TPS40200 Datasheet, PDF (24/39 Pages) Texas Instruments – WIDE INPUT RANGE NON-SYNCHRONOUS VOLTAGE MODE CONTROLLER

TPS40200
SLUS659 – FEBRUARY 2006
Soft Start Capacitor
The soft start interval is given (in pF) by the following equation:
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C_SS =
t _SS
´ 10³
R
´
ln
ç^ç_è^æ
V_SST
V_SST - 1.4
÷^÷_ø^ö
Where:
• R is an internal 105-kΩ charging resistor
• V_CC is the input voltage up to 8 volts where the charging
voltage is internally clamped. to 8 volts maximum
• V_OS = 700 mV, and because the input voltage is 12 volts, 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 volts 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 rising to 3.3 volt
B. Channel 2 is the soft start Pin
Figure 38. Soft Start Showing Output Delay and Controlled Rise To Programmed Output Voltage
24
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