ir3841 International Rectifier Corp., ir3841 Datasheet - Page 17

• Manufacturer Part Number: ir3841
• Description: Integrated 8a Synchronous Buck Regulator
• Manufacturer: International Rectifier Corp.

Application Information
Design Example:
The following example is a typical application for
IR3841. The application circuit is shown on page
23.
Enabling the IR3841
As explained earlier, the precise threshold of
the Enable lends itself well to implementation of
a UVLO for the Bus Voltage.
For a typical Enable threshold of V
For a V
is a good choice.
Programming the frequency
For F
Table. 1.
Output Voltage Programming
Output voltage is programmed by reference
voltage and external voltage divider. The Fb pin
is the inverting input of the error amplifier, which
is internally referenced to 0.7V. The divider is
ratioed to provide 0.7V at the Fb pin when the
output is at its desired value. The output voltage
is defined by using the following equation:
02/09/09
V
V
o
R
in
s
2
=
(min)
in (min)
= 600 kHz, select R
V
=
ref
R
V
V
I
F
ΔV
*
o
1
in
o
s
∗
R
=10.2V, R
V
=
IR3841
⎛
⎜
⎜
⎝
o
=
=
=
1
1
in(
8
R
1.8
Enable
+
≤
600
+
12
A
2
min
R
R
R
V
54mV
V
8
9
V
2
EN
kHz
)
⎞
⎟
⎟
⎠
(
−
=
13.2V
V
1
..........
V
=4.99K and R
EN
EN
..........
max)
=
..........
t
1.2
= 23.7 kΩ, using
..........
EN
(6)
..........
V
= 1.2 V
2
in
=750 ohm
R
R
(5)
.....(7)
1
2
When an external resistor divider is connected to
the output as shown in figure 11.
Equation (5) can be rewritten as:
For the calculated values of R8 and R9 see
feedback compensation section.
Soft-Start Programming
The soft-start timing can be programmed by
selecting the soft-start capacitance value. From
(1), for a desired start-up time of the converter,
the soft start capacitor can be calculated by
using:
Where T
For a start-up time of 3.5ms, the soft-start
capacitor will be 0.099μF. Choose a 0.1μF
ceramic capacitor.
Bootstrap Capacitor Selection
To drive the Control FET, it is necessary to
supply a gate voltage at least 4V greater than
the voltage at the SW pin, which is connected
the source of the Control FET . This is achieved
by using a bootstrap configuration, which
comprises the internal bootstrap diode and an
external bootstrap capacitor (C6). The operation
of the circuit is as follows: When the lower
MOSFET is turned on, the capacitor node
connected to SW is pulled down to ground. The
capacitor charges towards V
internal bootstrap diode, which has a forward
voltage drop V
bootstrap capacitor C6 is approximately given as
When the upper MOSFET turns on in the next
cycle, the capacitor node connected to SW rises
to the bus voltage V
C6 is appropriately chosen,
Fig. 11. Typical application of the IR3841 for
R
C
9
SS
=
V
R
(
start
c
μF
programming the output voltage
8
IR3624
≅
∗
IR3841
)
is the desired start-up time (ms).
V
⎛
⎜
⎜
⎝
=
V
cc
T
V
o
D
−
start
−
ref
V
. The voltage V
V
ref
Fb
D
(
in
⎞
⎟
⎟
⎠
ms
..........
. However, if the value of
..........
IR3841MPbF
)
×
0.02857
..........
..........
V
cc
OUT
..........
......
R
R
c
9
8
through the
..........
across the
(10)
....
(8)
(9)
17