7.3.8 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 125 ns in the LM43601. Minimum OFF-time, T_OFF-MIN, is the smallest duration that the HS switch can be off. T_OFF-MIN is typically 200 ns in the LM43601.

In CCM operation, T_ON-MIN and T_OFF-MIN limits 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 LM43601, frequency foldback scheme is employed to extend the maximum duty cycle when T_OFF-MIN is reached. The switching frequency will decrease once longer duty cycle is needed under low V_IN conditions. The switching frequency can be decreased to approximately 1/10 of the programmed frequency by R_T or the synchronization clock. Such wide range of frequency foldback allows the LM43601 output voltage to stay in regulation with a much lower supply voltage V_IN. This leads to a lower effective dropout voltage. See Typical Characteristics for more details.

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

At lower supply voltage, the switching frequency decreases once T_OFF-MIN is tripped. The minimum V_IN without frequency foldback can be approximated by:

Taking considerations of power losses in the system with heavy load operation, V_IN-MIN is higher than the result calculated in Equation 7. With frequency foldback, V_IN-MIN is lowered by decreased F_S. Figure 42 gives an example of how F_S decreases with decreasing supply voltage V_IN at dropout operation.

Figure 42. Switching Frequency Decreases in Dropout Operation
V_OUT = 5 V, F_S = 1 MHz