IRU3048CF IRF [International Rectifier], IRU3048CF Datasheet - Page 5

• Manufacturer Part Number: IRU3048CF
• Description: DUAL SYNCHRONOUS PWM CONTROLLER CIRCUITRY AND LDO CONTROLLER
• Manufacturer: IRF [International Rectifier]

THEORY OF OPERATION
Introduction
The IRU3048 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 is con-
figured as two independent controllers. The timing of the
IC is provided through an internal oscillator circuit. These
are two out of phase oscillators.
Soft-Start
The IRU3048 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.
Out of Phase Operation
The IRU3048 drives its two output stages 1808 out of
APPLICATION INFORMATION
Design Example:
The following example is a typical application for IRU3048
in current sharing mode. The schematic is Figure 9 on
page 12.
PWM Section
Output Voltage Programming
Output voltage is programmed by reference voltage and
external voltage divider. The Fb1 pin is the inverting input
of the error amplifier, which is internally referenced to
1.25V. The divider is ratioed to provide 1.25V at the Fb1
pin when the output is at its desired value. The output
voltage is defined by using the following equation:
Rev. 1.7
09/12/02
For Switcher
V
V
I
V
V
I
DV
f
OUT1
OUT2
S
IN1
OUT1
IN2
OUT2
= 200KHz
OUT
= 12V
= 5V
= 4A
= 4A
= 3.3V
= 1.8V
= 75mV
For Linear Regulator
V
V
I
OUT3
IN3
OUT3
= 3.3V
= 2A
= 2.5V
www.irf.com
phase. In application with single input voltage, the out of
phase operation reduces the input ripple current. This
results in much smaller RMS current in the input ca-
pacitor and reduction of input capacitors.
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.
Short-Circuit Protection
The outputs are protected against the short circuit. The
IRU3048 protects the circuit for shorted output by sens-
ing the output voltages. The IRU3048 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.
When an external resistor divider is connected to the
output as shown in Figure 3.
Equation (1) can be rewritten as:
Figure 3 - Typical application of the IRU3048 for
V
R
Will result to:
V
V
R
OUT1
6
OUT1
REF
8
=1K, R
= R
=1.25V
=3.3V
= V
8
programming the output voltage.
3
6
REF
IRU3048
(
=1.64K
3 1+
V
V
OUT1
( )
REF
Fb1
- 1
R
R
6
8
)
V
V
R
OUT2
REF
15
=1K, R
=1.25V
---(1)
=1.8V
V
OUT1
IRU3048
14
R
R
=442V
8
6
5