SC2542EVB SEMTECH [Semtech Corporation], SC2542EVB Datasheet - Page 19

• Manufacturer Part Number: SC2542EVB
• Description: High Performance Wide Input Range Dual Synchronous Buck Controller
• Manufacturer: SEMTECH [Semtech Corporation]

Top Switch
The RMS value of the top switch current is calculated as:
The conduction losses are then:
Rds(on) varies with temperature and gate-source voltage.
Curves showing Rds(on) variations can be found in manu-
facturers’ data sheet. From the Si4860 datasheet, Rds
(on) is less than 8m
However Rds(on) increases by 50% as the junction tem-
perature increases from 25°C to 110°C.
The switching losses can be estimated using the simple
formula:
where tr is the rise time and tf is the fall time of the
switching process. Different manufactures have differ-
ent definitions and test conditions for tr and tf. To clarify
these, we sketch the typical MOSFET switching charac-
teristics under clamped inductive mode in Figure 17
Figure 17. MOSFET switching characteristics
In Figure 17, 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
© 2005 Semtech Corp.
POWER MANAGEMENT
Applications Information (Cont.)
I
Q rms
1,
P
TS
=
=
P
1
2
TC
Ω
(
I
t
O
r
=
when Vgs is greater than 10V.
+
I
Q
t
, 1
f
D
rms
)(1
(1
+
2
R
+
DS
δ
2
(
)
ON
δ
12
I V f
O IN S
)
2
)
19
the switch current to reach its full-scale value Ids,.and
Qgd is the charge needed to charge gate-to-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:
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:
Vgsp is the Miller plateau voltage shown in Figure17.
Similarly an approximate expression for tf is:
Only a portion of the total losses Pg = QgVccfs is dissi-
pated in the MOSFET package. Here Qg is the total gate
charge specified in the datasheet. The power dissipated
within the MOSFET package is:
The total bottom switch losses are then:
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.
Bottom Switch
The RMS current in bottom switch is given by:
The conduction losses are then:
I
Q rms
R
T
T
2,
P
P
r
GT
f
T
TG
=
=
=
=
=
=
(
(
P
I
Q
R
Q
O
TC
R
R
gs
GI
gs
V
GT
G
2
2
(1
+
cc
+
+
+
V
P
−
Q
−
R
Q
gsp
TS
Q
D
V
G
GE
gd
gd
V
gsp
)(1
+
CC
)
+
)
P
R
R
+
R
f
TG
gt
gt
S
δ
12
G
2
)
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