8.3.6 Minimum ON-time, Minimum OFF-time and Frequency Foldback at Dropout Conditions

Minimum ON-time, T_{ON_MIN}, is the smallest duration of time that the HS switch can be on. T_{ON_MIN} is typically 60 ns in the LMR23630-Q1. 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 the LMR23630-Q1. In CCM operation, T_{ON_MIN} and T_{OFF_MIN}  limit the voltage conversion range given a selected switching frequency.

The minimum duty cycle allowed is:

And the maximum duty cycle allowed is:

Given fixed T_{ON_MIN} and T_{OFF_MIN}, the higher the switching frequency the narrower the range of the allowed duty cycle. In the LMR23630-Q1, 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 V_IN conditions. Wide range of frequency foldback allows the LMR23630-Q1 output voltage stay in regulation with a much lower supply voltage V_IN. This leads to a lower effective dropout voltage.

Given an output voltage, the choice of the switching frequency affects the allowed input voltage range, solution size and efficiency. The maximum operation supply voltage can be found by:

Equation 5.

At lower supply voltage, the switching frequency will decrease once T_{OFF_MIN} is tripped. The minimum V_IN without frequency foldback can be approximated by:

Equation 6.

Taking considerations of power losses in the system with heavy load operation, V_{IN_MAX} is higher than the result calculated in Equation 5. With frequency foldback, V_{IN_MIN} is lowered by decreased f_SW.

Figure 20. Frequency Foldback at Dropout (V_OUT = 5 V, f_SW = 400 kHz)