Datasheet
( )
SS / SD
SS / SD START
FB
I
C t F
V
æ ö
= ´
ç ÷
è ø
t
START
w 2p L C
O
Ǹ
(seconds)
TPS40054
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TPS40057
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SLUS593H –DECEMBER 2003–REVISED JULY 2012
PROGRAMMING SOFT START
The TPS4005x uses a closed-loop soft-start system to ensure a controlled ramp of the output during startup. The
reference voltage used for the startup is derived in the following manner. A capacitor (C
SS/SD
) is connected to the
SS/SD pin. There is a ramped voltage generated at this pin by charging C
SS/SD
with a current source. A value of
0.85 V is subtracted from the voltage at the SS/SSD pin and is applied to a non-inverting input of the error
amplifier. This is the effective soft-start ramp voltage, V
SSRMP
. The error amplifier also has the 0.7-V reference
(V
FB
) voltage applied to a non-inverting input. The structure of the error amplifier input stage is such that the
lower of V
FB
or V
SSRMP
becomes the dominant voltage that the error amplifier uses to regulate the FB pin. This
provides a clean, closed-loop startup while V
SSRMP
is lower than V
FB
and a precision reference regulated supply
as V
SSRMP
climbs above V
FB
. To ensure a controlled ramp-up of the output voltage, the soft-start time should be
greater than the L-C
O
time constant as described in Equation 12.
where
• t
START
is the startup ramp time in s (12)
There is a direct correlation between t
START
and the input current required during start-up. The faster t
START
, the
higher the input current required during start-up. This relationship is described in more detail in the section titled,
Programming the Current Limit which follows. The soft-start capacitance, C
SS/SD
, is described in Equation 13.
For applications in which the V
IN
supply ramps up slowly (typically between 50 ms and 100 ms), it may be
necessary to increase the soft-start time to between approximately 2 ms and 5 ms to prevent nuisance UVLO
tripping. The soft-start time should be longer than the time that the V
IN
supply transitions between 6 V and 7 V.
where
• I
SS/SD
is the soft-start charge current (typical value is 2.35 μA)
• V
FB
is the feedback reference voltage (typical value is 0.7 V) (13)
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