MP1583 – 3A, 23V, 385KHz STEP-DOWN CONVERTER
Output Rectifier Diode
The output rectifier diode supplies the current to
the inductor when the high-side switch is off.
Use a Schottky diode to reduce losses due to
the diode forward voltage and recovery times.
Choose a diode whose maximum reverse voltage
rating is greater than the maximum input voltage,
and whose current rating is greater than the
maximum load current. Table 2 lists example
Schottky diodes and manufacturers.
Table 2—Diode Selection Guide
Diode
SK33
SK34
B330
B340
MBRS330
MBRS340
Voltage/Current Manufacture
Rating
30V, 3A
40V, 3A
30V, 3A
40V, 3A
30V, 3A
40V, 3A
Diodes Inc.
Diodes Inc.
Diodes Inc.
Diodes Inc.
On Semiconductor
On Semiconductor
The input capacitor can be electrolytic, tantalum
or ceramic. When using electrolytic or tantalum
capacitors, a small, high quality ceramic
capacitor (i.e. 0.1μF) should be placed as close
to the IC as possible.
When using ceramic capacitors, make sure that
they have enough capacitance to provide
sufficient charge to prevent excessive voltage
ripple at the input. The input voltage ripple
caused by capacitance can be estimated by:
Δ
V
IN
=
I
LOAD
V
OUT
⎛
V
OUT
×
× ⎜
1
−
f
S
×
C
1
V
IN
⎜
V
IN
⎝
⎞
⎟
⎟
⎠
Where C1 is the input capacitance value.
Output Capacitor
The output capacitor is required to maintain the
DC output voltage. Ceramic, tantalum or low
ESR electrolytic capacitors are recommended.
Low ESR capacitors are preferred so as to
keep the output voltage ripple low. The output
voltage ripple can be estimated by:
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
will also suffice.
Since the input capacitor absorbs the input
switching current it requires an adequate ripple
current rating. The RMS current in the input
capacitor can be estimated by:
Δ
V
OUT
=
V
OUT
⎛
V
OUT
× ⎜
1
−
f
S
×
L
⎜
V
IN
⎝
⎞
⎞ ⎛
1
⎟
⎟ × ⎜
R
ESR
+
⎟ ⎜
8
×
f
S
×
C
2
⎟
⎠
⎠ ⎝
Where L is the inductor value, C2 is the output
capacitance value and R
ESR
is the equivalent
series resistance (ESR) value of the output
capacitor.
In the case of ceramic capacitors, the
impedance at the switching frequency is
dominated by the capacitance, which is the
main cause for the output voltage ripple. For
simplification, the output voltage ripple can be
estimated by:
I
C
1
=
I
LOAD
×
V
OUT
⎛
V
OUT
×⎜
1
−
⎜
V
IN
⎜
V
IN
⎝
⎞
⎟
⎟
⎟
⎠
The worst-case condition occurs at V
IN
= 2V
OUT
,
where:
ΔV
OUT
=
⎛
V
× ⎜
1
−
OUT
2
V
IN
8
×
f
S
×
L
×
C
2
⎜
⎝
V
OUT
⎞
⎟
⎟
⎠
I
C
1
=
I
LOAD
2
For simplification, choose an input capacitor
whose RMS current rating is greater than half of
the maximum load current.
In the case of tantalum or electrolytic capacitors,
the ESR dominates the impedance at the
switching frequency. For simplification, the
output ripple can be approximated to:
ΔV
OUT
=
V
OUT
⎛
V
OUT
× ⎜
1
−
f
S
×
L
⎝
V
IN
⎞
⎟ ×
R
ESR
⎠
MP1583 Rev. 3.1
6/20/2011
www.MonolithicPower.com
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7
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