SC2446 Semtech Corporation, SC2446 Datasheet - Page 17

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

The conduction losses are then
where R
If the input voltage to output voltage ratio is high (e.g.
V
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
Once the power losses P
MOSFET’s are known, thermal and package design 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.
POWER MANAGEMENT
Application Information (Cont.)
in
ja
=12V, V
2004 Semtech Corp.
depends on the die to substrate bonding, packaging
o
C) is not exceeded under the worst-case condition.
ds(on)
o
ja
=1.5V), the duty ratio D will be small. Since the
) should satisfy
is the channel resistance of bottom MOSFET.
P
bs
P
b
=P
P
P
ja
bg
bc
1
2
bc
t (
=I
+P
loss
r
T
Q2,rms
R
R
, j
max
bs
for the top (P
gt
t
g
+P
f
P
)(
Q
2
loss
1
R
bg
g
T
V
.
ds(on)
, a
cc
2
max
I )
. f
s
o
.
V
.
d
f
s
t
) and bottom (P
j,max
, usually
b
)
17
Integrated Power MOSFET Drivers
In SC2446 there are four internally integrated gate
drivers to drive all the MOSFETs in dual channels. With
the device bipolar process, emitter-follower based
Darlington bipolar transistors are used for the output
stage. The key advantage of the Darlington configuration
is that the total current gain is greatly improved which
leads to larger driving current I
reduce the MOSFETs switching losses. In order to
estimate the losses associated with the gate driver, we
first measured the gate driver waveform (typical
waveforms of V
with 2.2 current limit resistor.
It is clear that the saturation voltage is not a constant. It
changes with the driving current in a nonlinear fashion.
A simple formula to calculate the losses with a reasonable
accuracy is not available. But, we use a curve fitting
technique to estimate the power losses in gate driver.
First, the saturation voltage v
Where, V
constant related to the fall time of v
in Fig. 12, V
~50 ns. With these parameters, the approximated v
plotted as in Figure 13 a).
Figure 12. Measured gate driver output waveforms
cc
is the gate driver collector voltage, T
cc
=12V, T
ce
and I
v
ce
1
) t (
=0.5T
gs
) as shown in Figure12.
V
cc
f
2
with T
ce
(t) is approximated as
www.DataSheet4U.com
1
gs
2
(
. This in turn will help
T
t
1
)
f
2
.
being measured as
ce
. For the example
www.semtech.com
SC2446
1
is a time
ce
(t) is