FUJITSU MB3887 - Manual (Page 16)

MB3887
16
■ 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 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 1mA from the reference voltage VREF
terminal (pin 6) .
(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 17) .
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 9) , 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 7) and -INE1 terminal (pin 8) , providing stable phase compensation to the system.
(4) Error amplifier block (Error Amp2)
This amplifier (Error Amp2) detects voltage drop of 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 FB2
terminal (pin 5) to the
−
INE2 terminal (pin 4) of the error amplifier, enabling stable phase compensation to the
system.
(5) Error amplifier block (Error Amp3)
This error amplifier (Error Amp3) 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 inverted 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 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 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.
(6) Current detection amplifier block (Current Amp1)
The current detection amplifier (Current Amp1) detects a voltage drop which occurs between both ends of the
output sense resistor (R
S
) due to the flow of the charge current, using the
+
INC1 terminal (pin 13) and
−
INC1
terminal (pin 12) . Then it outputs the signal amplified by 20 times to the error amplifier (Error Amp1) at the next
stage.