nx2141 Microsemi Corporation, nx2141 Datasheet - Page 18

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

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. For example, two IRF7822 are used in appli-
cation. They have the following parameters: V
=18A,R
loss:conduction loss, switching loss.
tion temperature increases, K is R
dependency. As a result, R
the worst case, in which K approximately equals to 1.4
at 125
not exceed package rating or overall system thermal
budget.
conduction at the switching transition. The total
switching loss can be approximated.
and T
is switching frequency. Swithing loss P
dependent.
ered when choosing the proper power MOSFET.
MOSFET gate driver loss is the loss generated by dis-
charg
circuits.It is proportional to frequency and is defined as:
charge,Q
side gate source voltage.
Rev. 1.6
05/15/07
is the high side gate source voltage, and V
P
where the R
where I
gate
P
P
P
The NX2141 requires two N-Channel power
Conduction loss is simply defined as:
Switching loss is mainly caused by crossover
Also MOSFET gate driver loss should be consid-
where Q
P
F
i
There are two factors causing the MOSFET power
HCON
LCON
TOTAL
o
ng the gate capacitor and is dissipated in driver
SW
C according to datasheet. Conduction loss should
which can be found in mosfet datasheet, and F
DSON
LGATE
(Q
=I
=I
=P
OUT
=6.5m ,Q
OUT
HGATE
OUT
2
1
HCON
is the low side MOSFETs gate charge,V
HGATE
2
2
is output current, T
V
DS(ON)
IN
(1 D) R
D R
V
P
is the high side MOSFETs gate
I
HGS
LCON
OUT
will increases as MOSFET junc-
GATE
DS(ON)
Q
T
=44nC.
DS(ON)
SW
DS(ON)
LGATE
K
F
should be selected for
S
K
V
SW
DS(ON)
LGS
is the sum of T
) F
SW
temperature
is frequency
S
LGS
DS
is the low
=30V, I
...(20)
.
...(22)
..(21)
HGS
D
R
S
mum power dissipation of the driver device.
Over Current Limit Protection
achieved by sensing current through the low side
MOSFET. For NX2141, 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.
situation, then the current limit for MOSFET IRF7822 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 environment is
generated by the power components due to the switch-
ing power. Small signal components are connected to
sensitive pins or nodes. A multilayer layout which in-
cludes power plane, ground plane and signal 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 to
reduce the EMI radiated by the power loop due to the
high switching currents through them.
ripple current and a high frequency decoupling ceramic
cap which usually is 1uF
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
2. Low ESR capacitor which can handle input RMS
DSON
I
I
This power dissipation should not exceed maxi-
SET
SET
of the low side mosfet. When synchronous
320mV
R
320mV/R
DSON
1.4 6.5m
320mV
DSON
DSON
need to be practically
is calculated in the worst case
35A
NX2141
touch-
18