sc4508a Semtech Corporation, sc4508a Datasheet - Page 11

• Manufacturer Part Number: sc4508a
• Description: Sc4508a Buck Or Buck-boost Inverting Current-mode Controller
• Manufacturer: Semtech Corporation

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
MOSFET selection also depends on applications. In many
applications, either switching loss or conduction loss
dominates for a particular MOSFET. For buck and buck-
boost converters with high input to output voltage ratios,
the MOSFET is hard switched but conducts with very low
duty cycle. For such applications, MOSFETs with low C
should be used.
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
The RMS value of the MOSFET switch current is calculated
as:
The conduction losses are then
Idc is average inductor current. In buck converter, it is
also load current. In buck-boost, it is load current divided
by 1-D.
R
Curves showing R
Application Information (Cont.)
POWER MANAGEMENT
g
g
ds(on)
2007 Semtech Corp.
and low R
has to be made.
f
varies with temperature and gate-source voltage.
, and
Cg 100 Rds
Cg 200 Rds
Cg 500 Rds
Figure 5. Figure of Merit curves.
(
(
(
,
,
,
ds(on
50
1
)
)
)
. Usually a trade-off between R
40
20
0
I
1
0
Qrms
P
ds(on)
tc
= I
FOM:100*10^{-12}
FOM:200*10^{-12}
FOM:500*10^{-12}
I
Qrms
variations can be found in
dc
On-resistance (mOhm)
5
2
D
R
1 (
ds(on)
Rds
10
12
2
)
15
G
20
20
.
r
ds(on
and fall
ds(on)
and
g
,
11
manufacturers’ data sheet. From the FDS6675
datasheet, R
than 10V. However R
junction temperature increases from 25
The switching losses can be estimated using the simple
formula:
where t
switching process. Different manufacturers have
different definitions and test conditions for t
clarify these, we sketch the typical MOSFET switching
characteristics under clamped inductive mode in Figure
6.
In Figure 6,
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
gsp
gs1
gs2
gd
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
is the rise time and t
gs
r
is the total resistance from the driver supply
= t
to the threshold voltage V
ds(on)
3
P
-t
ts
1
and t
is less than 14m
g
within the MOSFET i.e.
1
2
t
r
ds
t (
R
r
gi
is falling.
r
gt
, external resistance R
(
ds(on)
can be approximated as:
Q
= R
t
gs
V
f
)(
2
cc
gi
1
increases by 30% as the
+R
Q
V
gd
ge
2
gsp
f
I )
+R
R )
is the fall time of the
dc
gt
V
g
ds
when V
in
gs_th
.
f
SC4508A
and
s
o
www.semtech.com
,
f
C to 110
is:
gs
r
ge
and t
is greater
and the
o
1
C.
f
, t
. To
3
].