5.3 Pulse Frequency Mode of Operation (PFM)

The MCP16367 selects the best operating switching mode (PFM or PWM) for high efficiency across a wide range of load currents; while in PFM, the duty cycle is determined by a fixed peak current. This allows for the output voltage to increase slightly above the typical regulation point. When the output voltage increases and the feedback voltage exceeds 810 mV typical, the MCP16367 stops switching and enters the Sleep mode. The part resumes normal operation when the output voltage decreases. Switching to PFM mode at light load currents, combined with the very low I_Q current when not switching, results in obtaining very high efficiency at very low loads. During the sleep period (between two switching packets), the MCP16367 draws 18 µA (typical) from the supply line. The switching pulse packets represent a small percentage of the total running cycle, and the overall average current drawn from power line is small.

The disadvantages of PFM/PWM mode are higher output voltage ripple and variable PFM mode frequency. The PFM mode threshold is a function of the input voltage, output voltage and load.