sc2545 Semtech Corporation, sc2545 Datasheet - Page 14

• Manufacturer Part Number: sc2545
• Description: Sc2545 High Performance Wide Input Range Dual Synchronous Buck Controller
• Manufacturer: Semtech Corporation

T T T T T op Switc
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
switching process. Different manufactures have different
definitions and test conditions for t
these, we sketch the typical MOSFET switching
characteristics under clamped inductive mode in Figure
9.
© 2005 Semtech Corp.
POWER MANAGEMENT
Applications Information (Cont.)
op Switc
op Switc
op Switc
op Switch h h h h
ds(on)
ds(on)
Volts
Volts
varies with temperature and gate-source voltage.
is less than 8m
Figure 9. MOSFET switching characteristics
r
t0
t0
Vgs th
Vgs th
is the rise time and t
Qgs1 Qgs2
Qgs1 Qgs2
Vds
Vds
ds(on)
P
t1
t1
tc
= I
t2
t2
increases by 50% as the junction
P
Q1,rms
ts
ds(on)
=
Qgd
Qgd
Ω
2
2
1
R
t (
ds(on)
variations can be found in
when V
r
+
Miller plateau
Miller plateau
.
t3
t3
t
Ids
Ids
f
)(
f
o
C to 110
gs
is the fall time of the
1
+
is greater than 10V.
2
δ
r
Vgs
Vgs
I )
and t
o
V
o
in
C.
. f
s
f
. To clarify
Gate charge
Gate charge
14
In Figure 9, Q
gate-to-source voltage V
Q
current to reach its full-scale value I
charge needed to charge gate-to-drain (Miller) capacitance
when V
Switching losses occur during the time interval [t
Defining t
where R
rail to the gate of the MOSFET. It includes the gate driver
internal impedance R
gate resistance R
V
Similarly an approximate expression for t
Only a portion of the total losses P
in 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 propor-
tional 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.
gsp
gs2
is the Miller plateau voltage shown in Figure 9.
is the additional gate charge required for the switch
ds
gt
is falling.
r
is the total resistance from the driver supply
= t
3
gs1
-t
1
is the gate charge needed to bring the
and t
P
g
t
t
within the MOSFET :
r
= P
=
R
P
t
f
tg
gt
gi
r
(
=
tc
Q
, external resistance R
can be approximated as
= R
=
+P
gs
(
gs
V
Q
R
R
to the threshold voltage V
2
cc
ts
gi
gs
gt
g
+P
+
+R
2
−
Q
Q
V
+
tg
V
ge
g
gsp
gd
V
.
Q
gsp
g
+R
cc
is the total gate charge
R )
gd
g
. f
g
= Q
R )
s
gt
.
.
gt
g
ds
.
V
,
cc
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SC2545
.
f
and Q
f
is
s
is dissipated
ge
gd
and the
is the
1
, t
gs_th
3
].
.