SC2545TSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2545TSTRT Datasheet PDF : 24 Pages
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SC2545
POWER MANAGEMENT
Applications Information (Cont.)
Top Switch
The RMS value of the top switch current is calculated as
The conduction losses are then
Ptc
=
I R . 2
Q1,rms ds(on)
IgnatFeig-tuor-seo9u,rcQegsv1oisltathgee
gate charge needed to bring the
Vgs to the threshold voltage Vgs_th.
Qgs2 is the additional gate charge required for the switch
current to reach
charge needed to
cihtsarfguellg-sactea-lteo-dvaraluine(MIdsil,l.earn) dcaQpagdciitsanthcee
when Vds is falling.
Switching losses occur during the time interval [t1, t3].
Defining tr = t3-t1 and tr can be approximated as
Rds(on) varies with temperature and gate-source voltage.
Curves showing Rds(on) variations can be found in
manufacturers’ data sheet. From the Si4860 datasheet,
Rds(on) is less than 8m Ω when Vgs is greater than 10V.
However Rds(on) increases by 50% 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
)Io
Vin
fs
.
where tr is the rise time and tf is the fall time of the
switching process. Different manufactures have different
definitions
these, we
and test
sketch
tchoendtityipoincsafloMr tOr
SaFnEd Ttf.s wToi
clarify
tching
characteristics under clamped inductive mode in Figure
9.
Vds
Volts
Ids
tr
=
(Qgs2 + Qgd )R gt
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 Rgi, external resistance Rge and the
gate resistance Rg within the MOSFET :
Rgt = Rgi+Rge+Rg.
Vgsp is the Miller plateau voltage shown in Figure 9.
Similarly an approximate expression for tf is
tf
=
(Qgs2 + Qgd )Rgt .
Vgsp
Only a portion of the total losses Pg = QgVccfs is dissipated
in the MOSFET package. Here Qg is the total gate charge
specified in the datasheet. The power dissipated within
the MOSFET package is
Vgs th
Miller plateau
Vgs
Ptg
=
Rg
Rgt
QgVcc fs.
The total power loss of the top switch is then
Qgs1 Qgs2
Qgd
t0 t1 t2
t3
Gate charge
Figure 9. MOSFET switching characteristics
Pt = Ptc+Pts+Ptg.
If the input supply of the power converter varies over a
wide range, then it will be necessary to weigh the
relative importance of conduction and switching losses.
This is because conduction losses are inversely propor-
tional to the input voltage. Switching loss however
increases with the input voltage. The total power loss
of MOSFET should be calculated and compared for
high-line and low-line cases. The worst case is then
used for thermal design.
© 2005 Semtech Corp.
14
www.semtech.com
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