Datasheet

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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-EP
www.ti.com SGLS371A –JANUARY 2007–REVISED JANUARY 2013
Programming the Soft-Start Time
An external capacitor, C
SS
, connected from the soft-start (SS) pin to ground controls the TPS40200 soft-start
interval. An internal charging resistor connected to VDD produces a rising reference voltage, which is connected
though 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 30. 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 30. Soft-Start Circuit
The slow-start time should be more (slower) than the time constant of the output LC filter. This time constraint
may be expressed as:
(5)
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:
(6)
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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