SLUSDX1A September 2020 – August 2021 TPS563211
PRODUCTION DATA
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 9. With frequency foldback, V_{IN_VIN} is lowered by decreased f_{SW}, as shown in Figure 7-4.