Datasheet
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÷
ø
ö
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ç
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-
´´=
4.1V
V
lnCRt
SST
SST
SScSS
OOS
CL2t ´´p³
Css
+
105 kW
2
+
3
4
V
SST
V (offset)
SST
COMP
FB
SS
700 mV
+
Ideal
Diodes
700 mV
Error Amplifier
TPS40200
TPS40200-Q1
SLUS739E –SEPTEMBER 2006–REVISED JULY 2013
www.ti.com
Programming the Soft-Start Time
An external capacitor (C
SS
) connected from the SS pin to ground controls the TPS40200 soft-start interval. An
internal charging resistor connected to V
DD
produces a rising reference voltage that is connected through a 700-
mV offset to the reference input of the TPS40200 error amplifier. When the soft-start capacitor voltage (V
CSS
) is
below 150 mV, there is no switching activity. When V
CSS
rises above the 700-mV offset, the error amplifier starts
to follow V
SST
– 700 mV and uses this rising voltage as a reference. When V
CSS
reaches 1.4 V, the internal
reference takes over, and further increases have no effect. An advantage of initiating a slow start in this fashion
is that the controller cannot overshoot, because its output follows a scaled version of the controller's reference
voltage. A conceptual drawing of the circuit that produces these results is shown in Figure 33. A consequence of
the 700-mV offset is that the controller does not start switching until the V
CSS
has charged up to 700 mV. The
output remains at 0 V during the resulting delay. When V
CCS
exceeds the 700-mV offset, the TPS40200 output
follows the soft-start time constant. Once above 1.4 V, the 700-mV internal reference takes over, and normal
operation begins.
Figure 33. Soft-Start Circuit
The slow-start time should be longer (slower) than the time constant of the output LC filter. This time constraint
may be expressed as:
(2)
The calculation of the soft-start interval is simply the time it takes the RC network to exponentially charge from
0 V to 1.4 V. An internal 105-kΩ charging resistor is connected from the SS pin to V
SST
. For applications where
the voltage is above 8 V, an internal regulator clamps the maximum charging voltage to 8 V.
The result of this is a formula for the start up time, as given by:
Where:
t
SS
= required soft-start time in seconds
C
SS
= soft-start capacitor value in F
R
c
= internal soft-start charging resistor (105 kΩ nominal)
V
SST
= input voltage up to a maximum of 8 V
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