nx2142 Microsemi Corporation, nx2142 Datasheet - Page 9

• Manufacturer Part Number: nx2142
• Description: Single Channel Pwm Controller With Feedforward And 5v Bias Regulator
• Manufacturer: Microsemi Corporation

APPLICATION INFORMATION
Symbol Used In Application Information:
Design Example
Output Inductor Selection
ductor 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. Usually 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 application requirements. The inductor value
can be calculated by using the following equations:
DO3316P-682HC is a good choice.
Rev. 1.1
10/28/07
I
V
V
DI
Power stage design requirements:
V
V
I
DV
DV
DV
F
V
F
OUT
OUT
OUT
IN
S
INMIN
OUT
S
INMAX
RIPPLE
=600kHz
RIPPLE
RIPPLE
TRAN
=3A
=5V
The selection of inductor value is based on in-
where k is between 0.2 to 0.4.
Select k=0.3, then
Choose L
L
L
=8V
I
=20V
L
RIPPLE
OUT
OUT
<=150mV @ 1.5A step
OUT
<=50mV
=
=6.9uH
=
- Output voltage ripple
- Input voltage
- Output voltage
- Output current
- Switching frequency
- Inductor current ripple
=k I
0.3 3A
V
20V-5V
INMAX
I
RIPPLE
OUT
OUTPUT
-V
=6.8uH, then coilcraft inductor
OUT
20V
5V
V
V
INMAX
OUT
600kHz
F
1
1
S
...(1)
Output Capacitor Selection
amount of the output voltage ripple allowed during
steady state(DC) load condition as well as specifica-
tion 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
tors.
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 capacitor, the ESR and inductor current
typically determines the output voltage ripple.
tiple capacitors in parallel are better than a big capaci-
tor. For example, for 50mV output ripple, Electrolytic
6ME1000WG with 30m
integer. Choose N =1.
ESR
Current Ripple is calculated as
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, Aluminum Electrolytic is cho-
If low ESR is required, for most applications, mul-
Number of Capacitor is calculated as
N
N =0.55
The number of capacitor has to be round up to a
N
I
RIPPLE
V
RIPPLE
30m
desire
=
E S R
=
20V-5V
=
V -V
6.8uH
50mV
IN
L
ESR
V
0.919A
OUT
I
RIPPLE
E
RIPPLE
V
OUT
R I P P L E
OUT
20V 600kHz
5V
I
is the value of output capaci-
I
R I P P L E
V
RIPPLE
V
OUT
0.919A
50mV
NX2142/2142A
are chosen.
IN
F
8 F
1
S
1
54m
I
S
RIPPLE
C
0.919A
OUT
.
..(5)
...(3)
...(4)
...(2)
9