MB3821 Datasheet, PDF (14/24 Pages) Fujitsu Component Limited. – 2-ch DC/DC Converter IC With Synchronous Rectifier

MB3821
s METHOD OF SETTING SOFT START TIME
To provide a soft start by preventing current surges at power-on, soft start capacitors (Cs1, Cs2) are connected to
both channels, the CS1 pin (pin 4) for CH1 and the CS2 pin (pin 21) for CH2.
When the IC is started (when the CTL pin (pin 18) goes to “H” level, and Vcc ≥ UVLO threshold voltage), transistors
Q2 and Q3 switch off and the CS1 and CS2 pins begin charging the external soft start capacitors (Cs1, Cs2) at 1 µA.
The error amplifier contributes to a soft start with the proportionate output voltage to the CS1 and CS2 pin voltage
regardless of the load current on the DC/DC converter.
The soft start time can be calculated by the following formula.
Soft start time (time to 100% output)
tS(sec) =: 1.26 × CS (µF)
Soft start circuit
1 µA
1 µA
FB1 6
−IN1 5
4
CS1
Cs1
Error
− ^Amp.1
+
+
1.26 V
Q2
Output stage
8
OUT1−1
Output stage
11
OUT2−1
FB2 19
−IN2 20
21
CS2
Cs2
Q3
CSCP
22
1 µA
CSCP
Q1
Q4
Error
− ^Amp.2
+
+
1.26 V
SCP Comp.
bias
S
R
Latch
2.1 V
UVLO
Output stage
17
OUT1−2
Output stage
14
OUT2−2
bias
Ref Power
(2.5V) ^ON/OFF
18
CTL
14