nx2114 Microsemi Corporation, nx2114 Datasheet - Page 14

• Manufacturer Part Number: nx2114
• Description: 300khz & 600khz Synchronous Pwm Controller
• 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. In this design example, two Fairchild FDS6294
are used. They have the following parameters: V
I
loss: conduction loss, switching loss and gate driver loss.
ing the gate capacitor and is dissipated in driver circuits.
It is proportional to frequency and is defined as:
charge, Q
V
the low side gate source voltage.
power dissipation should not exceed maximum power
dissipation of the driver device.
tion temperature increases, K is R
dependency. As a result, R
the worst case, in which K equals to 1.43 at 125
according to FDS6294 datasheet
the result of P
not exceed package rating or overall system thermal
budget.
duction at the switching transition. The total switching
loss can be approximated.
where I
is switching frequency. Swithing loss P
Rev. 4.0
06/20/06
P
D
HGS
=13A, R
gate
P
P
P
is the high side gate source voltage, and V
Gate driver loss is the loss generated by discharg-
where Q
According to equation (20), P
Conduction loss is simply defined as:
H C O N
L C O N
where the R
Switching loss is mainly caused by crossover con-
T O T A L
There are three factors causing the MOSFET power
P
The NX2114 requires two N-Channel power
OUT
SW
(Q
DSON
LGATE
is output current, T
HGATE
= I
= I
= P
O U T
O U T
1
2
=14.4m ,Q
TOTAL
HGATE
is the low side MOSFETs gate charge,
H C O N
DS(ON)
2
2
V
V
IN
HGS
is 0.75W. Conduction loss should
is the high side MOSFETs gate
(1
D
will increases as MOSFET junc-
I
P
OUT
Q
L C O N
GATE
R
D )
DS(ON)
LGATE
D S ( O N )
SW
=10nC.
T
SW
is swithing time,and F
R
.
should be selected for
Using equation (21),
D S (O N )
V
DS(ON)
GATE
LGS
F
K
S
SW
) F
=0.03W. This
temperature
is frequency
K
S
.
..(22)
DS=
...(21)
...(20)
LGS
30V,
o
is
C
S
dependent.
Soft Start, Enable and shut Down
digital counter with 1024 cycles. For NX2114 with 300kHz
operation, the start up time is about 3.5ms. For NX2114A
with 600kHz operation, the start up time is about half of
NX2114, 1.75mS.
pulling COMP pin below 0.3V. The function is illustrated
in the following diagram. During the normal operation,
the lowest COMP voltage is clamped to be about 700mV
, the COMP voltage is higher than 0.3V. If external switch
with 10
COMP is below 0.3V, the digital soft start will be reset to
zero. All the drivers will be off. The synchronous buck is
shut off. When external switch is released, and COMP
is above 0.3V, a soft start will initiates and system starts
from the beginning.
pulling down COMP pin.
Feedback Under Voltage Shut Down
Out (FB UVLO ) to provide short circuit protection. Ba-
sically, NX2114 has a comparator compare the feedback
voltage with the FB UVLO threshold 0.4V.
The NX2114 has a digital start up. It is based on
Figure 14 - Enable and Shut down NX2114 by
NX2114 relies on the Feedback Under Voltage Lock
NX2114/NX2114A can be enabled or disabled by
OFF
R
dson
ON
Compensation
Network
or less to pull down COMP pin, when
comp
FB
NX2114/2114A
0.6
2114
1.3V
Clamp
0.3V
Shut
down
14