MP1580 – 2A, 380KHz STEP-DOWN CONVERTER
MP1580 Rev. 3.0 www.MonolithicPower.com 6
10/10/2007 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2007 MPS. All Rights Reserved.
TM
Input Capacitor
The input current to the step-down converter is
discontinuous, 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 will also suffice.
The input capacitor value should be greater
than 10F. 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.
To ensure stable operation, C1 should be
placed as close to the IN pin as possible.
Alternately, a smaller high quality ceramic
0.1F capacitor may be placed closer to the IN
pin and a larger capacitor placed further away.
If using this technique, it is recommended that
the larger capacitor be a tantalum or electrolytic
type capacitor. All ceramic capacitors should be
placed close to the MP1580.
Output Capacitor
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 oscillator frequency is dominated by the
capacitance, so the output voltage ripple is
mostly independent of the ESR. The output
voltage ripple is estimated to be:
2
LC
INRIPPLE
f
f
V4.1V
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
××≅
Where V
RIPPLE
is the output ripple voltage, f
LC
is
the resonant frequency of the LC filter and f is
the oscillator frequency.
In the case of tantalum or low-ESR electrolytic
capacitors, the ESR dominates the impedance
at the oscillator frequency, therefore the output
ripple is calculated as:
ESRRIPPLE
RIV ×Δ≅
Where V
RIPPLE
is the output voltage ripple and
R
ESR
is the equivalent series resistance of the
output capacitors.
Output Rectifier Diode
The output rectifier diode supplies the current to
the inductor when the upper transistor M1 is off.
To reduce losses due to the diode forward
voltage and recovery times, use a Schottky
rectifier.
Table 2 provides the Schottky rectifier part
numbers based on the maximum input voltage
and current rating.
Table 2—Schottky Rectifier Selection Guide
2A Load Current
V
IN
(Max)
Part Number Vendor
15V 30BQ015 4
B220 1
SK23 6
20V
SR22 6
20BQ030 4
B230 1
SK23 6
SR23 3, 6
26V
SS23 2, 3
Table 3 lists some rectifier manufacturers.
Table 3—Schottky Diode Manufacturers
Vendor Web Site
Diodes, Inc. www.diodes.com
Fairchild Semiconductor www.fairchildsemi.com
General Semiconductor www.gensemi.com
International Rectifier www.irf.com
On Semiconductor www.onsemi.com
Pan Jit International www.panjit.com.tw
Choose a rectifier that has a maximum reverse
voltage rating greater than the maximum input
voltage, and a current rating greater than the
maximum load current.
