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 LMR50410. 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 LMR50410, a frequency foldback scheme is employed once 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. 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 2.

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 3. In such condition, the frequency can be as low as approximately 133 kHz. Wide range of frequency foldback allows for the LMR50410 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 = 1 V

fSW = 700 kHz

Figure 8-4 Frequency Foldback at T_{ON_MIN}

VOUT = 5 V

fSW = 700 kHz

Figure 8-5 Frequency Foldback at T_{OFF_MIN}