MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
MP1591 Rev. 2.3
www.MonolithicPower.com
6
9/21/2007
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The input capacitor value should be greater
than 10
μ
F. The capacitor can be electrolytic,
tantalum or ceramic. However, since it absorbs
the input switching current it requires an
adequate ripple current rating. Its RMS current
rating should be greater than approximately 1/2
of the DC load current.
For insuring stable operation C1 should be
placed as close to the IC as possible.
Alternately, a smaller high quality ceramic
0.1
μ
F capacitor may be placed closer to the IC
and a larger capacitor placed farther away. If
using this technique, it is recommended that the
larger capacitor be a tantalum or electrolytic
type. All ceramic capacitors should be placed
close to the MP1591.
Output Capacitor (C5)
The output capacitor is required to maintain the
DC output voltage. Low ESR capacitors are
preferred to keep the output voltage ripple low.
The characteristics of the output capacitor also
affect the stability of the regulation control
system. Ceramic, tantalum or low ESR
electrolytic capacitors are recommended. In the
case of ceramic capacitors, the impedance at
the switching frequency is dominated by the
capacitance, and so the output voltage ripple is
mostly independent of the ESR. The output
voltage ripple is estimated to be:
2
SW
LC
IN
RIPPLE
f
f
V
4
.
1
V
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
×
×
≅
Where V
RIPPLE
is the output ripple voltage, f
LC
is
the resonant frequency of the LC filter, f
SW
is the
switching frequency.
In the case of tantalum or low-ESR electrolytic
capacitors, the ESR dominates the impedance
at the switching frequency, and so the output
ripple is calculated as:
ESR
RIPPLE
R
I
V
×
Δ
≅
Where V
RIPPLE
is the output voltage ripple and
R
ESR
is the equivalent series resistance of the
output capacitors.
Output Rectifier Diode (D1)
The output rectifier diode supplies the current to
the inductor when the high-side switch is off. To
reduce losses due to the diode forward voltage
and recovery times, use a Schottky rectifier.
Table 2 provides some recommended Schottky
rectifiers based on the maximum input voltage
and current rating.
Table 2—Diode Selection Guide
2A Load Current
3A Load Current
V
IN
(Max)
Part
Number
Vendor
Part
Number
Vendor
15V 30BQ15 4
B220 1 B320 1
SK23 6 SK33
1,
6
20V
SR22 6 SS32 3
20BQ030 4 B330 1
B230 1 B340L 1
SK23 6
MBRD330
4,
5
SR23 3,
6 SK33 1,
6
30V
SS23 2,
3 SS33 2,
3
21DQ04 4 B340L 1
MBRS240L
5 MBRS340
4
SK24 6 SK34
1,
6
34V
SS24 2,
3 SS34 2,
3
Table 3 lists manufacturer’s websites.
Table 3—Schottky Diode Manufacturers
#
Vendor Web
Site
1
Diodes, Inc.
www.diodes.com
2
Fairchild Semiconductor www.fairchildsemi.com
3
General Semiconductor
www.gensemi.com
4
International Rectifier
www.irf.com
5
On Semiconductor
www.onsemi.com
6
Pan Jit International
www.panjit.com.tw
Choose a rectifier whose maximum reverse
voltage rating is greater than the maximum
input voltage, and whose current rating is
greater than the maximum load current.