TM
MP2354 鈥?2A, 23V, 380KHz STEP-DOWN CONVERTER
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鈥擨nductor Selection Guide
Package
Dimensions
(mm)
W
L
H
7.0
7.3
5.5
5.5
6.7
10.1
5.0
7.6
10.0
9.7
9.4
9.4
7.8
8.0
5.7
5.7
6.7
10.0
5.0
7.6
10.0
1.5
13.0
13.0
5.5
5.2
5.5
5.5
3.0
3.0
3.0
5.1
4.3
4.0
3.0
5.1
For simplification, choose the input capacitor
whose RMS current rating greater than half of
the maximum load current.
The input capacitor can be electrolytic, tantalum
or ceramic. When using electrolytic or tantalum
capacitors, a small, high quality ceramic
capacitor, i.e. 0.1碌F, 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 input. The
input voltage ripple caused by capacitance can
be estimated by:
鈭?/div>
V
IN
=
鈳?/div>
I
LOAD
V
V
脳
OUT
脳 鈳?/div>
1
鈭?/div>
OUT
鈳?/div>
f
S
脳
C1 V
IN
鈳?/div>
V
IN
鈳?/div>
鈳?/div>
鈳?/div>
鈳?/div>
Vendor/
Model
Sumida
CR75
CDH74
CDRH5D28
CDRH5D28
CDRH6D28
CDRH104R
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
Toko
D53LC
Type A
D75C
D104C
D10FL
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 to keep the
output voltage ripple low. The output voltage
ripple can be estimated by:
鈭?/div>
V
OUT
=
V
OUT
鈳?/div>
V
脳 鈳?/div>
1
鈭?/div>
OUT
鈳?/div>
f
S
脳
L
鈳?/div>
V
IN
鈳?/div>
鈳?鈳?/div>
1
鈳?/div>
鈳?脳 鈳?/div>
R
ESR
+
鈳?鈳?/div>
8
脳
f
S
脳
C2
鈳?/div>
鈳?鈳?/div>
鈳?/div>
Coilcraft
DO3308
DO3316
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
脳
V
OUT
鈳?/div>
V
OUT
鈳?/div>
鈳?/div>
脳鈳?/div>
1
鈭?/div>
V
IN
鈳?/div>
V
IN
鈳?/div>
鈳?/div>
鈳?/div>
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. The output
voltage ripple is mainly caused by the
capacitance. For simplification, the output
voltage ripple can be estimated by:
鈭哣
OUT
=
V
OUT
8
脳
f
S
2
鈳?/div>
V
脳 鈳?/div>
1
鈭?/div>
OUT
鈳?/div>
V
IN
脳
L
脳
C2
鈳?/div>
鈳?/div>
鈳?/div>
鈳?/div>
鈳?/div>
The worst-case condition occurs where:
I
C1
I
=
LOAD
2
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:
鈭哣
OUT
=
V
OUT
鈳?/div>
V
脳 鈳?/div>
1
鈭?/div>
OUT
鈳?/div>
f
S
脳
L
鈳?/div>
V
IN
鈳?/div>
鈳?脳
R
ESR
鈳?/div>
鈳?/div>
The characteristics of the output capacitor also
affect the stability of the regulation system. The
7
MP2354 Rev. 1.4
1/6/2006
www.MonolithicPower.com
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