TM
MP1570 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
peak inductor current is below the maximum
switch current limit. The inductance value can
be calculated by:
L
=
⎞
V
OUT
⎛
V
× ⎜
1
−
OUT
⎟
f
S
×
∆I
⎜
V
IN
⎟
⎝
⎠
Where V
IN
is the input voltage, f
S
is the 340KHz
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:
I
LP
=
I
LOAD
+
⎛
V
OUT
V
× ⎜
1
−
OUT
2
×
f
S
×
L
⎜
V
IN
⎝
⎞
⎟
⎟
⎠
Optional Schottky Diode
During the transition between high-side switch
and low-side switch, the body diode of the low-
side power MOSFET conducts the inductor
current. The forward voltage of this body diode
is high. An optional Schottky diode may be
paralleled between the SW pin and GND pin to
improve overall efficiency. Table 2 lists example
Schottky diodes and their Manufacturers.
Table 2—Diode Selection Guide
Part Number
B130
SK13
MBRS130
Voltage/Current
Rating
30V, 1A
30V, 1A
30V, 1A
Vendor
Diodes, Inc.
Diodes, Inc.
International
Rectifier
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.
Table 1—Inductor Selection Guide
Package
Dimensions
(mm)
W
L
H
7.0 7.8
7.3 8.0
5.5 5.7
5.5 5.7
6.7 6.7
10.1 10.0
5.0
5.0
5.5
5.2
5.5
5.5
3.0
3.0
3.0
5.1
4.3
4.0
3.0
5.1
Vendor/
Model
Sumida
CR75
CDH74
CDRH5D28
CDRH5D28
CDRH6D28
CDRH104R
Toko
D53LC
Type A
D75C
D104C
D10FL
Coilcraft
DO3308
DO3316
Core
Type
Open
Open
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Open
Open
Open
Core
Material
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Input Capacitor
The input current to the step-down converter is
discontinuous, therefore a capacitor is required
to supply the AC current to the step-down
converter while maintaining the DC input
voltage. Use low ESR capacitors for the best
performance. Ceramic capacitors are preferred,
but tantalum or low-ESR electrolytic capacitors
may also suffice. Choose X5R or X7R
dielectrics when using ceramic capacitors.
Since the input capacitor (C1) absorbs the input
switching current it requires an adequate ripple
current rating. The RMS current in the input
capacitor can be estimated by:
I
C1
=
I
LOAD
×
V
OUT
⎛
V
OUT
⎞
⎟
×⎜
1
−
⎜
V
IN
⎝
V
IN
⎟
⎠
The worst-case condition occurs at V
IN
= 2V
OUT
,
where:
I
C1
=
I
LOAD
2
7.6 7.6
10.0 10.0
9.7 1.5
9.4
9.4
13.0
13.0
For simplification, choose the input capacitor
whose RMS current rating greater than half of
the maximum load current.
MP1570 Rev. 1.5
1/31/2006
www.MonolithicPower.com
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© 2006 MPS. All Rights Reserved.
6
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