nx2124 Microsemi Corporation, nx2124 Datasheet - Page 12

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

at 75% of the LC double pole.
at half the swithing frequency.
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 and picture show the relationship
between
value of R
Rev.1.8
02/28/08
R =
4.Calculate R
Choose R
5. Calculate C
Choose C
6. Calculate C
Choose C
Output voltage is set by reference voltage and
where R
See compensator design for R
C =
R =
C =
3
=
=14.6k
1
1
2
=
=6.3nF
1.5V 2
V
=
= 7 2 p F
V
5V
1
2
V
2
OSC
value can be set by voltage divider.
OUT
V
in
p
R
O U T
1.8V
0.8V
2
2
,
1
1
R
is part of the compensator, and the
1
2
1 4 . 7 k
=68pF.
3
=6.8nF.
R
V
14.7k
1
=14.7k
3
-V
REF
3
V
3
R
1
2
R E F
value by the following equation.
R E F
F
by setting compensator pole
by setting compensator zero F
ESR
F
and voltage divider. .
30kHz 1.5uH
z
F
6.5m
s
O
1
1
0.75 2.3kHz
L
3 0 0 k H z
g
1
m
...(18)
V
V
1
OUT
REF
and R
2.0mA/V
1
2
selection.
F
p
Z
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:
the result of input RMS current is 4.3A.
ommended. One Sanyo OS-CON 16SP270M 16V 270uF
18m
capacitors.
Power MOSFETs Selection
MOSFETs. The selection of MOSFETs is based on
maximum drain source voltage, gate source voltage,
maximum current rating, MOSFET on resistance and
power dissipation. The main consideration is the power
loss contribution of MOSFETs to the overall converter
efficiency. In this design example, two IRFR3706 are
used. They have the following parameters: V
=75A,R
loss:conduction loss, switching loss.
V
Input capacitors are usually a mix of high frequency
For higher efficiency, low ESR capacitors are rec-
The power stage requires two N-Channel power
Conduction loss is simply defined as:
IN
I
D
There are two factors causing the MOSFET power
R2
R1
RMS
with 4.4A RMS rating are chosen as input bulk
= 5V, V
DSON
Vout
Figure 8 - Voltage divider
V
Voltage divider
V
=9m ,Q
OUT
I
OUT
IN
Vref
OUT
Fb
=1.8V, I
D
GATE
=23nC.
1- D
OUT
NX2124/2124A
=9A, using equation (19),
...(19)
DS
=30V, I
12
D