Datasheet

LTC3780
19
3780ff
For more information www.linear.com/LTC3780
applicaTions inForMaTion
Power MOSFET Selection and
Efficiency Considerations
The LTC3780 requires four external N-channel power MOS-
FETs, two for the top switches (switch A and D, shown in
Figure 1) and two for the bottom switches (switch B and C
shown in Figure 1). Important parameters for the power
MOSFETs are the breakdown voltage V
BR,DSS
, threshold
voltage V
GS,TH
, on-resistance R
DS(ON)
, reverse transfer
capacitance C
RSS
and maximum current I
DS(MAX)
.
The drive voltage is set by the 6V INTV
CC
supply. Con-
sequently, logic-level threshold MOSFETs must be used
in LTC3780 applications. If the input voltage is expected
to drop below 5V, then the sub-logic threshold MOSFETs
should be considered.
In order to select the power MOSFETs, the power dis
-
sipated by the device must be known. For switch A, the
maximum power dissipation happens in boost mode, when
it remains on all the time. Its maximum power dissipation
at maximum output current is given by:
P
V
V
IR
A BOOST
OUT
IN
OUT MAX TDSON,(
)(
)
•••=






2
ρ
where ρ
T
is a normalization factor (unity at 25°C) ac-
counting for the significant variation in on-resistance with
temperature, typically about 0.4%/°C as shown in Figure 9.
For a maximum junction temperature of 125°C, using a
value ρ
T
= 1.5 is reasonable.
Switch B operates in buck mode as the synchronous
rectifier. Its power dissipation at maximum output current
is given by:
P
VV
V
IR
BBUCK
IN OUT
IN
OUT MAX TDSON
,()(
)
–
•••=
2
ρ
Switch C operates in boost mode as the control switch.
Its power dissipation at maximum current is given by:
P
VVV
V
IR
kV
I
V
Cf
CBOOST
OUT IN OUT
IN
OUT MAX TDSON
OUT
OUT MAX
IN
RSS
,()(
)
()
–
•••
•• ••
=
()
+
2
2
3
ρ
where C
RSS
is usually specified by the MOSFET manufactur-
ers. The constant k, which accounts for the loss caused
by reverse recovery current, is inversely proportional to
the gate drive current and has an empirical value of 1.7.
For switch D, the maximum power dissipation happens in
boost mode, when its duty cycle is higher than 50%. Its
maximum power dissipation at maximum output current
is given by:
P
V
V
V
V
I
DBOOST
IN
OUT
OUT
IN
OUT MAX
,(
)
•• •=






2
ρ
TTDSON
R•
()
For the same output voltage and current, switch A has the
highest power dissipation and switch B has the lowest
power dissipation unless a short occurs at the output.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
T
J
= T
A
+ P • R
TH(JA)
The R
TH(JA)
to be used in the equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the case to the ambient temperature (R
TH(JC)
). This value
of T
J
can then be compared to the original, assumed value
used in the iterative calculation process.
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON-RESISTANCE (Ω)
1.0
1.5
150
3780 F09
0.5
0
0
50
100
2.0
Figure 9. Normalized R
DS(ON)
vs Temperature