Datasheet

TPS40200
2
4
SS
FB
C
SS
105 kW
V
SST
3
COMP
+
+
+
Error Amplifier
700 mV
700 mV
V
SST(offset)
Ideal
Diodes
UDG-05083
³ p ´ ´
S OUT OUT
t 2 L C
æ ö
= ´ ´ ´
ç ÷
-
è ø
SST
SS C SS
SST
V
t R C ln
V 1.4
TPS40200
www.ti.com SLUS659F –FEBRUARY 2006–REVISED MARCH 2012
Programming the Soft-Start Time
An external capacitor C
SS
, connected from the SS pin to ground controls the soft-start interval. An internal
charging resistor connected to V
DD
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 reference
voltage. A conceptual drawing of the circuit that produces these results is shown in Figure 29. A consequence of
the 700-mV offset is that the controller does not start switching until V
CSS
has risen 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 29. Soft-Start Circuit
The slow-start time should be longer (slower) than the time constant of the output L-C filter. This time constraint
may be expressed as described in Equation 5.
(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 shown in Equation 6.
where
• t
SS
is the required soft-start time in seconds
• C
SS
is the soft-start capacitor value in F
• R
c
is the internal soft-start charging resistor (105 kΩ nominal)
• V
SST
is the input voltage up to a maximum of 8 V (6)
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