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TPS630241YFFT Datasheet(HTML) 13 Page - Texas Instruments
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PFM mode at light load
Heavy Load transient step
Absolute Voltage drop
SLVSCK8A – NOVEMBER 2014 – REVISED DECEMBER 2014
Device Functional Modes (continued)
9.4.2 Power Save Mode Operation
Figure 4. Power Save Mode Operation
Depending on the load current, in order to provide the best efficiency over the complete load range, the device
works in PWM mode at load currents of approximately 350mA or higher. 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 about 350mA, the device switches off again and the
same operation is repeated. Instead, if at the next clock cycle, the load is above 350mA, the device automatically
switches to PWM mode.
In order 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 50mV pk-pk, (typically 30mV pk-pk), with 20µF effective output
capacitance. In order to avoid a critical voltage drop when switching from 0A to full load, the output voltage in
PFM mode is typically 1.3% 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.
Copyright © 2014, Texas Instruments Incorporated
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