iru3047 International Rectifier Corp., iru3047 Datasheet - Page 5

• Manufacturer Part Number: iru3047
• Description: Dual Synchronous Controller With Current Sharing Circuitry Controller
• Manufacturer: International Rectifier Corp.

THEORY OF OPERATION
Introduction
The IRU3047 is designed for multi-outputs applications.
It includes two synchronous buck controllers and a lin-
ear regulator controller. The two synchronous controller
operates with fixed frequency voltage mode and can be
configured as two independent controller or 2-phase con-
troller with current sharing. The timing of the IC is pro-
vided through an internal oscillator circuit. These are two
out of phase oscillators with 200KHz switching frequency.
Independent Mode
In this mode the IRU3047 provides two independent out-
puts with either common or different input voltages. The
output voltage of the individual channel is set and con-
trolled by the output of the error amplifier, this is the
amplified error signal from the sensed output voltage and
the reference voltage. This voltage is compared to the
ramp signal and generates fixed frequency pulses of vari-
able duty-cycle, which drives the two N-channel exter-
nal MOSFETs.
Current Sharing Mode
In the current sharing mode, the two converter’s outputs
tied together and provide one single output (see Figure
1). In this mode, one control loop acts as a master and
sets the output voltage as a regular Voltage Mode buck
controller and the other control loop acts as a slave and
monitors the current information for current sharing. The
current sharing is programmable and sets by using two
external resistors in output currents’ path. The slave's
error amplifier, error amplifier 2 (see the block diagram)
measures the voltage drops across the current sense
resistors, the differential of these signals is amplified
and compared with the ramp signal and generate the
fixed frequency pulses of variable duty cycle to match
the output currents.
Out of Phase Operation
The IRU3047 drives its two output stages 180
phase. In 2-phase configuration, the two inductor ripple
currents cancel each other and result to a reduction of
the output current ripple and contributes to a smaller
output capacitors for the same ripple voltage require-
ment.
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In application with single input voltage, the 2-phase con-
figuration reduces the input ripple current. This results in
much smaller RMS current in the input capacitor and
reduction of input capacitor.
Soft-Start
The IRU3047 has a programmable soft start to control
the output voltage rise and limit the current surge at the
start-up. To ensure correct start-up, the soft-start se-
quence initiates when the Vcc, VcH1 and VcH2 rise
above their threshold and generates the Power On Re-
set (POR) signal. Soft-start function operates by sourc-
ing an internal current to charge an external capacitor to
about 3V. Initially, the soft-start function clamps the E/
A’s output of the PWM converter. As the charging volt-
age of the external capacitor ramps up, the PWM sig-
nals increase from zero to the point the feedback loop
takes control.
Shutdown
The converter can be shutdown by pulling the soft-start
pin below 0.5V. This can be easily done by using an
external small signal transistor. During shutdown the
MOSFET drivers and the LDO controller turn off.
Power Good
The IRU3047 provides a power good signal. This is an
open collector output and it is pulled low if the output
voltages are not within the specified threshold. This pin
can be left floating if not used.
Short-Circuit Protection
The outputs are protected against the short circuit. The
IRU3047 protects the circuit for shorted output by sens-
ing the output voltages. The IRU3047 shuts down the
PWM signals and LDO controller, when the output volt-
ages drops below the set values.
Under-Voltage Lockout
The under-voltage lockout circuit assures that the
MOSFET driver outputs and LDO controller remain in
the off state whenever the supply voltages drop below
set parameters. Normal operation resumes once the
supply voltages rise above the set values.
IRU3047
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