SC2446 Semtech Corporation, SC2446 Datasheet - Page 15

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

Sometimes meeting tight input voltage ripple
specifications may require the use of larger input
capacitance. At full load, the peak-to-peak input voltage
ripple due to the ESR is
The peak-to-peak input voltage ripple due to the capacitor
is
From these two expressions, C
input voltage ripple specification. In a multi-phase
converter, channel interleaving can be used to reduce ripple.
The two step-down channels of the SC2446 operate at
180 degrees from each other. If both step-down channels
in the SC2446 are connected in parallel, both the input
and the output RMS currents will be reduced.
Ripple cancellation effect of interleaving allows the use of
smaller input capacitors. When converter outputs are
connected in parallel and interleaved, smaller inductors
and capacitors can be used for each channel. The total
output ripple-voltage remains unchanged. Smaller
inductors speeds up output load transient.
When two channels with a common input are interleaved,
the total DC input current is simply the sum of the individual
DC input currents. The combined input current waveform
depends on duty ratio and the output current waveform.
Assuming that the output current ripple is small, the
following formula can be used to estimate the RMS value
of the ripple current in the input capacitor.
Let the duty ratio and output current of Channel 1 and
Channel 2 be D
If D
If D
If D
I
I
POWER MANAGEMENT
Cin
Cin
Application Information (Cont.)
2004 Semtech Corp.
1
1
1
>0.5 and (D
>0.5 and D
<0.5 and D
D
. 0
1
I 5
I
o
1
o
2
1
2
D
D (
2
I
1
2
2
o
1
1
, D
v
<0.5, then
2
-0.5) < D
< (D
2
ESR
.
0
2
5 .
and I
1
I )(
-0.5) < 0.5, then
R
v
o
C
esr
1
2
o1
<0.5, then
1 (
, I
I
o
C
DI
2
IN
o2
)
in
2
2
o
f
, respectively.
can be found to meet the
s
)
. I
,
o
D (
2
D
1
0
5 .
I )
o
2
2
.
15
If D
Choosing Power MOSFET’s
Main considerations in selecting the MOSFET’s are power
dissipation, cost and packaging. Switching losses and
conduction losses of the MOSFET’s are directly related to
the total gate charge (C
(R
the product of the total gate charge and on-resistance is
used as a figure of merit (FOM). Transistors with the same
FOM follow the same curve in Figure 10.
The closer the curve is to the origin, the lower is the FOM.
This means lower switching loss or lower conduction loss
or both. It may be difficult to find MOSFET’s with both low
C
has to be made.
MOSFET selection also depends on applications. In many
applications, either switching loss or conduction loss
dominates for a particular MOSFET. For synchronous buck
converters with high input to output voltage ratios, the top
MOSFET is hard switched but conducts with very low duty
cycle. The bottom switch conducts at high duty cycle but
switches at near zero voltage. For such applications,
MOSFET’s with low C
I
I
Cin
Cin
g
ds(on)
and low R
1
>0.5 and D
). In order to judge the performance of MOSFET’s,
D (
. 0
Cg 100 Rds
Cg 200 Rds
Cg 500 Rds
Figure 10. Figure of Merit curves.
I 5
1
(
(
(
o
1
2
D
ds(on
,
,
,
2
D
50
. Usually a trade-off between R
1
)
)
)
2
2
1
I (
I )(
> 0.5, then
40
20
o
0
1
o
1
1
0
I
g
o
I
o
2
are used for the top switch and
2
)
FOM:100*10^{-12}
FOM:200*10^{-12}
FOM:500*10^{-12}
2
)
2
g
On-resistance (mOhm)
5
) and channel on-resistance
D (
1 (
1
www.DataSheet4U.com
D
Rds
10
D
2
2
I )
o
1
2
0
5 .
15
1 (
I )
www.semtech.com
o
SC2446
2
2
D
.
1
20
20
I )
ds(on
o
2
2
.
and C
g