nx2124 Microsemi Corporation, nx2124 Datasheet - Page 6

• Manufacturer Part Number: nx2124
• Description: 300khz Synchronous Pwm Controller
• Manufacturer: Microsemi Corporation

APPLICATION INFORMATION
Symbol Used In Application Information:
Design Example
schematic is figure 1.
Output Inductor Selection
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:
152HC with L=1.5uH is a good choice.
Rev.1.8
02/28/08
DV
DI
F
DV
DV
V
I
F
V
I
V
V
OUT
OUT
S
S
IN
OUT
IN
OUT
RIPPLE
=300kHz
The following is typical application for NX2124, the
RIPPLE
RIPPLE
DROOP
= 5V
=9A
=1.8V
The selection of inductor value is based on induc-
where k is between 0.2 to 0.4.
Select k=0.3, then
Choose inductor from COILCRAFT DO5010P-
Current Ripple is recalculated as
L
I
L
L
RIPPLE
OUT
OUT
OUT
<=100mV @ 9A step
<=20mV
- Working frequency
- Inductor current ripple
- Output voltage ripple
- Output current
- Input voltage
- Output voltage
=
=
=1.4uH
=k I
5V-1.8V 1.8V
V -V
0.3 9A
IN
I
RIPPLE
OUTPUT
OUT
V
5V
V
OUT
IN
300kHz
F
1
S
1
...(1)
Output Capacitor Selection
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
condition is determined by equation(3).
series resistance,C
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.
sen as output capacitors, the ESR and inductor current
typically determines the output voltage ripple.
tiple capacitors in parallel are better than a big capaci-
tor. For example, SANYO electrolytic capacitor
16ME1500WG is chosen.
integer. Choose N =2.
I
RIPPLE
ESR
Output capacitor is basically decided by the
The amount of voltage ripple during the DC load
Where ESR is the output capacitors' equivalent
Typically when large value capacitors are selected
For this example,electrolytic capacitors are cho-
If low ESR is required, for most applications, mul-
Number of Capacitor is calculated as
N
N =1.7
The number of capacitor has to be round up to a
If ceramic capacitors are chosen as output ca
N
V
=
=
RIPPLE
13m
desire
V -V
5V-1.8V 1.8v
IN
1.5uH
E S R
L
OUT
20mV
=
OUT
ESR
V
I
2.56A
RIPPLE
E
RIPPLE
V
OUT
R I P P L E
V
5v
V
OUT
is the value of output capacitors.
IN
I
I
R I P P L E
RIPPLE
20mV
2.56A
NX2124/2124A
300kHz
F
1
S
1
8 F
7.8m
I
S
RIPPLE
2.56A
C
OUT
.
...(2)
...(3)
...(4)
..(5)
6