Datasheet
95%
5%
t
RAMP
t
Start
EN
V
OUT
TPS65708
SLVSAE1A –OCTOBER 2010– REVISED FEBRUARY 2011
www.ti.com
SOFT START
The step-down converter in TPS65708 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 13. 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
IN
min = V
O
max + I
O
max (R
DS(on)
max + R
L
)
With
I
O
max = maximum output current plus inductor ripple current
R
DS(on)
max = maximum high side switch R
DS(on)
.
R
L
= DC resistance of the inductor
V
O
max = nominal output voltage plus maximum output voltage tolerance
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