MB3887_06 Datasheet, PDF (17/34 Pages) Fujitsu Component Limited. – DC/DC Converter IC for Charging Li-ion battery

MB3887
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 (pin 11) is 17 V or less.
Battery charging voltage : VO
VO (V) = (R3 + R4) / R4 × 4.2 (V)
B ^VO
R3
−INE3
16
R4
11
OUTD
<Error Amp3>
−
+
+
4.2 V
22
CS
■ 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) × RS (Ω)
■ METHOD OF SETTING THE TRIANGULAR WAVE OSCILLATION FREQUENCY
The triangular wave oscillation frequency can be set by the timing resistor (RT) connected the RT terminal (pin 17) .
Triangular wave oscillation frequency : fOSC
fOSC (kHz) =: 13630 / RT (kΩ)
17