TPS40200-Q1
SLUS739D –SEPTEMBER 2006–REVISED JULY 2011
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Programming the Soft-Start Time
An external capacitor (CSS) connected from the SS pin to ground controls the TPS40200 soft-start interval. An
internal charging resistor connected to VDD 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
(VCSS) is below 150 mV, there is no switching activity. When VCSS rises above the 700-mV offset, the error
amplifier starts to follow VSST – 700 mV and uses this rising voltage as a reference. When VCSS 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 VCSS has charged up to
700 mV. The output remains at 0 V during the resulting delay. When VCCS 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.
TPS40200
VSST
105 kW
700mV
Ideal
SS
VSST (offset)
Diodes
2
Error Amplifier
+
Css
FB
700mV
4
3
COMP
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:
tS ³ 2p ´ LO ´ CO
(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 VSST. 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:
æ
ç
ç
è
ö
÷
÷
ø
VSST
Where:
tSS = required soft-start time in seconds
tSS = Rc ´ CSS ´ ln
VSST - 1.4
CSS = soft-start capacitor value in F
Rc = internal soft-start charging resistor (105 kΩ nominal)
VSST = input voltage up to a maximum of 8 V
16
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