MB3817_03 Datasheet, PDF (18/28 Pages) Fujitsu Component Limited. – Switching Regulator Controller

MB3817
s METHOD OF SETTING SOFT START TIME
To protect against surge currents when the IC is turned on, a soft start setting can be made by connecting a soft
start capacitor (CS) to the CS terminal (pin 13) .
When the IC starts up (CTL terminal (pin 15) to “H” level, Vcc ≥ UVLO threshold voltage VTH) the transistor Q1
turns off and the soft start capacitor (CS) connected to the CS terminal begins charging from the charge current
ICHG which is −1.0 µA.
At this time, if the CS terminal voltage is less than 0.9 V, the soft start comparator circuit output goes to “H” level,
transistor Q2 turns on and the CSCP terminal (pin 14) holds input standby voltage VSTB which is 50 mV so that
the short protection circuit is not activated. When the CS terminal voltage is greater than or equal to 0.9 V,
transistor Q2 turns off, the PWM comparator circuit compares the CS terminal voltage with the triangular wave
and changes the ON duty of the OUTPUT terminal, thus achieving a soft start. Note that the soft start time is
determined by the following formula.
Soft start time (time before output ON duty reaches 50%)
tS [ms] =: 1.2 × CS [µF]
• Soft start circuit
CS
13
1 µA
Q1
Soft Start Comp.
−
+
(0.9 V)
bias
RS
Latch
UVLO
PWM Comp.
OFF
current
setting
block
Outside putting
PNP transistor
9
VCC
Q4
10
OUT
Q6
RE
D1
11
Q5
(0.5V)
VE
1 µA
Q2 Q3
14
CSCP
18