculated in Equation 6. With frequency foldback, V_{IN_MIN} is lowered by decreased f_SW, as shown in Figure 16 .

Figure 16. Frequency Foldback at Dropout (V_OUT = 5 V)