LTC1871-1 LINER [Linear Technology], LTC1871-1 Datasheet - Page 18

• Manufacturer Part Number: LTC1871-1
• Description: High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3780
Power MOSFET Selection and
Effi ciency Considerations
The LTC3780 requires four external N-channel power
MOSFETs, 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
voltage V
capacitance C
The drive voltage is set by the 6V INTV
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:
where ρ
counting for the signifi cant 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
18
P
A BOOST
,
T
T
Figure 9. Normalized R
= 1.5 is reasonable.
GS,TH
is a normalization factor (unity at 25°C) ac-
2.0
1.5
1.0
0.5
0
=
–50
RSS
⎛
⎜
⎝
, on-resistance R
V
V
OUT
and maximum current I
IN
JUNCTION TEMPERATURE (°C)
0
I
OUT MAX
(
50
DS(ON)
)
DS(ON)
⎞
⎟
⎠
2
vs Temperature
100
ρ
, reverse transfer
T
BR,DSS
3780 F09
CC
DS(MAX)
R
150
DS ON
supply. Con-
, threshold
(
)
.
Switch B operates in buck mode as the synchronous
rectifi er. Its power dissipation at maximum output current
is given by:
Switch C operates in boost mode as the control switch. Its
power dissipation at maximum current is given by:
P
where C
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:
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:
The R
the R
the case to the ambient temperature (R
of T
used in the iterative calculation process.
C BOOST
,
P
P
T
J
B BUCK
D BOOST
J
,
,
can then be compared to the original, assumed value
TH(JC)
= T
TH(JA)
RSS
A
=
+ P • R
=
for the device plus the thermal resistance from
is usually specifi ed by the MOSFET manufactur-
+
(
=
to be used in the equation normally includes
V
V
k V
OUT
V
IN
V
OUT
IN
–
V
TH(JA)
OUT
IN
–
V
V
IN
OUT
V V
⎛
⎝ ⎜
IN
2
3
V
)
V
OUT
I
IN
OUT MAX
OUT
I
OUT MAX
V
(
IN
I
(
OUT MAX
I
OUT MAX
)
(
)
2
(
C
RSS
)
TH(JC)
ρ
⎞
⎠ ⎟
T
)
2
2
R
ρ
f
ρ
). This value
DS ON
T T
T
(
R
R
DS ON
)
DS ON
(
3780fe
(
)
)