MB3889 Fujitsu Microelectronics, Inc., MB3889 Datasheet - Page 14

• Manufacturer Part Number: MB3889
• Description: 2-channel Dc/dc Converter Ic With Overcurrent Protection Symmetrical-phase Type
• Manufacturer: Fujitsu Microelectronics, Inc.

14
MB3889
1. DC/DC Converter Functions
(5) Output block
(1) Reference voltage block (Ref)
(2) Triangular-wave oscillator block (OSC)
(3) Error amplifier block (Error Amp1, Error Amp2)
(4) PWM comparator (PWM Comp.) block
FUNCTIONS
The reference voltage circuit generates a temperature-compensated reference voltage (typically 3.5 V) using
the voltage supplied from the power supply terminal (pin 24) . The voltage is used as the reference voltage for
the IC’s internal circuitry.
The reference voltage can be used to supply a load current of up to 1 mA to an external device through the
VREF terminal (pin 8) .
The triangular waveform oscillator incorporates a timing capacitor connected respectively to the RT terminal
(pin 5) to generate triangular oscillation waveforms CT1 (amplitude of 1.8 V to 3.0 V) and CT2 (amplitude of 1.8
V to 3.0 V in antiphase with CT1). The symmetrical-phase system using the two opposite-phase triangular waves
reduces the input ripple current, resulting in a smaller input capacitor.
The triangular oscillation waveforms are input to the IC’s internal PWM comparator.
The error amplifier detects the DC/DC converter output voltage and outputs PWM control signals. By connecting
a feedback resistor and capacitor between the output terminal and inverted input terminal, it is possible to create
any desired level of loop gain, thereby providing stable phase compensation to the system.
Also, it is possible to prevent rush current at power supply start-up by connecting a soft-start capacitor to the
CS1 terminal (pin 3) or CS2 terminal (pin 13), the non-inverted input terminal for Error Amp. The use of Error
Amp for soft-start detection makes it possible for a system to operate on a fixed soft-start time that is independent
of the output load on the DC/ DC converter.
The PWM comparator is a voltage-pulse width modulator that controls the output duty depending on the
input/output voltage.
Main side : Turns the output transistor on in the intervals in which the error amplifier output voltage is higher
than the triangular wave voltage.
Synchronous rectification side : Turns the output transistor on in the intervals in which the error amplifier output
voltage is lower than the triangular wave valtage.
The output circuits on the main side and on the synchronous rectification side are both in the totem pole
configuration, capable of driving an external N-ch MOS FET.
In addition, because the output drive ability (700 mA Max : Duty
large and the FET of low ON resistor can be used.
5 ) is high, the gate
source capacity is