SC2544TSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2544TSTRT' PDF : 24 Pages View PDF
SC2544
POWER MANAGEMENT
Applications Information (Cont.)
Top Switch
The RMS value of the top switch current is calculated
as
IQ1,rms = Io
D(1+
δ2
12
).
The conduction losses are then
In Figure 9, Q is the gate charge needed to bring
gs1
the gate-to-source voltage Vgs to the threshold voltage
V gs_th.
Q is the additional gate charge required for the
gs2
switch
is the
current
charge
nteoerdeeadchtoitschfaurllg-secagaletev-atolu-deraIdisn,.a(MndillQergd)
capacitance when Vds is falling.
Ptc =
I R . 2
Q1,rms ds(on)
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
dtsfwi.ffiTtecorhecinnltagrdicfeyhfaitnrhiateicostene,sriwsatenicdssktueentscdthecrothncedlaittmyioppniecsdaflionMrdOturScatFinEvdeT
mode in Figure 9.
V ds
V o lts
Id s
Switching losses occur during the time interval [t ,
1
t3].
Defining
tr
=
t3-t1
and
t
r
can
be
approximated
as
tr
=
(Q gs2 + Q gd )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 R , external resistance R
gi
ge
and the gate resistance Rg within the MOSFET :
Rgt = Rgi+Rge+Rg.
V is the Miller plateau voltage shown in Figure 9.
gsp
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
V gs th
M iller plateau
V gs
Ptg
=
Rg
R gt
QgVcc fs.
The total power loss of the top switch is then
Q gs1 Q gs2
Q gd
t0 t1 t2
t3
G ate charge
Figure 9. MOSFET switching characteristics
P = P +P +P .
t
tc ts tg
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 proportional 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
Share Link: 
