Minimum on-time, tON-MIN, is the smallest duration of time that the HS switch can be on. tON-MIN is typically 70 ns in the LM76002/LM76003. Minimum off-time, tOFF-MIN, is the smallest duration that the HS switch can be off. tOFF-MIN is typically 100 ns in the LM76002/LM76003. 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 LM76002/LM76003, 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 a wide range of frequency foldback allows the LM76002/LM76003 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 operational 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:
Considering power losses in the system with heavy load operation, VIN-MIN is higher than the result calculated in Equation 10. With frequency foldback, VIN-MIN is lowered by decreased fSW. When the device is operating in auto mode at voltages near maximum rated input voltage and light load conditions, an increased output voltage ripple during load transient may be observed. For this reason TI recommends that device operating point be calculated with sufficient operational margin so that minimum on-time condition is not triggered.