MB3875 Datasheet, PDF (14/27 Pages) Fujitsu Component Limited. – DC/DC Converter IC for Charging

MB3875/MB3877
s FUNCTIONAL DESCRIPTION
1. DC/DC Converter Unit
(1) Reference voltage block (Ref)
The reference voltage generator uses the voltage supplied from the Vcc terminal (pin 18) to generate a temper-
ature-compensated, stable voltage (=: 4.2 V) used as the reference supply voltage for the IC’s internal circuitry.
The reference voltage can be output, up to 1 mA, to an external device through the VREF terminal (pin 5).
(2) Triangular wave oscillator block(OSC)
The triangular wave oscillator generates a triangular waveform with a frequency setting resistor connected to
the internal frequency setting capacitor via the RT terminal (pin 17).
The triangular wave is input to the PWM comparator on the IC.
(3) Error amplifier block (Error Amp. 1)
This error amplifier (Error Amp. 1) detects a voltage drop in the AC adapter and outputs a PWM control signal.
In addition, an arbitrary loop gain can be set by connecting a feedback resistor and capacitor from the FB1
terminal (pin 9) to the -INE1 terminal (pin 11) of the error amplifier, enabling stable phase compensation to the
system.
(4) Error amplifier block (Error Amp. 2)
This error amplifier (Error Amp. 2) detects the output signal from the current detector amplifier
(Current Amp. 2), compares it with the +INE2 terminal (pin 7), and outputs a PWM control signal to control the
charge current.
In addition, an arbitrary loop gain can be set by connecting a feedback resistor and capacitor from the FB2
terminal (pin 3) to the -INE2 terminal (pin 6) of the error amplifier, enabling stable phase compensation to the
system.
(5) Error amplifier block (Error Amp. 3)
This error amplifier (Error Amp. 3) detects the output voltage from the DC/DC converter and outputs the PWM
control signal. The error amplifier inverting input pin is connected to the output voltage setting resistor in the IC,
eliminating the need for an external resistor for setting the output voltage. The MB3875 and MB3877 are set to
output voltage of 12.6 V (for a 3-cell battery) and 16.8 V (for a 4-cell battery), respectively; these ICs are suitable
for use in equipment that uses a lithium-ion battery.
In addition, an arbitrary loop gain can be set by connecting a feedback resistor and capacitor from the FB3
terminal (pin 15) to the -INE3 terminal (pin 16) of the error amplifier, enabling stable phase compensation to the
system.
Connecting a soft-start capacitor to the CS terminal (pin 22) prevents surge currents when the IC is turned on.
Using an error amplifier for soft start detection makes the soft start time constant, independent of the output load.
(6) Current detector amplifier block (Current Amp. 2)
The current detection amplifier (Current Amp. 2) detects a voltage drop which occurs between both ends of the
output sense resistor (RS) due to the flow of the charge current, using the +INC2 terminal (pin 24) and −INC2
terminal (pin 1). Then it outputs the signal amplified by 25 times to the error amplifier (Error Amp. 2) at the next
stage.
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