FUJITSU MB3887 - Manual (Page 18)

MB3887
18
3. Soft-Start Function
Soft-start block (SOFT)
Connecting a capacitor to the CS terminal (pin 22) prevents rush currents when the IC is turned on. Using an
error amplifier for soft-start detection makes the soft-start time constant, being independent of the output load
of the DC/DC converter.
■ SETTING THE CHARGING VOLTAGE
The charging voltage (DC/DC output voltage) can be set by connecting external voltage setting resistors (R3,
R4) to the
−
INE3 terminal (pin 16) . Be sure to select a resistor value that allows you to ignore the on-resistor
(35
Ω
, 1mA) of the internal FET connected to the OUTD terminal (pin 11) . In standby mode, the charging
voltage is applied to OUTD termial. Therefore, output voltage must be adjusted so that voltage applied to OUTD
terminal is 17 V or less.
Battery charging voltage : V
O
V
O
(V)
=
(R3
+
R4)
/
R4
×
4.2 (V)
■ METHOD OF SETTING THE CHARGING CURRENT
The charge current (output limit current) value can be set with the voltage at the
+
INE1 terminal (pin 9) .
If a current exceeding the set value attempts to flow, the charge voltage drops according to the set current value.
Battery charge current setting voltage :
+
INE1
+
INE1 (V)
=
20
×
I1 (A)
×
R
S
(
Ω
)
■ METHOD OF SETTING THE TRIANGULAR WAVE OSCILLATION FREQUENCY
The triangular wave oscillation frequency can be set by the timing resistor (R
T
) connected the RT terminal (pin 17) .
Triangular wave oscillation frequency : f
OSC
f
OSC
(kHz) := 13630
/
R
T
(k
Ω
)
−
+
+
4.2 V
R3
V O
R4
− INE3
OUTD
CS
16
B
11
22