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MP2363 – 3A, 27V, 365KHz STEP-DOWN CONVERTER
MP2363 Rev. 1.0
6/15/2006
www.MonolithicPower.com
6
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
2006 MPS. All Rights Reserved.
APPLICATION INFORMATION
COMPONENT SELECTION (Refer to the
Typical Application Circuit on page 10)
Setting the Output Voltage
The output voltage is set using a resistive
voltage divider from the output voltage to FB
pin. The voltage divider divides the output
voltage down to the feedback voltage by the
ratio:
2
R
1
R
2
R
+
V
V
OUT
FB
=
Where V
FB
is the feedback voltage and V
OUT
is
the output voltage.
Thus the output voltage is:
2
R
2
R
1
R
92
.
V
OUT
+
×
=
A typical value for R2 can be as high as 100k
,
but a typical value is 10k
. Using that value, R1
is determined by:
)
k
)(
92
.
V
(
18
.
1
R
OUT
×
=
Inductor
The inductor is required to supply constant
current to the output load while being driven by
the switched input voltage. A larger value
inductor will result in less ripple current that will
result in lower output ripple voltage. However,
the larger value inductor will have a larger
physical size, higher series resistance, and/or
lower saturation current. A good rule for
determining the inductance to use is to allow
the peak-to-peak ripple current in the inductor
to be approximately 30% of the maximum
switch current limit. Also, make sure that the
peak inductor current is below the maximum
switch current limit. The inductance value can
be calculated by:
×
×
=
IN
OUT
V
L
S
OUT
I
V
1
f
V
L
Where V
IN
is the input voltage, f
S
is the 365KHz
switching frequency, and
I
L
is the peak-to-
peak inductor ripple current.
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated by:
×
×
×
+
=
IN
OUT
V
S
OUT
f
LOAD
LP
I
V
1
L
2
V
I
Where I
LOAD
is the load current and f
S
is the
365KHz switching frequency.
Table 1 lists a number of suitable inductors
from various manufacturers. The choice of
which style inductor to use mainly depends on
the price vs. size requirements and any EMI
requirement.
Table 1—Inductor Selection Guide
Package
Dimensions
(mm)
W
L
Vendor/
Model
Core
Type
Core
Material
H
Sumida
CR75
CDH74
CDRH5D28 Shielded
CDRH5D28 Shielded
CDRH6D28 Shielded
CDRH104R Shielded
Toko
D53LC
Type A
D75C
D104C
D10FL
Coilcraft
DO3308
DO3316
Open
Open
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
7.0
7.3
5.5
5.5
6.7
10.1 10.0
7.8
8.0
5.7
5.7
6.7
5.5
5.2
5.5
5.5
3.0
3.0
Shielded
Ferrite
5.0
5.0
3.0
Shielded
Shielded
Open
Ferrite
Ferrite
Ferrite
7.6
10.0 10.0
9.7
7.6
5.1
4.3
4.0
1.5
Open
Open
Ferrite
Ferrite
9.4 13.0
9.4 13.0
3.0
5.1
Output Rectifier Diode
The output rectifier diode supplies the current to
the inductor when the high-side switch is off. To
reduce losses due to the diode forward voltage
and recovery times, use a Schottky diode.