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%
95%
5%
t
RAMP
t
Startup
EN
V
OUT
Vin
min
+ Vout
max
) Iout
max
ǒ
RDSon
max
) R
L
Ǔ
TPS62420, TPS62421
SLVS676C –JUNE 2006–REVISED JUNE 2010 www.ti.com
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.
Figure 27. Soft Start
100% Duty Cycle Low Dropout Operation
The converters offer a low input to output voltage difference while still maintaining operation with the use of the
100% duty cycle mode. In this mode the P-channel switch is constantly turned on. This is particularly useful in
battery-powered applications to achieve longest operation time by taking full advantage of the whole battery
voltage range; i.e., the minimum input voltage to maintain regulation depends on the load current and output
voltage, and can be calculated as:
(3)
With:
Iout
max
= maximum output current plus inductor ripple current
RDSon
max
= maximum P-channel switch RDSon
R
L
= DC resistance of the inductor
Vout
max
= nominal output voltage plus maximum output voltage tolerance
With decreasing load current, the device automatically switches into pulse skipping operation in which the power
stage operates intermittently based on load demand. By running cycles periodically the switching losses are
minimized and the device runs with a minimum quiescent current maintaining high efficiency.
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