NX2124/2124A
APPLICATION INFORMATION
Symbol Used In Application Information:
V
IN
V
OUT
I
OUT
F
S
- Input voltage
- Output voltage
- Output current
- Working frequency
∆
I
RIPPLE
=
=
V
IN
-V
OUT
V
OUT
1
×
×
L
OUT
V
IN
F
S
...(2)
5V-1.8V 1.8v
1
×
×
=
2.56A
1.5uH
5v 300kHz
DV
RIPPLE
- Output voltage ripple
DI
RIPPLE
- Inductor current ripple
Output Capacitor Selection
Output capacitor is basically decided by the
amount of the output voltage ripple allowed during steady
state(DC) load condition as well as specification for the
load transient. The optimum design may require a couple
of iterations to satisfy both condition.
Based on DC Load Condition
The amount of voltage ripple during the DC load
condition is determined by equation(3).
Design Example
The following is typical application for NX2124, the
schematic is figure 1.
V
IN
= 5V
V
OUT
=1.8V
F
S
=300kHz
I
OUT
=9A
DV
RIPPLE
<=20mV
DV
DROOP
<=100mV @ 9A step
∆
V
RIPPLE
=
ESR
× ∆
I
RIPPLE
+
∆
I
RIPPLE
8
×
F
S
×
C
OUT
...(3)
Where ESR is the output capacitors' equivalent
series resistance,C
OUT
is the value of output capacitors.
Typically when large value capacitors are selected
such as Aluminum Electrolytic,POSCAP and OSCON
types are used, the amount of the output voltage ripple
is dominated by the first term in equation(3) and the
second term can be neglected.
For this example,electrolytic capacitors are cho-
sen as output capacitors, the ESR and inductor current
typically determines the output voltage ripple.
Output Inductor Selection
The selection of inductor value is based on induc-
tor ripple current, power rating, working frequency and
efficiency. Larger inductor value normally means smaller
ripple current. However if the inductance is chosen too
large, it brings slow response and lower efficiency. Usu-
ally the ripple current ranges from 20% to 40% of the
output current. This is a design freedom which can be
decided by design engineer according to various appli-
cation requirements. The inductor value can be calcu-
lated by using the following equations:
ESR
desire
=
∆
V
RIPPLE
20mV
=
=
7.8m
Ω
∆
I
RIPPLE
2.56A
...(4)
If low ESR is required, for most applications, mul-
tiple capacitors in parallel are better than a big capaci-
tor. For example, SANYO electrolytic capacitor
16ME1500WG is chosen.
V -V
V
1
L
OUT
=
IN OUT
×
OUT
×
∆
I
RIPPLE
V
IN
F
S
I
RIPPLE
=k
×
I
OUTPUT
where k is between 0.2 to 0.4.
Select k=0.3, then
...(1)
N
=
E S R
E
× ∆
I
R I P P L E
∆
V
R IPPLE
.
..(5)
5V-1.8V 1.8V
1
L
OUT
=
×
×
0.3
×
9A
5V 300kHz
L
OUT
=1.4uH
Choose inductor from COILCRAFT DO5010P-
152HC with L=1.5uH is a good choice.
Current Ripple is recalculated as
Rev.1.8
02/28/08
Number of Capacitor is calculated as
N
=
13m
Ω×
2.56A
20mV
N =1.7
The number of capacitor has to be round up to a
integer. Choose N =2.
If ceramic capacitors are chosen as output ca
6
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