MB3817 Datasheet, PDF (18/29 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 pin (pin 13).
When the IC starts up (CTL pin (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 pin begins charging from the charge current ICHG which
is –1.0 µA.
At this time, if the CS pin voltage is less than 0.9 V, the soft start comparator circuit output goes to “H” level,
transistor Q2 turns on and the CSCP pin (pin 14) holds input standby voltage VSTB which is 50 mV so that the
short protection circuit is not activated. When the CS pin voltage is greater than or equal to 0.9 V, transistor Q2
turns off, the PWM comparator circuit compares the CS pin voltage with the triangular wave and changes the
ON duty of the OUTPUT pin, 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
1µ
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
1µ
Q2 Q3
14
CSCP
18