nx2141 Microsemi Corporation, nx2141 Datasheet - Page 17

• Manufacturer Part Number: nx2141
• Description: Single Channel Mobile Pwm Controller With Feedforward And Enable
• Manufacturer: Microsemi Corporation

nal selection of R
swithing frequency, here F
at 75% of the LC double pole.
at half the swithing frequency.
Output Voltage Calculation
ternal 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 22, which shows the relation-
ship between
Rev. 1.6
05/15/07
Choose R
R =
1.Set R
2. Set crossover frequency at 1/20 of the
3.Calculate R
4. Calculate C
Choose C
5. Calculate C
Choose C
Output voltage is set by reference voltage and ex-
C =
3
C =
=
=0.8k
1
10
2
1
V
=
=70nF
V
=
= 5 3 0 p F
OSC
2
2
in
2
2
2.5V
0.8V
3
V
equal to10k . Using equation 18, the fi-
=1k .
1
2
OUT
2
=68nF.
=560pF.
1
R
1k
1
R
1k
is 4.7k
3
1
10kHz 2.2uH
20.5m
R
3
1
2
3
,
value by the following equation.
V
ESR
by setting compensator zero F
by setting compensator pole
F
REF
F
O
F
z
0.75 2.9kHz
1
1
s
O
3 0 0 k H z
and voltage divider. .
L
=10kHz.
g
1
m
V
V
2.5mA/V
OUT
REF
1
F
Z
p
of R
Input Capacitor Selection
ceramic capacitors and bulk capacitors. Ceramic ca-
pacitors bypass the high frequency noise, and bulk ca-
pacitors supply switching current to the MOSFETs. Usu-
ally 1uF ceramic capacitor is chosen to decouple the
high frequency noise.The bulk input capacitors are de-
cided by voltage rating and RMS current rating. The RMS
current in the input capacitors can be calculated
as:
input RMS current is 3.4A.
recommended. One Sanyo OSCON CAP 25SVP56M
25V 56uF 28m with 3.8A RMS rating are chosen as
input bulk capacitors.
1
value can be set by voltage divider.
where R
Input capacitors are usually a mix of high frequency
For higher efficiency, low ESR capacitors are
V
R =
I
D
R2
R1
RMS
INMIN
Vout
1
Figure 22 - 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
OUT
...(18)
=10A, the result of
...(19)
NX2141
17