MB3886PFV FUJITSU [Fujitsu Component Limited.], MB3886PFV Datasheet - Page 12

• Manufacturer Part Number: MB3886PFV
• Description: 2-Channel DC/DC Converter IC with Overcurrent Protection Symmetrical-Phase Type
• Manufacturer: FUJITSU [Fujitsu Component Limited.]

12
MB3886
1. DC/DC Converter Functions
(1) Reference voltage Block
(2) Triangular waveform oscillator Block
(3) Error Amp. Block (Error Amp.)
(4) PWM comparator Block (PWM Comp.)
(5) Output Block
FUNCTIONAL DESCRIPTION
The reference voltage circuit generates a temperature-compensated reference voltage (typically 3.5 V) using
the voltage supplied from the power supply terminal (pin 26) . 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 27) .
The triangular waveform oscillator incorporates a timing capacitor and a timing resistor connected respectively
to the CT terminal (pin 4) and RT terminal (pin 5) to generate oscillating triangular waveforms CT1 (amplitude
of 1.3 V to 2.1 V) and CT2 (amplitude of 1.3 V to 2.1 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 oscillating triangular waveforms are input to the IC’s internal PWM comparator and can be output to an
external device through the CT terminal.
The error amplifier detects the DC/DC converter output voltage and outputs PWM control signals. It supports
a wide range of in-phase input voltages from 0 V to “Vcc 1.8 V”, allowing easy setting from the external power
supply.
In addition, an arbitrary loop gain can be set by connecting a feedback resistor and capacitor from the output
terminal to inverting input terminal of the error amplifier, enabling stable phase compensation to be provided for
the system.
The PWM comparator is a voltage-pulse width modulator that controls the output duty depending on the input
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 sides : 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