nx2142 Microsemi Corporation, nx2142 Datasheet - Page 17

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

25V 56uF 28m
as input bulk 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 STM6912 are
used. They have the following parameters: V
=6A,R
power loss:conduction loss, switching loss.
tion temperature increases, K is R
dependency. As a result, R
for the worst case, in which K approximately equals to
1.4 at 125
should not exceed package rating or overall system
thermal budget.
duction at the switching transition. The total switching
loss can be approximated.
and T
F
quency dependent.
ered when choosing the proper power MOSFET.
MOSFET gate driver loss is the loss generated by dis-
char g i
circuits.It is proportional to frequency and is defined
as:
Rev. 1.1
10/28/07
S
P
is switching frequency. Swithing loss P
where the R
where I
gate
P
P
P
The NX2142 requires two N-Channel power
Conduction loss is simply defined as:
Switching loss is mainly caused by crossover con-
Also MOSFET gate driver loss should be consid-
P
F
HCON
LCON
TOTAL
DSON
ng the gate capacitor and is dissipated in driver
There are two factors causing the MOSFET
SW
which can be found in mosfet datasheet, and
(Q
=I
=I
=P
o
=57m ,Q
C according to datasheet. Conduction loss
OUT
OUT
OUT
2
HGATE
1
HCON
2
2
is output current, T
V
DS(ON)
IN
(1 D) R
D R
with 3.8A RMS rating are chosen
V
P
I
HGS
GATE
LCON
OUT
will increases as MOSFET junc-
DS(ON)
=6.3nC.
Q
T
DS(ON)
SW
LGATE
DS(ON)
K
F
S
K
SW
V
should be selected
DS(ON)
LGS
is the sum of T
) F
temperature
S
DS
SW
.
..(24)
...(23)
=30V, I
...(25)
is fre-
D
R
charge,Q
charge,V
low side gate source voltage.
mum power dissipation of the driver device.
Over Current Limit Protection
achieved by sensing current through the low side
MOSFET. For NX2142, the current limit is decided by
the R
FET is on, and the voltage on SW pin is below 320mV,
the over current occurs. The over current limit can be
calculated by the following equation.
case situation, then the current limit for MOSFET
STM6912 is
Layout Considerations
frequency switching converters. Layout will affect noise
pickup and can cause a good design to perform with
less than expected results.
the layout which are power components and small sig-
nal components. Power components usually consist of
input capacitors, high-side MOSFET, low-side
MOSFET, inductor and output capacitors. A noisy en-
vironment is generated by the power components due
to the switching power. Small signal components are
connected to sensitive pins or nodes. A multilayer lay-
out which includes power plane, ground plane and sig-
nal plane is recommended .
layer connected by wide, copper filled areas. The input
capacitor, inductor, output capacitor and the MOSFETs
should be close to each other as possible. This helps
is the high side gate source voltage, and V
where Q
Over current Limit for step down converter is
The MOSFET R
The layout is very important when designing high
There are two sets of components considered in
Layout guidelines:
1. First put all the power components in the top
DSON
I
I
This power dissipation should not exceed maxi-
SET
SET
HGS
of the low side mosfet. When synchronous
LGATE
320mV
R
320mV/R
DSON
HGATE
is the low side MOSFETs gate
1.2 57m
is the high side MOSFETs gate
320mV
DSON
DSON
NX2142/2142A
is calculated in the worst
4.6A
LGS
is the
17