Datasheet

EN
95%
5%
V
OUT
t
Start
t
RAMP
TPS65720
TPS65721
SLVS979 –OCTOBER 2009 www.ti.com
Dynamic Voltage Positioning
This feature reduces the voltage under/overshoots at load steps from light to heavy load and vice versa. It is
active in Power Save Mode and regulates the output voltage 1% higher than the nominal value. This provides
more headroom for both the voltage drop at a load step, and the voltage increase at a load throw-off.
Soft Start
The step-down converter in TPS65720 has an internal soft start circuit that controls the ramp up of the output
voltage. The output voltage ramps up from 5% to 95% of its nominal value within typical 250μs. This limits the
inrush current in the converter during ramp up and prevents possible input voltage drops when a battery or high
impedance power source is used.
Figure 26. Soft Start
100% Duty Cycle Low Dropout Operation
The device starts to enter 100% duty cycle mode once the input voltage comes close to the nominal output
voltage. In order to maintain the output voltage, the High Side MOSFET switch is turned on 100% for one or
more cycles. With further decreasing VIN the High Side MOSFET switch is turned on completely. In this case the
converter offers a low input-to-output voltage difference. This is particularly useful in battery-powered applications
to achieve longest operation time by taking full advantage of the whole battery voltage range. The minimum input
voltage to maintain regulation depends on the load current and output voltage, and can be calculated as:
V
INmin
= V
Omax
+ I
Omax
× ®
DS(on)max
+ R
L
)
With:
I
Omax
= maximum output current plus inductor ripple current
R
DS(on)max
= maximum high side switch RDSon.
R
L
= DC resistance of the inductor
V
Omax
= nominal output voltage plus maximum output voltage tolerance
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