MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
MP1591 Rev. 2.3
www.MonolithicPower.com
5
9/21/2007
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© 2007 MPS. All Rights Reserved.
APPLICATION INFORMATION
COMPONENT SELECTION
Setting the Output Voltage
The output voltage is set using a resistive
voltage divider from the output voltage to FB.
The voltage divider divides the output voltage
down 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
(
230
.
1
V
OUT
+
×
=
A typical value for R2 can be as high as 100k
Ω
,
but 10k
Ω
is recommended. Using that value, R1
is determined by:
)
230
.
1
V
(
18
.
8
1
R
OUT
−
×
≅
For example, for a 3.3V output voltage, R2 is
10k
Ω
, and R1 is 17k
Ω
.
Inductor (L1)
The inductor is required to supply constant
current to the output load while being driven by
the switched input voltage. A larger value
inductor results in less ripple current that results
in lower output ripple voltage. However, the
larger value inductor has a larger physical size,
higher series resistance, and/or lower
saturation current. Choose an inductor that
does not saturate under the worst-case load
conditions. A good rule to use for determining
the inductance is to allow the peak-to-peak
ripple current in the inductor to be
approximately 30% of the maximum load
current that the IC can provide. Also, make sure
that the peak inductor current (the load current
plus half the peak-to-peak inductor ripple
current) is below the 2.3A minimum current limit.
The inductance value can be calculated by the
equation:
)
I
f
V
(
)
V
V
(
V
1
L
IN
OUT
IN
OUT
Δ
×
×
−
×
=
Where V
IN
is the input voltage, f is the switching
frequency and
Δ
I is the peak-to-peak inductor
ripple current.
Table 1 lists a number of suitable inductors
from various manufacturers.
Table 1—Inductor Selection Guide
Package
Dimensions
(mm)
Vendor/
Model
Core
Type
Core
Material
W
L
H
Sumida
CR75 Open
Ferrite
7.0
7.8
5.5
CDH74 Open Ferrite
7.3
8.0
5.2
CDRH5D28 Shielded
Ferrite 5.5
5.7
5.5
CDRH5D28 Shielded
Ferrite 5.5
5.7
5.5
CDRH6D28 Shielded
Ferrite 6.7
6.7
3.0
CDRH104R Shielded
Ferrite 10.1 10.0 3.0
Toko
D53LC
Type A
Shielded
Ferrite 5.0
5.0
3.0
D75C Shielded
Ferrite 7.6
7.6
5.1
D104C Shielded
Ferrite 10.0 10.0 4.3
D10FL Open Ferrite
9.7
1.5
4.0
Coilcraft
DO3308 Open Ferrite 9.4 13.0 3.0
DO3316 Open Ferrite 9.4 13.0 5.1
Input Capacitor (C1)
The input current to the step-down converter is
discontinuous, and so a capacitor is required to
supply the AC current to the step-down
converter while maintaining the DC input
voltage. A low ESR capacitor is required to
keep the noise at the IC to a minimum. Ceramic
capacitors are preferred, but tantalum or low
ESR electrolytic capacitors may also suffice.