Depending on the load current, the device works in PWM mode at load currents of approximately 350 mA or higher to provide the best efficiency over the complete load range. At lighter loads, the device switches automatically into Power Save Mode to reduce power consumption and extend battery life. The PFM/PWM pin is used to select between the two different operation modes. To enable Power Save Mode, the PFM/PWM pin must be set low.
During Power Save Mode, the part operates with a reduced switching frequency and lowest supply current to maintain high efficiency. The output voltage is monitored with a comparator at every clock cycle by the thresholds comp low and comp high. When the device enters Power Save Mode, the converter stops operating and the output voltage drops. The slope of the output voltage depends on the load and the output capacitance. When the output voltage reaches the comp low threshold, at the next clock cycle the device ramps up the output voltage again, by starting operation. Operation can last for one or several pulses until the comp high threshold is reached. At the next clock cycle, if the load is still lower than 150 mA, the device switches off again and the same operation is repeated. If at the next clock cycle the load is above 150 mA, the device automatically switches to PWM mode.
To keep high efficiency in PFM mode, there is only one comparator active to keep the output voltage regulated. The AC ripple in this condition is increased, compared to the PWM mode. The amplitude of this voltage ripple in the worst case scenario is 50 mV peak to peak, (typically 30 mV peak-to-peak), with 10 µF of effective output capacitance. To avoid a critical voltage drop when switching from 0 A to full load, the output voltage in PFM mode is typically 1.5% above the nominal value in PWM mode. This is called Dynamic Voltage Positioning and allows the converter to operate with a small output capacitor and still have a low absolute voltage drop during heavy load transients.
Power Save Mode is disabled by setting the PFM/PWM pin high.