SC2446 Semtech Corporation, SC2446 Datasheet - Page 16

• Manufacturer Part Number: SC2446
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

MOSFET’s with low R
MOSFET power dissipation consists of
a) conduction loss due to the channel resistance R
b) switching loss due to the switch rise time t
t
c) the gate loss due to the gate resistance R
Top Switch:
The RMS value of the top switch current is calculated as
The conduction losses are then
R
Curves showing R
manufacturers’ data sheet. From the Si4860 datasheet,
R
However R
temperature increases from 25
The switching losses can be estimated using the simple
formula
where t
process. Different manufactures have different definitions
and test conditions for t
the typical MOSFET switching characteristics under clamped
inductive mode in Figure 11.
In Figure 11,
Q
voltage V
POWER MANAGEMENT
Application Information (Cont.)
f
, and
ds(on)
ds(on)
gs1
2004 Semtech Corp.
is the gate charge needed to bring the gate-to-source
varies with temperature and gate-source voltage.
is less than 8m
r
Figure 11. MOSFET switching characteristics
is the rise time and t
gs
to the threshold voltage V
ds(on)
P
ts
increases by 50% as the junction
I
Q
1
2
P
, 1
ds(on)
ds(on)
t (
tc
rms
r
= I
r
and t
are used for the bottom switch.
Q1,rms
when V
variations can be found in
t
I
o
f
f
)(
is the fall time of the switching
D
1
f
2
. To clarify these, we sketch
1 (
R
o
C to 110
ds(on)
2
gs
I )
12
2
o
is greater than 10V.
.
. )
V
in
gs_th
Gate charge
. f
s
o
C.
,
r
and fall time
G
.
ds(on)
,
16
Q
current to reach its full-scale value I
Q
capacitance when V
Switching losses occur during the time interval [t
Defining t
where R
to the gate of the MOSFET. It includes the gate driver internal
impedance R
resistance R
V
Similarly an approximate expression for t
Only a portion of the total losses P
the MOSFET package. Here Q
specified in the datasheet. The power dissipated within
the MOSFET package is
The total power loss of the top switch is then
If the input supply of the power converter varies over a
wide range, then it will be necessary to weigh the relative
importance of conduction and switching losses. This is
because conduction losses are inversely proportional
to the input voltage. Switching loss however increases
with the input voltage. The total power loss of MOSFET
should be calculated and compared for high-line and
low-line cases. The worst case is then used for thermal
design.
Bottom Switch:
The RMS current in bottom switch can be shown to be
gsp
gs2
gd
is the charge needed to charge gate-to-drain (Miller)
is the Miller plateau voltage shown in Figure 11.
is the additional gate charge required for the switch
gt
r
is the total resistance from the driver supply rail
= t
g
3
within the MOSFET i.e.
-t
gi
1
, external resistance R
I
and t
Q
P
, 2
t
t
tg
rms
r
f
ds
P
R
t
is falling.
r
= P
gt
(
(
R
R
can be approximated as
Q
Q
= R
I
gt
g
o
gs
gs
tc
V
Q
+P
2
cc
2
1 (
gi
V
g
+R
V
ts
gsp
Q
Q
cc
+P
D
V
g
ge
gd
gd
. f
gsp
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)(
is the total gate charge
s
+R
g
tg
R )
R )
1
= Q
.
ds
gt
g
gt
.
12
.
g
and
.
.
2
V
. )
cc
ge
f
f
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s
is
SC2446
is dissipated in
and the gate
1
, t
3
].