Datasheet

LTC3785
13
3785fc
applicaTions inForMaTion
eters 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 4.5V V
CC
supply.
Consequently, logic-level threshold MOSFETs must be used
in LTC3785 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:
PA(BOOST)=
V
OUT
V
IN
•I
OUT(MAX)






2
• ρT •R
DS(ON)
where ρT is a normalization factor (unity at 25°C) ac-
counting for the significant variation in on-resistance with
te
mperature, 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
rectifier. Its power dissipation at maximum output current
is given by:
PB(BUCK)=
V
IN
– V
OUT
V
IN
•I
OUT(MAX)
2
• ρT •R
DS(ON)
Switch C operates in boost mode as the control switch. Its
power dissipation at maximum current is given by:
PC(BOOST)=
V
OUT
– V
IN
( )
• V
OUT
V
IN
2
•I
OUT(MAX)
2
• ρT
• R
DS(ON)
+k • V
OUT
3
•
I
OUT(MAX)
V
IN
•C
RSS
• f
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 1.0.
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:
PD BOOST
( )
=
V
OUT
V
IN
•I
OUT(MAX)
2
• ρT •R
DS(ON)
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:
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(CA)
). This value
of T
J
can then be compared to the original, assumed value
used in the iterative calculation process.
SCHOT
TKY 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 significantly reduces reverse recovery
current between switch D turn off and switch C turn on,
which improves converter efficiency and reduces switch C
voltage stress. In order for D2 to be effective,
it must be
located in very close proximity to SWD.
Figure 4. Normalized R
DS(ON)
vs Temperature
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON-RESISTANCE
1.0
1.5
150
3785 F04
0.5
0
0
50
100
2.0