SC2463 Semtech, SC2463 Datasheet - Page 16

• Manufacturer Part Number: SC2463
• Description: High Performance Quad Output Switching Regulator
• Manufacturer: Semtech

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
6.
Where,
Q
voltage V
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
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
POWER MANAGEMENT
Applications Information (Cont.)
gsp
gs1
gs2
gd
2007 Semtech Corp.
is the charge needed to charge gate-to-drain (Miller)
is the Miller plateau voltage shown in Figure 11.
is the gate charge needed to bring the gate-to-source
is the additional gate charge required for the switch
Figure 6. MOSFET switching characteristics
r
gt
gs
is the rise time and t
r
is the total resistance from the driver supply
= t
to the threshold voltage V
3
P
-t
ts
1
and t
g
t
t
within the MOSFET i.e.
r
f
2
1
ds
t (
R
gi
is falling.
r
gt
r
(
(
, external resistance R
Q
Q
can be approximated as
= R
gs
gs
V
t
2
cc
2
f
gi
)(
V
+R
1
gsp
Q
Q
V
ge
gd
gd
gsp
f
+R
2
R )
R )
is the fall time of the
I )
g
o
= Q
gt
g
gt
.
V
ds
.
.
r
in
gs_th
.
and
g
. f
and t
V
s
cc
,
f
f
is
s
is dissipated
f
. To clarify
ge
and the
1
, t
3
].
16
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
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 how-
ever 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
The conduction losses are then
where R
If the input voltage to output voltage ratio is high (e.g.
V
the bottom switch conducts with duty ratio (1-D), the
corresponding conduction losses can be quite high.
Due to non-overlapping conduction between the top and
the bottom MOSFET’s, the internal body diode or the
external Schottky diode across the drain and source
terminals always conducts prior to the turn on of the
bottom MOSFET. The bottom MOSFET switches on with
only a diode voltage between its drain and source
terminals. The switching loss
is negligible due to near zero-voltage switching.
The gate losses are estimated as
The total bottom switch losses are then
in
=12V, V
If the input supply of the power converter varies
ds(on)
o
=1.5V), the duty ratio D will be small. Since
is the channel resistance of bottom MOSFET.
P
I
Q
P
bs
P
, 2
tg
b
rms
=P
P
P
P
bg
t
bc
1
2
= P
bc
R
R
t (
=I
+P
I
gt
r
g
o
Q2,rms
R
tc
R
Q
bs
+P
1 (
gt
t
g
+P
g
f
V
)(
Q
ts
2
cc
1
+P
D
R
bg
g
V
. f
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g
)(
ds(on)
.
s
cc
tg
1
is the total gate charge
2
.
I )
. f
s
o
.
V
12
2
d
. )
f
s
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SC2463