Vo

Vo+1.3%*Vo

PFM mode at light load

current

PWM mode

Comparator High

Comparator low

Heavy Load transient step

Absolute Voltage drop

with positioning

30mV ripple

TPS630250

TPS630251, TPS630252

www.ti.com

SLVSBJ9B – MAY 2014 – REVISED MARCH 2015

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 30 mV pk-pk), with 2-µF 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.

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