SNVSA46B June 2014 – January 2018 LM46001
Minimum ON-time, tON-MIN, is the smallest duration of time that the HS switch can be on. Minimum ON-time (tON-MIN) is typically 125 ns in the LM46001. Minimum OFF-time, tOFF-MIN, is the smallest duration that the HS switch can be off. TOFF-MIN is typically 200 ns in the LM46001.
In CCM operation, tON-MIN and tOFF-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 tON-MIN and tOFF-MIN, the higher the switching frequency the narrower the range of the allowed duty cycle. In the LM46001, a frequency foldback scheme is employed to extend the maximum duty cycle when tOFF-MIN is reached. The switching frequency decreases once longer duty cycle is needed under low VIN conditions. The switching frequency can be decreased to approximately 1/10 of the programmed frequency by RT or the synchronization clock. Such wide range of frequency foldback allows the LM46001 output voltage to stay in regulation with a much lower supply voltage VIN. 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 tOFF-MIN is tripped. The minimum VIN without frequency foldback can be approximated by:
Taking considerations of power losses in the system with heavy load operation, VIN-MIN is higher than the result calculated in Equation 7 . With frequency foldback, VIN-MIN is lowered by decreased FS. Figure 42 gives an example of how FS decreases with decreasing supply voltage VIN at dropout operation.