SNVSA47C February 2016 – December 2020 LM5165
If RDSON1 and RDSON2 are the high-side and low-side MOSFET on-state resistances, respectively, and RDCR is the inductor DC resistance, the duty cycle in COT (CCM) or PFM mode is given by Equation 3.
The LM5165 offers a low input voltage to output voltage dropout by engaging the high-side MOSFET at 100% duty cycle. In COT mode, a frequency foldback feature effectively extends maximum duty cycle to 100% during low dropout conditions or load-on transients. Based on the 4-mV FB comparator dropout hysteresis, the duty cycle extends as needed at low input voltage conditions, corresponding to lower switching frequency. The PWM on-time extends based on the requirement that the FB voltage exceeds the dropout hysteresis during a given on-time. 100% duty cycle operation is eventually reached as the input voltage decreases towards the output setpoint. The output voltage stays in regulation at a lower supply voltage, thus achieving an extremely low dropout voltage.
Note that PFM mode operation provides an inherently natural transition to 100% duty cycle if needed for low dropout applications.
Use Equation 4 to calculate the minimum input voltage to maintain output regulation.