FUJITSU MB39A105 - Manual (Page 13)

MB39A105
13
■ ■ ■ ■ 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
(C
SCP
) to the CSCP terminal (pin 2). When IC starts (V
CC
≥
UVLO threshold voltage), the external soft-start
capacitors (C
SCP
) connected to CSCP terminal are charged at 11
µ
A. 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
×
C
SCP
(
µ
F)
t
:= 0.8 V
:= 0.5 V
:= 0 V
CSCP terminal voltage
Error Amp block
−
INE voltage
Soft-start time (ts)
−
+
+
V O
1
2
8
R1
R2
− INE
VREF
CSCP
C SCP
FB
Error Amp
UVLO
(0.5 V)
11
µ
A
L priority
• Soft-Start Circuit