MB3821 Datasheet, PDF (12/24 Pages) Fujitsu Component Limited. – 2-ch DC/DC Converter IC With Synchronous Rectifier

MB3821
s FUNCTIONAL DESCRIPTION
1. DC/DC Converter Function
(1) Reference voltage circuit (Ref)
The reference voltage circuit generates a temperature-compensated reference voltage (≅ 2.50 V) using the voltage
supplied from the power supply terminal (pin 23). This voltage is used as the reference voltage for the internal
circuits of the IC. The reference voltage can also be supplied to an external device from the VREF terminal (pin 24)
up to a maximum current of 1mA.
(2) Triangular-wave oscillator circuit (OSC)
By connecting a frequency setting capacitor and a resistor to the CT (pin 1) and the RT (pin 2) terminals, it is possible
to generate any desired triangular oscillation waveform.
The triangular wave is input to the PWM comparator within the IC.
(3) Error amplifier
This amplifier detects the output voltage of the DC/DC converter and outputs a PWM control signal accordingly.
The system can be provided with stable phase compensation by connecting a feedback resistor and capacitor
between the FB pin and the -IN pin of the error amplifier to create the desired level of loop gain.
Also, by connecting soft start capacitance to the CS terminal, which is the non inverted input pin for the error
amplifier, it is possible to prevent current surges when the power supply is started. By using the error amplifier for
soft start detection, it is possible to operate with a fixed soft start interval independent of the output load on the DC/
DC converter.
(4) PWM comparators (PWM Comp.1, PWM Comp.2)
PWM Comp.1 and PWM Comp.2 are voltage-pulse width modulators that control the output duty according to input
voltage.
PWM Comp.1 controls the pulse width on the main side output circuit, and PWM Comp.2 controls the pulse width
on the synchronous rectifier side output circuit. The triangular wave generated by the triangular wave oscillator is
compared to the error amplifier output voltage, and in the intervals when the error amplifier voltage is higher than
the triangular wave, the main side output transistor is switched on and the synchronous rectifier side output transistor
is switched off.
Also, PWM Comp.1 is set to a maximum duty cycle of approximately 90 % (normal mode).
(5) Output circuit (Drive)
The output circuits is comprised of a totem-pole configuration on both the main side and synchronous rectifier side,
and can drive an external N-ch MOSFET.
(6) Mode select circuit (SEL)
The SEL terminal (pin 7) can set either channel to normal mode or low power mode.
In low power mode the triangular oscillator frequency is set to approximately 1/10 of normal mode, reducing the
internal power consumption of the chip and enabling high efficiency power supply at light load levels.
(7) Power supply control circuit (CTL)
The CTL terminal (pin 18) is used for power supply on/off control (standby power consumption is 10 µA or less).
2. Protection Functions
(1) Under Voltage Lockout Circuit (UVLO)
Power-on surge states or sudden drops in supply voltage can cause a control IC to operate abnormally, leading to
destruction or damage to system elements. The under voltage lockout circuit detects the internal reference voltage
level from the supply voltage, and shuts off the output transistors so that the inactive interval becomes 100%, holding
the CSCP terminal (pin 22) voltage at “L” level.
Operation is restored as soon as the supply voltage exceeds the under voltage lockout circuit threshold voltage.
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