MB3885 Datasheet, PDF (13/29 Pages) Fujitsu Component Limited. – 1-channel DC/DC Converter IC with Synchronous Rectifier

MB3885
s FUNCTION DESCRIPTION
1. DC/DC Converter Function
(1) Reference Voltage Block
The reference voltage circuit takes the voltage feed from the power supply terminal (pin 10) and generates a
temperature compensated reference voltage (2.5 V typ.) , for use as the reference voltage for the power supply
control unit.
Also, an external load current can be obtained from the power supply at the VREF terminal (pin 18) , up to a
maximum of 1 mA.
(2) Triangular Wave Oscillator Block
A triangular waveform with amplitude 1.3 V to 1.9 V can be generated by connecting a timing capacitor and
resistor to the CT terminal (pin 4) and RT terminal (pin 5) , respectively.
The triangular oscillator waveform can be input to the IC’s internal PWM comparator, as well as supplied externally
from the CT terminal.
(3) Error Amp Block (Error Amp.)
The error Amp. is an amplifier that detects the output voltage from the DC/DC converter and outputs a PWM
control signal. The error Amp. has a broad in-phase input voltage range of 0 to Vcc−1.8 V that can be easily set
by the external power supply.
In addition, an arbitrary loop gain can be set up by connecting a feedback resistor and capacitor between the
error Amp. output pin and inverter input pin, providing stable phase compensation to the system.
Also, power-on rush current can be prevented by connecting a soft start capacitor between the error Amp. non-
inverted input pins CS terminal (pin 6) . The soft start function operates with a stable soft start time that is not
dependent on the output load of the DC/DC converter.
(4) PWM Comparator Block (PWM Comp.)
This is a voltage - pulse width modulator that controls the output duty according to the input voltage.
Main side :
Turns the output FET on in the intervals in which the error Amp. output voltage
is higher than the triangular wave voltage.
Synchronous rectifier side : Turns the output FET on in the intervals in which the triangular wave voltage
the is lower than error Amp. voltage.
(5) Output Block
The output block has totem pole configuration on both the main side and synchronous rectifier side, and can
drive an external N-ch MOSFET.
Also, the high output drive capability (700 mA Max. : duty ≤ 5%) provides high gate-source capacitor, enabling
the use of low on-resistor FET devices.
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