SC4508A データシートの表示（PDF） - Semtech Corporation
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SC4508A Datasheet PDF : 21 Pages
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SC4508A
POWER MANAGEMENT
Application Information (Cont.)
50
40
Cg(100, Rds)
Cg(200, Rds)
Cg(500, Rds) 20
1 00
5
10
15
20
1
Rds
20
On-resistance (mOhm)
FOM:100*10^{-12}
FOM:200*10^{-12}
FOM:500*10^{-12}
Figure 5. Figure of Merit curves.
manufacturers’ data sheet. From the FDS6675
datasheet, Rds(on) is less than 14mΩ when Vgs is greater
than 10V. However Rds(on) increases by 30% as the
junction temperature increases from 25oC to 110oC.
The switching losses can be estimated using the simple
formula:
Pts
=
1
2
(t
r
+
tf )(1+
δ
2
)Idc
Vinfs
where tr is the rise time and tf is the fall time of the
switching process. Different manufacturers have
different definitions and
clarify these, we sketch
test
the
ctyopnidciatiloMnsOfSoFrEtTr
asnwditctfh.inTog
characteristics under clamped inductive mode in Figure
6.
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 and low R . Usually a trade-off between R and
g
ds(on
ds(on
Cg 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 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 Cg
should be used.
MOSFET power dissipation consists of:
a) conduction loss due to the channel resistance Rds(on),
b) switching loss due to the switch rise time tr and fall
time tf, and
c) the gate loss due to the gate resistance RG.
The RMS value of the MOSFET switch current is calculated
as:
IQrms = Idc
D(1 +
δ2
12
)
The conduction losses are then
Ptc
=
I2
Qrms
Rds(on)
Idc is average inductor current. In buck converter, it is
also load current. In buck-boost, it is load current divided
by 1-D.
Figure 6. MOSFET switching characteristics
In Figure 6,
Qgs1 is the gate charge needed to bring the gate-to-source
voltage Vgs to the threshold voltage Vgs_th,
Qgs2 is the additional gate charge required for the switch
current to reach its full-scale value
Qgd is the charge needed to charge
Igdas.taen-tdo-drain
(Miller)
capacitance when Vds is falling.
Switching losses occur during the time interval [t1, t3].
Defining tr = t3-t1 and tr can be approximated as:
tr
=
(Qgs2 + Qgd )Rgt
Vcc − Vgsp
where Rgt is the total resistance from the driver supply
rail to the gate of the MOSFET. It includes the gate driver
internal impedance R , external resistance R and the
gi
ge
gate resistance R within the MOSFET i.e.
g
Rgt = Rgi+Rge+Rg
Rds(on) varies with temperature and gate-source voltage.
Curves showing Rds(on) variations can be found in
Vgsp is the Miller plateau voltage shown in Figure 11.
Similarly an approximate expression for tf is:
2007 Semtech Corp.
11
www.semtech.com
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