iru3047 International Rectifier Corp., iru3047 Datasheet - Page 6

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

IRU3047
APPLICATION INFORMATION
Design Example:
The following example is a typical application for IRU3047
in current sharing mode. The schematic is Figure 12 on
page 16.
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:
When an external resistor divider is connected to the
output as shown in Figure 3.
Equation (1) can be rewritten as:
If the high value feedback resistors are used, the input
bias current of the Fb pin could cause a slight increase
in output voltage. The output voltage set point can be
more accurate by using precision resistor.
6
Figure 3 - Typical application of the IRU3047 for
V
V
V
I
DV
f
V
R
OUT
S
IN1(MASTER)
IN2(SLAVE)
OUT1
OUT1
6
= 200KHz
= R
OUT
= 12A
= 1.5V
= V
5
= 75mV
programming the output voltage.
3
= 5V
REF
(
= 12V
IRU3047
3 1+
V
V
OUT1
REF
(
- 1
Fb1
R
R
6
5
)
)
Will result to:
V
V
R
R
OUT1
REF
5
6
= 1K
= 1K
---(1)
V
= 1.25V
OUT1
= 2.5V
R
R
5
6
www.irf.com
Soft-Start Programming
The soft-start timing can be programmed by selecting
the soft start capacitance value. The start up time of the
converter can be calculated by using:
For a start-up time of 75ms, the soft-start capacitor will
be 1mF. Choose a ceramic capacitor at 1mF.
Boost Supply Vc
To drive the high-side switch it is necessary to supply a
gate voltage at least 4V greater than the bus voltage.
This is achieved by using a charge pump configuration
as shown in Figure 1. The capacitor is charged up to
approximately twice the bus voltage. A capacitor in the
range of 0.1mF to 1mF is generally adequate for most
applications.
Sense Resistor Selection
These resistors will determine the current sharing
between two channels. The relationship between the
Master and Slave output currents is expressed by:
Input Capacitor selection
The input filter capacitor should be based on how much
ripple the supply can tolerate on the DC input line. The
ripple current generated during the on time of control
MOSFET should be provided by input capacitor. The RMS
value of this ripple is expressed by:
R
I
t
Where:
C
For an equal current sharing, R
Choose R
Where:
D is the Duty Cycle, simply D=V
I
I
For V
Results to: I
And for V
Results to: I
RMS
RMS
OUT
START
SEN1
SS
= I
is the soft-start capacitor (mF)
is the RMS value of the input capacitor current.
is the output current for each channel.
3I
IN1
= 753Css (ms)
OUT
MASTER
=12V, I
IN2
SEN1
=5V, I
RMS1
RMS2
D3(1-D)
=R
= R
OUT1
=2.43A
SEN2
=3A
SEN2
OUT2
=6A and D1=0.208
=5mV
3I
=6A and D2=0.5
SLAVE
---(4)
---(2)
SEN1
OUT
---(3)
=R
/V
SEN1
IN.
09/09/02
Rev. 1.0