nx2124 Microsemi Corporation, nx2124 Datasheet - Page 13

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

tion temperature increases, K is R
dependency. As a result, R
the worst case, in which K approximately equals to 1.4
at 125
tion loss should not exceed package rating or overall
system thermal budget.
duction at the switching transition. The total switching
loss can be approximated.
and T
is switching frequency. Switching 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.
mum power dissipation of the driver device.
Over Current Limit Protection
achieved by sensing current through the low side
MOSFET. For NX2124, the current limit is decided by
the R
FET is on, and the voltage on SW pin is below 360mV,
the over current occurs. The over current limit can be
calculated by the following equation.
Rev.1.8
02/28/08
is the high side gate source voltage, and V
P
where I
gate
where the R
Switching loss is mainly caused by crossover con-
Also MOSFET gate driver loss should be consid-
where Q
Over current Limit for step down converter is
DSON
P
P
P
P
F
i
This power dissipation should not exceed maxi-
ng the gate capacitor and is dissipated in driver
o
HCON
LCON
TOTAL
SW
which can be found in mosfet datasheet, and F
C according to IRFR3706 datasheet
LGATE
(Q
of the low side mosfet. When synchronous
=I
=I
=P
OUT
HGATE
2
1
OUT
OUT
is the low side MOSFETs gate charge,V
HCON
HGATE
is output current, T
2
2
V
IN
DS(ON)
(1 D) R
D R
V
P
is the high side MOSFETs gate
I
HGS
OUT
LCON
will increases as MOSFET junc-
DS(ON)
Q
T
DS(ON)
SW
LGATE
DS(ON)
K
F
should be selected for
S
V
SW
K
DS(ON)
LGS
is the sum of T
) F
SW
temperature
is frequency
S
LGS
.
is the low
Conduc-
.
...(22)
...(20)
..(21)
HGS
R
S
consideration K=1.5, then
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
ing the drain pin of the upper MOSFET, a plane connec-
tion is a must.
as to the load as possible and plane connection is re-
quired.
the high-side MOSFET need to be connected thru a plane
ans as close as possible. A snubber nedds to be placed
as close to this junction as possible.
If 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
3. The output capacitors should be placed as close
4. Drain of the low-side MOSFET and source of
5. Source of the lower MOSFET needs to be con-
I
I
SET
SET
K R
K R
320mV
360mV
DSON
DSON
DSON
1.5 9m
360mV
=9m , the worst case thermal
need to be practically
NX2124/2124A
26.7A
touch-
13