Datasheet

3
SST
SST
SS
SS
10
4.1V
V
lnR
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TPS40200
SLUS659F –FEBRUARY 2006–REVISED MARCH 2012 www.ti.com
Soft Start Capacitor
The soft start interval is given (in pF) by the following equation:
where
• R is an internal 105-kΩ charging resistor
• V
CC
is the input voltage up to 8 V where the charging voltage
is internally clamped. to 8 V maximum
• V
OS
= 700 mV, and because the input voltage is 12 V, V
SST
= 8
V.
The oscilloscope picture below shows the expected delay at the output (middle trace) until the soft-start node
(bottom trace) reaches 700 mV. At this point, the output rises following the exponential rise of the soft-start
capacitor voltage until the soft-start capacitor reaches 1.4 V and the internal 700-mV reference takes over. This
total time is approximately 1 ms, which agrees with the calculated value of 0.95 ms where the soft start
capacitance is 0.047 μF.
A. Channel 1 is the output voltage (V
OUT
) rising to 3.3 V
B. Channel 2 is the soft start pin (SS)
Figure 35. Soft Start Showing Output Delay and Controlled Rise To Programmed Output Voltage
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