LTC3785-1 Linear Technology, LTC3785-1 Datasheet - Page 13

• Manufacturer Part Number: LTC3785-1
• Description: Buck-Boost Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
(switches B and C shown in Figure 1). Important param-
eters for the power MOSFETs are the breakdown voltage
V
reverse transfer capacitance C
I
Consequently, logic-level threshold MOSFETs must be used
in LTC3785-1 applications. If the input voltage is expected
to drop below 5V, then sub-logic threshold MOSFETs should
be considered. In order to select the power MOSFETs, the
power dissipated 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 maxi-
mum power dissipation at maximum output current is
given by:
where ρT is a normalization factor (unity at 25°C) ac-
counting for the signifi cant variation in on-resistance with
temperature, typically about 0.4%/°C as shown in Figure 4.
For a maximum junction temperature of 125°C, using a
value ρT = 1.5 is reasonable.
Switch B operates in buck mode as the synchronous
rectifi er. Its power dissipation at maximum output current
is given by:
DS(MAX)
BR(DSS)
PA BOOST
PB BUCK
(
(
. The drive voltage is set by the 4.35V V
, threshold voltage V
Figure 4. Normalized R
2.0
1.0
0.5
1.5
)
0
–50
=
)
=
V
⎛
⎝ ⎜
IN
V
–
JUNCTION TEMPERATURE (°C)
V
V
OUT
IN
IN
0
V
OUT
•
I
OUT MAX
GS(TH)
•
DS(ON)
50
I
OUT MAX
RSS
(
(
, on-resistance R
and maximum current
vs Temperature
)
100
⎞
⎠ ⎟
2
)
2
•
•
ρ
37851 F04
ρ
T R
T R
150
•
•
CC
DS O
DS ON
supply.
DS(ON)
(
(
N N )
)
,
Switch C operates in boost mode as the control switch. Its
power dissipation at maximum current is given by:
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 1.0.
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:
Typically, 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.
SCHOTTKY DIODE (D1, D2) SELECTION
Optional Schottky diodes D1 and D2 shown in the Block
Diagram conduct during the dead time between the conduc-
tion of the power MOSFET switches. They are intended to
prevent the body diode of synchronous switches B and D
from turning on and storing charge during the dead time.
In particular, D2 signifi cantly reduces reverse recovery
current between switch D turn off and switch C turn on,
which improves converter effi ciency and reduces switch
C voltage stress. In order for D2 to be effective, it must
be located in very close proximity to SWD.
PC BOOST
T
(
PD BOOST
J
J
TH(JC)
can then be compared to the original, assumed value
= 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
R
=
OUT
TH(JA)
DS ON
V
(
V
OUT
–
IN
V
V
)
IN
+
IN
•
2
k V
)
I
OUT MAX
•
•
V
OUT
OUT
(
3
•
)
LTC3785-1
•
I
2
OUT MAX
I
OUT MAX
•
TH(CA)
ρ
(
V
T R
(
IN
•
)
). This value
2
DS ON
)
•
•
(
ρ
C
13
T
RSS
)
37851f
•
f