sc2441a Semtech Corporation, sc2441a Datasheet - Page 31

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

In Figure 21,
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
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 loss is 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 calculated
POWER MANAGEMENT
Applications Information
gsp
gs1
gs2
gd
2006 Semtech Corp.
is the charge needed to charge gate-to-drain (Miller)
is the gate charge needed to bring the gate-to-source
is the additional gate charge required for the switch
is the Miller plateau voltage shown in Figure 21.
gt
gs
r
is the total resistance from the driver supply
= t
to the threshold V
3
-t
1
. t
g
P
r
t
t
within the MOSFET i.e.
tg
r
f
can be approximated as
ds
P
=
=
=
R
t
gi
is falling.
= P
gt
(Q
R
(
R
, external resistance R
Q
= R
gt
g
gs2
gs
V
tc
Q
2
+P
cc
gi
+
V
g
+
+R
V
-
gsp
ts
Q
gs_th
Q
cc
+P
V
ge
gd
gd
. f
gsp
g
s
+R
,
)R
tg
R )
is the total gate charge
g
.
= Q
gt
g
gt
.
ds
.
.
.
and
g
V
cc
f
f
is
s
is dissipated
ge
and the
1
, t
3
].
31
The conduction loss is 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 loss is estimated as
The total bottom switch loss is then
Once the power losses P
(P
at component and system level should be done to verify
that the maximum die junction temperature (T
125
The equivalent thermal impedance from junction to
ambient (
material, the thermal contact surface, thermal compound
property, the available effective heat sink area and the
air flow condition (free or forced convection). Actual
temperature measurement of the prototype should be
carried out to verify the thermal design.
Integrated Power MOSFET Drivers
There are four internal MOSFET drivers in a dual-
channel step-down converter.
ja
in
b
=12V, V
) MOSFET’s are known, thermal and package design
depends on the die to substrate bonding, packaging
o
C) is not exceeded under the worst-case conditions.
ds(on)
o
ja
=1.5V), the duty ratio D will be small. Since
) should satisfy
is the channel resistance of bottom MOSFET.
P
I
Q
bs
P
, 2
b
rms
q
=
=P
P
P
ja
bg
bc
1
2
=
bc
£
t (
=I
=
+P
I
r
T
o
Q2,rms
R
loss
+
R
, j
bs
max
1 (
gt
t
g
+P
f
P
)(
Q
for the top (P
-
2
-
loss
1
D
R
bg
g
T
+
V
)(
ds(on)
.
, a
cc
1
2
d
max
I )
. f
+
s
o
,
V
d
12
.
2
d
. )
f
SC2441A
s
www.semtech.com
t
) and bottom
j,max
, usually