SC2463 Semtech, SC2463 Datasheet - Page 15

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

Choosing P
Choosing P
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
If D
Choosing P
Choosing Po o o o o w w w w w er MOSFET
Choosing P
Main considerations in selecting the MOSFETs are power
dissipation, cost and packaging. Switching losses and
conduction losses of the MOSFETs 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 5.
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 MOSFETs with both low
C
C
I
I
I
POWER MANAGEMENT
Applications Information (Cont.)
Cin
Cin
Cin
g
g
2007 Semtech Corp.
ds(on)
and low R
has to be made.
1
1
1
1
>0.5 and D
>0.5 and D
<0.5 and D
>0.5 and (D
I
Cin
). In order to judge the performance of MOSFETs,
D (
. 0
. 0
I 5
I 5
1
o
o
1
1
2
2
D
D
ds(on
1
2
I
o
D (
D
1
2
1
2
2
2
1
. Usually a trade-off between R
2
1
, D
er MOSFET
er MOSFETs s s s s
er MOSFET
1
er MOSFET
<0.5, then
I (
I )(
< (D
-0.5) < D
> 0.5, then
o
D
1
o
0
2
1
2
5 .
I
and I
o
I
1
2
o
I
I )(
-0.5) < 0.5, then
2
o
2
2
)
.
o
2
)
1
2
g
o1
) and channel on-resistance
2
<0.5, then
D (
, I
I
o
1 (
2
o2
1
)
2
, respectively.
D
D
2
D (
2
I )
o
2
1
2
0
5 .
D
1 (
I )
1
o
2
2
D
0
.
5 .
1
I )
I )
o
o
2
2
ds(on
2
2
.
.
and
15
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,
MOSFETs with low C
MOSFETs with low R
MOSFET power dissipation consists of
a) conduction loss due to the channel resistance R
b) switching loss due to the switch rise time t
time t
c) the gate loss due to the gate resistance R
Top Switch:
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
than 10V. However R
junction temperature increases from 25
The switching losses can be estimated using the simple
formula
ds(on)
f
varies with temperature and gate-source voltage.
, and
Figure 5. Figure of Merit curves
Cg 100 Rds
Cg 200 Rds
Cg 500 Rds
(
(
(
ds(on)
,
,
,
50
is less than 8mW when V
1
)
)
)
I
Q
P
40
20
g
ds(on)
ds(on)
, 1
0
rms
tc
are used for the top switch and
1
0
= I
ds(on)
are used for the bottom switch.
variations can be found in
Q1,rms
I
o
FOM:100*10^{-12}
FOM:200*10^{-12}
FOM:500*10^{-12}
On-resistance (mOhm)
increases by 50% as the
5
D
2
1 (
R
www.DataSheet4U.com
ds(on)
12
Rds
10
2
. )
.
o
www.semtech.com
SC2463
15
C to 110
gs
G
.
is greater
r
20
20
and fall
o
C.
ds(on)
,