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MP2355 – 3A, 23V, 380KHz STEP-DOWN CONVERTER
MP2355 Rev. 1.5
5/1/2006
www.MonolithicPower.com
6
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
2006 MPS. All Rights Reserved.
TM
APPLICATIONS INFORMATION
MP2355_TAC_F02
MP2355
LX
VIN
BST
3
2
4
6
7
5
1
8
FB
SS
COMP
GND
RUN
OUTPUT
3.3V / 3A
INPUT
4.75V to 23V
OPEN
AUTOMATIC
STARTUP
C6
OPEN
C4
10nF
C3
4.7nF
C5
10nF
D1
B330A
Figure 2—MP2355 with Murata 22
μ
F, 10V Ceramic Output Capacitor
COMPONENT SELECTION
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
V
V
OUT
FB
+
2
R
1
R
=
Thus the output voltage is:
2
R
2
R
1
R
22
.
V
OUT
+
×
=
Where V
FB
is the feedback voltage and V
OUT
is
the output voltage.
A typical value for R2 can be as high as 100k
,
but a typical value is 10k
. Using that value, R1
is determined by:
V
(
18
.
1
R
OUT
×
=
For example, for a 3.3V output voltage, R2 is
10k
, and R1 is 17k
.
)
k
)(
22
.
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 380KHz
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
I
LP
I
V
1
L
2
V
Where I
LOAD
is the load current.
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.
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