MB3874/MB3876

15

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 21) 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

as well as a signal to the dynamically controlled charging detection block (MASK Comp.).

In addition, an arbitrary loop gain can be set by connecting a feedback resistor and capacitor from the FB1

terminal (pin 7) to the

−INE1 terminal (pin 8) of the error amplifier, enabling stable phase compensation to the

system.

(4) Error amplifier block (Error Amp.2, 3)

These error amplifiers (Error Amp.2, Error Amp.3) detect the output signals from the current detector amplifiers

(Current Amp.1, Current Amp.2), compare them with the +INE2 terminal (pin 4) and +INE3 terminal (pin 9), and

output PWM control signals to control the charge current.

In addition, these amplifiers allow an arbitrary loop gain to be set by connecting a feedback resistor and capacitor

from the FB2 terminal (pin 2) to

−INE2 terminal (pin 3) and from the FB3 terminal (pin 11) to −INE3 terminal (pin

10) of the error amplifiers, enabling stable phase compensation to the system.

(5)

Error amplifier block (Error Amp.4, 5)

This error amplifier (Error Amp.4, Error Amp.5) detects the output voltage from the switching rerulator and outputs

the PWM control signal. The error amplifier inverted 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 MB3874 and MB3876

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 FB4

terminal (pin 15) to the

−INE4 terminal (pin 16) to the −INE5 terminal (pin 14) 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.1, 2)

The current detection amplifier (Current Amp.1, Current Amp.2) detects a voltage drop which occurs between

both ends of the output sense resistor (RS1) due to the flow of the charge current, using the +INC1 terminal

(pin 24) and

−INC1 terminal (pin 1). Then it outputs the signal amplified by 25 times to the error amplifier (Error Amp.2) at

the next stage.The amplifiers also detect a voltage drop which occurs at both ends of the output sense resistor