nx2142 Microsemi Corporation, nx2142 Datasheet - Page 16

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

final selection of R
swithing frequency, here F
at 75% of the LC double pole.
at half the swithing frequency.
Rev. 1.1
10/28/07
Choose R
R =
2.Set R
3. Set crossover frequency at 1/10~ 1/5 of the
4.Calculate R
5. Calculate C
Choose C
6. Calculate C
Choose C
C =
C =
3
=
=5k
F
1
10
2
ESR
=
=14nF
V
1
V
=
= 5 4 p F
OSC
2
2
in
2
2
2.5V
0.8V
3
5.7kHz
2
2
equal to10k . Using equation 18, the
=5k .
1
2
2
=15nF.
=52pF.
1
R
5 k
R
5k
1
1
3
3
3
R
60kHz 2.2uH
20.5m
1
2
is 4.7k
value by the following equation.
ESR C
20.5m
by setting compensator pole
ESR
by setting compensator zero F
F
F
1
F
O
z
1
1
0.75 2.9kHz
s
6 0 0 k H z
1
O
L
=60kHz.
OUT
g
1360uF
1
m
V
V
2.5mA/V
OUT
REF
1
F
Z
p
Output Voltage Calculation
external voltage divider. The reference voltage is fixed
at 0.8V. The divider consists of two ratioed resistors
so that the output voltage applied at the Fb pin is 0.8V
when the output voltage is at the desired value. The
following equation applies to figure 15, which shows
the relationship between
vider.
of R
Input Capacitor Selection
quency ceramic capacitors and bulk capacitors. Ce-
ramic capacitors bypass the high frequency noise, and
bulk capacitors supply switching current to the
MOSFETs. Usually 1uF ceramic capacitor is chosen
to decouple the high frequency noise.The bulk input
capacitors are decided by voltage rating and RMS cur-
rent rating. The RMS current in the input capacitors
can be calculated
as:
input RMS current is 3.4A.
recommended. One Sanyo OSCON CAP 25SVP56M
1
Output voltage is set by reference voltage and
where R
Input capacitors are usually a mix of high fre-
For higher efficiency, low ESR capacitors are
value can be set by voltage divider.
V
R =
I
D
R2
R1
RMS
INMIN
Vout
1
Figure 15 - Voltage divider
V
V
V
= 8V, V
R
INMIN
I
OUT
O U T
OUT
Vref
2
Fb
2
is part of the compensator, and the value
-V
V
OUT
R E F
D
R E F
=1.05V, I
1- D
NX2142/2142A
V
OUT
OUT
,
V
...(21)
REF
=10A, the result of
...(22)
and voltage di-
16