sc2441a Semtech Corporation, sc2441a Datasheet - Page 30

• Manufacturer Part Number: sc2441a
• Description: Sc2441a 1.8v To 20v Input 2-phase Synchronous Step-down Controllers With Step-up Converter
• Manufacturer: Semtech Corporation

If D
Power MOSFET Selection and Gate Drive
Main considerations in selecting the MOSFET’s are power
dissipation, cost and packaging. Switching losses and
conduction losses of the MOSFET’s are directly related
to the total gate charge (C
(R
the product of the total gate charge and on-resistance is
used as a figure of merit (FOM). Transistors with the same
FOM follow the same curve in Figure 20.
The closer the curve is to the origin, the lower is the FOM.
This means lower switching loss or lower conduction loss
or both. It is difficult to find MOSFET’s with both low C
and low R
has to be made.
MOSFET selection also depends on applications. In many
applications, either switching loss or conduction loss
dominates for a particular MOSFET. For synchronous buck
converters with high input to output voltage ratios, the
top MOSFET is hard switched but conducts with very low
duty cycle. The bottom switch conducts at high duty cycle
but switches at near zero voltage. For such applications,
MOSFET’s with low C
MOSFET’s with low R
I
POWER MANAGEMENT
Applications Information
Cin
ds(on)
2006 Semtech Corp.
1
»
>0.5 and D
). In order to judge the performance of MOSFET’s,
D (
Cg 100 Rds
Cg 200 Rds
Cg 500 Rds
Figure 20. Figure of merit curves.
1
(
(
(
+
ds(on)
D
,
,
,
2
. Usually a trade-off between R
-
50
1
)
)
)
2
1
I )(
> 0.5, then
40
20
0
o
1
1
0
+
ds(on)
g
I
o
are used for the top switch and
2
FOM:100*10^{-12}
FOM:200*10^{-12}
FOM:500*10^{-12}
)
are used for the bottom switch.
2
On-resistance (mOhm)
5
+
g
) and channel on-resistance
1 (
-
D
Rds
10
2
I )
o
1
2
+
15
1 (
-
D
1
ds(on)
20
20
I )
o
2
2
.
and C
g
g
30
The losses in power MOSFET’s consist of
a) conduction loss due to the channel resistance R
b) switching loss due to the switch rise time t
time t
c) the gate loss due to the gate resistance R
Top Switch:
The RMS value of the top switch current is
Its conduction loss is then
R
Curves showing R
manufacturers’ data sheet. From the Si7882DP
datasheet, R
than 5V. However R
junction temperature increases from 25°C to 125°C.
The switching losses can be estimated using the simple
formula
where t
switching process. To clarify these, we sketch the typical
MOSFET switching characteristics under clamped
inductive mode in Figure 21.
ds(on)
f
varies with temperature and gate-source voltage.
and
Figure 21. MOSFET switching characteristics
r
is the rise time and t
ds(on)
P
ts
is less than 4.5mOhm when V
=
I
Q
1
2
ds(on)
P
ds(on)
, 1
t (
rms
tc
r
= I
+
=
increases by nearly 40% as the
Q1,rms
variations can be found in
t
I
o
f
)(
D
1
2
1 (
+
R
f
+
ds(on)
2
d
is the fall time of the
I )
d
12
2
o
. )
.
V
in
SC2441A
. f
s
www.semtech.com
G
gs
.
is greater
r
and fall
ds(on)
,