Datasheet

V
OUT_NOM
+1%
PWMMode
medium/heavyload
PFMMode
lightload
Smooth
increasedload
PWMMode
medium/heavyload
PFMMode
lightload
Fastloadtransient
PWMMode
medium/heavyload
COMP_LOWthreshold –1%
TPS62420-Q1
www.ti.com SLVSA56 –DECEMBER 2009
In order to keep the output voltage ripple in Power Save Mode low, the output voltage is monitored with a single
threshold comparator (skip comparator). As the output voltage falls below the skip comparator threshold (skip
comp) of 1.01 x VOUTnominal, the corresponding converter starts switching for a minimum time period of
typically 1μs and provides current to the load and the output capacitor. Therefore the output voltage increases
and the device maintains switching until the output voltage trips the skip comparator threshold (skip comp) again.
At this moment all switching activity is stopped and the quiescent current is reduced to minimum. The load is
supplied by the output capacitor until the output voltage has dropped below the threshold again. Hereupon the
device starts switching again. The Power Save Mode is exited and PWM Mode entered in case the output
current exceeds the current IOUT_PFM_leave, or if the output voltage falls below a second comparator
threshold, called skip comparator low (Skip Comp Low) threshold. This skip comparator low threshold is set to
–2% below nominal Vout, and enables a fast transition from Power Save Mode to PWM Mode during a load step.
In Power Save Mode the quiescent current is reduced typically to 19μA for one converter and 32μA for both
converters active. This single skip comparator threshold method in Power Save Mode results in a very low output
voltage ripple. The ripple depends on the comparator delay and the size of the output capacitor. Increasing
output capacitor values minimizes the output ripple. The Power Save Mode can be disabled through the
MODE/DATA pin set to high. Both converters then operate in fixed PWM mode. Power Save Mode
Enable/Disable applies to both converters.
Dynamic Voltage Positioning
This feature reduces the voltage under/overshoots at load steps from light to heavy load and vice versa. It is
activated in Power Save Mode operation. It provides more headroom for both the voltage drop at a load step,
and the voltage increase at a load throw-off. This improves load transient behavior.
At light loads, in which the converter operate in PFM Mode, the output voltage is regulated typically 1% higher
than the nominal value. In case of a load transient from light load to heavy load, the output voltage drops until it
reaches the skip comparator low threshold set to –2% below the nominal value and enters PWM mode. During a
load throw off from heavy load to light load, the voltage overshoot is also minimized due to active regulation
turning on the N-channel switch.
Figure 26. Dynamic Voltage Positioning
Soft Start
The two converters have an internal soft start circuit that limits the inrush current during start-up. During soft
start, the output voltage ramp up is controlled as shown in Figure 27.
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