The minimum ON time (T_{ON_MIN}) is the shortest duration of time that the high-side switch can be turned on. T_{ON_MIN} is typically 60 ns for the LMR544xx. The minimum OFF time (T_{OFF_MIN}) is the shortest duration of time that the high-side switch can be off. T_{OFF_MIN} is typically 110 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:

The maximum duty cycle without frequency foldback allowed is:

Given a required output voltage, the maximum V_IN without frequency foldback can be found by:

Equation 4.

The minimum V_IN without frequency foldback can be calculated by:

Equation 5.

In the LMR544xx, a frequency foldback scheme is employed after the 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. After 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 2.

The frequency foldback scheme also works after larger duty cycle is needed under a low V_IN condition. The frequency decreases after the device hits its T_{OFF_MIN}, which extends the maximum duty cycle according to Equation 3. In such condition, the frequency can be as low as approximately 133 kHz. A wide range of frequency foldback allows for the LMR544xx output voltage to stay in regulation with a much lower supply voltage V_IN, which leads to a lower effective dropout.

With frequency foldback while maintaining a regulated output voltage, V_{IN_MAX} is raised and V_{IN_MIN} is lowered by decreased f_SW.

VOUT = 0.8V

fSW = 1100 kHz

Figure 7-4 Frequency Foldback at T_{ON_MIN}

VOUT = 5 V

fSW = 1100 kHz

Figure 7-5 Frequency Foldback at T_{OFF_MIN}