SLUSDX1 September 2020 TPS563211
Minimum on-time, TON_MIN, is the smallest duration of time that the high-side MOSFET can be on. TON_MIN is typically 45 ns in the device. Minimum off-time, TOFF_MIN, is the smallest duration that the high-side MOSFET can be off. TOFF_MIN is typically 105 ns in the device. In CCM operation, TON_MIN and TOFF_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 TOFF_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 TOFF_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, VIN_MAX is higher than the result calculated in Equation 9. With frequency foldback, VIN_VIN is lowered by decreased fSW, as shown in Figure 7-4.