MP1583 – 3A, 23V, 385KHz STEP-DOWN CONVERTER
APPLICATION INFORMATION
COMPONENT SELECTION
Setting the Output Voltage
The output voltage is set using a resistive
voltage divider from the output voltage to the
FB pin. The voltage divider divides the output
voltage down to the feedback voltage by the
ratio:
Choose an inductor that will not saturate under
the maximum inductor peak current.
The peak inductor current can be calculated by:
I
LP
=
I
LOAD
+
⎛
V
V
OUT
× ⎜
1
−
OUT
2
×
f
S
×
L
⎜
V
IN
⎝
⎞
⎟
⎟
⎠
V
FB
=
V
OUT
R
2
R
1
+
R
2
Where I
LOAD
is the load current.
Table 1 lists a number of suitable inductors
from various manufacturers. The choice of
which inductor to use mainly depends on the
price vs. size requirements and any EMI
requirements.
Table 1—Inductor Selection Guide
Package
Dimensions
(mm)
W
7.0
7.3
5.5
5.5
6.7
L
7.8
8.0
5.7
5.7
6.7
H
5.5
5.2
5.5
5.5
3.0
3.0
3.0
5.1
4.3
4.0
3.0
5.1
Where V
FB
is the feedback voltage and V
OUT
is
the output voltage.
Thus the output voltage is:
V
OUT
R
1
+
R
2
=
1.22
V
×
R
2
A typical value for R2 can be as high as 100kΩ,
but a typical value is 10kΩ. Using that value, R1
is determined by:
Vendor/
Model
Sumida
CR75
CDH74
Core
Type
Open
Open
Core
Material
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
R
1
=
8.18
×
(
V
OUT
−
1.22
V
)(
k
Ω
)
For example, for a 3.3V output voltage, R2 is
10kΩ, and R1 is 17kΩ.
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 and
lower output ripple voltage. However, larger
value inductors 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 inductor
peak-to-peak ripple current 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:
CDRH5D28 Shielded
CDRH5D28 Shielded
CDRH6D28 Shielded
CDRH104R Shielded
Toko
D53LC
Type A
D75C
D104C
D10FL
Coilcraft
DO3308
DO3316
Open
Open
Shielded
Shielded
Shielded
Open
10.1 10.0
5.0
7.6
9.7
9.4
9.4
5.0
7.6
1.5
13.0
13.0
10.0 10.0
L
=
⎛
V
V
OUT
× ⎜
1
−
OUT
f
S
×
ΔI
L
⎜
V
IN
⎝
⎞
⎟
⎟
⎠
Where V
IN
is the input voltage, f
S
is the 385KHz
switching frequency and
ΔI
L
is the peak-to-peak
inductor ripple current.
MP1583 Rev. 3.1
6/20/2011
www.MonolithicPower.com
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6
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