MB39A105 Datasheet, PDF (13/27 Pages) Fujitsu Component Limited. – 1-ch DC/DC Converter IC for low voltage

MB39A105
s SETTING THE SOFT-START TIMES
To prevent rush currents when the IC is turned on, you can set a soft-start by connecting soft-start capacitors
(CSCP) to the CSCP terminal (pin 2). When IC starts (VCC ≥ UVLO threshold voltage), the external soft-start
capacitors (CSCP) connected to CSCP terminal are charged at 11 µA. The error amplifier output (FB (pin 8) ) is
determined by comparison between the lower one of the potentials at two non-inverted input terminals (0.5 V
in an internal reference voltage, CSCP terminal voltages) and the inverted input terminal voltage (−INE (pin 1)
voltage).
The FB terminal voltage is decided for the soft-start period by the comparison between 0.5 V in an internal
reference voltage and the voltages of the CSCP terminal. The DC/DC converter output voltage rises in proportion
to the CSCP terminal voltage as the soft-start capacitor connected to the CSCP terminal is charged.
The soft-start time is obtained from the following formula:
Soft-start time: ts (time to output 100%)
ts (s) =: 0.045 × CSCP (µF)
=: 0.8 V
=: 0.5 V
=: 0 V
CSCP terminal voltage
Error Amp block −INE voltage
t
Soft-start time (ts)
• Soft-Start Circuit
VO
R1
−INE
1
VREF
R2
2
CSCP
CSCP
8
FB
11 µA
L priority
Error Amp
−
+
+
(0.5 V)
UVLO
13