MB3888 Datasheet, PDF (14/30 Pages) Fujitsu Component Limited. – DC/DC Converter IC for Charging Li-ion Battery

MB3888
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 16) to generate a tem-
perature-compensated, stable voltage (5.0 V Typ) used as the reference supply voltage for the IC’s internal
circuitry.
This terminal can also be used to obtain a load current to a maximum of 1 mA from the reference voltage VREF
terminal (pin 4) .
(2) Triangular wave oscillator block (OSC)
The triangular wave oscillator builds the capacitor for frequency setting into, and generates the triangular wave
oscillation waveform by connecting the frequency setting resistor with the RT terminal (pin 2) .
The triangular wave is input to the PWM comparator on the IC.
(3) Error amplifier block (Error Amp1)
This amplifier detects the output signal from the current detection amplifier (Current amp1) , compares this to
the +INE1 terminal (pin 13) , and outputs a PWM control signal to be used in controlling the charging current.
In addition, an arbitrary loop gain can be set up by connecting a feedback resistor and capacitor between the
FB1 terminal (pin 15) and −INE1 terminal (pin 14) , providing stable phase compensation to the system.
Connecting a soft-start capacitor to the CS terminal (pin 20) prevents rush 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.
(4) Error amplifier block (Error Amp2)
This amplifier (Error Amp2) detects the output voltage from the DC/DC converter and outputs the PWM control
signal. External output voltage setting resistors can be connected to the error amplifier inverse input terminal to
set the desired level of output voltage from 1 cell to 4 cells.
In addition, an arbitrary loop gain can be set by connecting a feedback resistor and capacitor from the FB2
terminal (pin 6) to the −INE2 terminal (pin 7) of the error amplifier, enabling stable phase compensation to the
system.
Connecting a soft-start capacitor to the CS terminal (pin 20) prevents rush 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.
(5) Current detector amplifier block (Current Amp)
The current detection amplifier (Current Amp) 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 +INC terminal (pin 11) and −INC
terminal (pin 10) . Then it outputs the signal amplified by 20 times to the error amplifier (Error Amp1) at the next
stage.
(6) PWM comparator block (PWM Comp.)
The PWM comparator circuit is a voltage-pulse width converter for controlling the output duty of the error
amplifiers (Error Amp1 and Error Amp2) and DTC terminal (pin 5) depending on their output voltage.
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