MP1591 – 2A, 32V, 330KHz 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 FB.
The voltage divider divides the output voltage
down by the ratio:
V
FB
=
V
OUT
×
R2
(R1
+
R2)
The inductance value can be calculated by the
equation:
L1
=
V
OUT
×
( V
IN
−
V
OUT
)
( V
IN
×
f
× ∆
I)
Where V
IN
is the input voltage, f is the switching
frequency and
∆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/
Core
Core
Model
Type
Material W
L
H
Sumida
CR75
CDH74
Open
Open
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
7.0
7.3
5.5
5.5
6.7
7.8
8.0
5.7
5.7
6.7
5.5
5.2
5.5
5.5
3.0
Where V
FB
is the feedback voltage and V
OUT
is
the output voltage.
Thus the output voltage is:
V
OUT
=
1.230
×
(R1
+
R2)
R2
A typical value for R2 can be as high as 100kΩ,
but 10kΩ is recommended. Using that value, R1
is determined by:
R1
≅
8.18
×
( V
OUT
−
1.230 )
CDRH5D28 Shielded
CDRH5D28 Shielded
CDRH6D28 Shielded
CDRH104R Shielded
Toko
D53LC
Type A
D75C
D104C
D10FL
Coilcraft
DO3308
DO3316
Open
Open
Shielded
Shielded
Shielded
Open
For example, for a 3.3V output voltage, R2 is
10kΩ, and R1 is 17kΩ.
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.
10.1 10.0 3.0
5.0
7.6
9.7
9.4
9.4
5.0
7.6
1.5
3.0
5.1
4.0
10.0 10.0 4.3
13.0 3.0
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.
MP1591 Rev. 2.3
9/27/2006
www.MonolithicPower.com
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5
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