Minimum ON-time, T_{ON_MIN}, is the smallest duration of time that the HS switch can be on. T_{ON_MIN} is typically 70 ns in the LV3842 . Minimum OFF-time, T_{OFF_MIN}, is the smallest duration that the HS switch can be off. T_{OFF_MIN} is typically 100 ns. In CCM operation, T_{ON_MIN} and T_{OFF_MIN} limit the voltage conversion ratio 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 LV3842, 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 low V_IN conditions. 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 about 133-kHz minimum. Wide range of frequency foldback allows the LV3842 output voltage stay in regulation with a much lower supply voltage V_IN. This leads to a lower effective drop-out voltage.

With frequency foldback, V_{IN_MAX} is raised, and V_{IN_MIN} is lowered to overcome T_{ON_MIN} or T_{OFF_MIN} limitation.

VOUT = 1 V

Figure 8-4 Frequency Foldback at T_{ON_MIN}

VOUT = 5 V

Figure 8-5 Frequency Foldback at T_{OFF_MIN}