IRDC3821A International Rectifier, IRDC3821A Datasheet - Page 16

• Manufacturer Part Number: IRDC3821A
• Description: BOARD EVAL SYNC BUCK CONVERTER
• Manufacturer: International Rectifier

11/04/08
Programming the Current-Limit
The Current-Limit threshold can be set by
connecting a resistor (R
low-side MOSFET to the OCSet pin. The
resistor can be calculated by using equation (3).
The
coefficient and it should be considered for the
worse case operation. This resistor must be
placed close to the IC, place a small ceramic
capacitor from this pin to power ground (PGnd)
for noise rejection purposes.
Setting the Power Good Threshold
Power Good threshold can be programmed by
using two external resistors (see figure 16).
The following formula can be used to set the
threshold:
Where:
0.38V is reference of the internal comparator
0.9*Vout is selectable threshold for power good,
for this design it is 1.62V.
Select R
Using (18): R
Select R
Use a pull up resistor (4.99K) from PGood pin to
Vcc.
R
where
υ
where
R
Note
I
I
Δ
Δ
I
R
I
SET
o
SET
DS(on)
SET
: i
i
1
2
DS(on)
OCSet
:
R
:
=10KOhm
=3.09K
Temperatur
=
Max
2
Inductor
=
=
I =
V (
=
:
(I
(9A
if
=
=
Use
L(critical
in
:
:
o
0
2
Output
10.5mΩ
R
5V
-
=3.06KOhm
∗
9 .
V
. 1
∗
7
has
o
*V
14.5
1.5)
=
)
) 5
is
)
. 0
=
ripple
∗
12.4KΩ
out
used
R
V
38
+
e
+
Current
in
OCSet
mΩ
-
Δ
Dependency
∗
2
V
2.1A
. 0
R
∗
υ
i
V
a
L
DS(on)
current
38
o
for
=
for
∗
∗
I
10.5mΩ
V
F
OCSet
=
SET
V
positive
s
low
15.6A
cc
*R
) from drain of the
1
-
side
-
∗
-(3)
1
--(
.5
MOSFET
19
=
temperature
15.75mΩ
)
Layout Consideration
The layout is very important when designing high
frequency switching converters. Layout will affect
noise pickup and can cause a good design to
perform with less than expected results.
Start to place the power components, making all
the connection in the top layer with wide, copper
filled areas.
The inductor, output capacitor and the IR3821A
should be as close to each other as possible.
This helps to reduce the EMI radiated by the
power traces due to the high switching currents
through them. Place the input capacitor directly to
the Vin pin of IR3821A. To reduce the ESR
replace the single input capacitor with two
parallel units.
The feedback part of the system should be kept
away from the inductor and other noise sources.
The
capacitors for Vcc and Vc should be close to their
respective pins. It is important to place the
feedback
resistors and compensation components close to
Fb and Comp pins.
In a multilayer PCB use one layer as a power
ground plane and have a control circuit ground
(analog ground), to which all signals are
referenced. The goal is to localize the high
current path to a separate loop that does not
interfere with the more sensitive analog control
function. These two grounds must be connected
together on the PC board layout at a single point.
The Power QFN is a thermally enhanced
package. Based on thermal performance it is
recommended to use at least a 4-layers PCB. To
effectively remove heat from the device the
exposed pad should be connected to the ground
plane using vias.
critical
components
bypass
IR3821AMPbF
components
including
PD-60332
such
feedback
as
16