MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
The input capacitor value should be greater
than 10µF. 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
0.1µF 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:
⎛
f
V
RIPPLE
≅
1.4
×
V
IN
× ⎜
LC
⎜
f
⎝
SW
⎞
⎟
⎟
⎠
2
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
V
IN
(Max)
15V
20V
2A Load Current
Part
Vendor
Number
30BQ15
4
B220
1
SK23
6
SR22
6
20BQ030
4
B230
1
SK23
6
SR23
3, 6
SS23
2, 3
21DQ04
4
MBRS240L
5
SK24
6
SS24
2, 3
3A Load Current
Part
Vendor
Number
B320
SK33
SS32
B330
B340L
MBRD330
SK33
SS33
B340L
MBRS340
SK34
SS34
1
1, 6
3
1
1
4, 5
1, 6
2, 3
1
4
1, 6
2, 3
30V
34V
Table 3 lists manufacturer’s websites.
Table 3—Schottky Diode Manufacturers
#
1
2
3
4
5
6
Vendor
Diodes, Inc.
Fairchild Semiconductor
General Semiconductor
International Rectifier
On Semiconductor
Pan Jit International
Web Site
www.diodes.com
www.fairchildsemi.com
www.gensemi.com
www.irf.com
www.onsemi.com
www.panjit.com.tw
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:
V
RIPPLE
≅ ∆
I
×
R
ESR
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.
Where V
RIPPLE
is the output voltage ripple and
R
ESR
is the equivalent series resistance of the
output capacitors.
MP1591 Rev. 2.3
9/27/2006
www.MonolithicPower.com
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6
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