Datasheet

LT3844
19
3844fb
APPLICATIONS INFORMATION
two losses. Calculate the maximum conduction losses of
the MOSFET:
PDC
I
DC
R
COND MAX
OUT MAX
MAX
DS ON
=
−
⎛
⎝
⎜
⎞
⎠
⎟
()
()
•
1
Note that R
DS(ON)
has large positive temperature depen-
dence. The MOSFET manufacturer’s data sheet contains
a curve, R
DS(ON)
vs Temperature. Calculate the maximum
transition losses:
P
kV I C f
DC
TRAN
OUT OUT MAX RSS SW
=
()( )
()
()()
−
2
1
()
(
MMAX
)
where k is a constant inversely related to the gate driver
current, approximated by k = 2 for LT3844 applications.
The total maximum power dissipation of the MOSFET is
the sum of these two loss terms:
P
FET(TOTAL)
= P
COND
+ P
TRAN
To achieve high supply effi ciency, keep the P
FET(TOTAL)
to
less than 3% of the total output power. Also, complete
a thermal analysis to ensure that the MOSFET junction
temperature is not exceeded.
T
J
= T
A
+ P
FET(TOTAL)
• θ
JA
where θ
JA
is the package thermal resistance and T
A
is the
ambient temperature. Keep the calculated T
J
below the
maximum specifi ed junction temperature, typically 150°C.
Note that when V
OUT
is high (>20V), the transition losses
may dominate. A MOSFET with higher R
DS(ON)
and lower
C
RSS
may provide higher effi ciency. MOSFETs with higher
voltage V
DSS
specifi cation usually have higher R
DS(ON)
and lower C
RSS
.
Choose the MOSFET V
DSS
specifi cation to exceed the
maximum voltage across the drain to the source of the
MOSFET, which is V
OUT
plus the forward voltage of the
rectifi er, typically less than 1V.
The internal V
CC
regulator is capable of sourcing up to
40mA which limits the maximum total MOSFET gate
charge, Q
G
, to 40mA / f
SW
. The Q
G
vs V
GS
specifi cation
is typically provided in the MOSFET data sheet. Use Q
G
at
V
GS
of 8V. If V
CC
is back driven from an external supply,
the MOSFET drive current is not sourced from the internal
regulator of the LT3844 and the Q
G
of the MOSFET is not
limited by the IC. However, note that the MOSFET drive
current is supplied by the internal regulator when the
external supply back driving V
CC
is not available such as
during start-up or short-circuit.
The manufacturer’s maximum continuous drain current
specifi cation should exceed the peak switch current which is
the same as the inductor peak current, I
L(MAX)
+ ΔI
L
/2.
During the supply start-up, the gate drive levels are set by
the V
CC
voltage regulator, which is approximately 8V. Once
the supply is up and running, the V
CC
can be back driven
by an auxiliary supply such as V
OUT
. It is important not
to exceed the manufacturer’s maximum V
GS
specifi cation.
A standard level threshold MOSFET typically has a V
GS
maximum of 20V.
Boost Converter: Rectifi er Selection
The rectifi er is selected based upon the forward voltage,
reverse voltage and maximum current. A Schottky diode
is recommended for its low forward voltage and yields the
lowest power loss and highest effi ciency. The maximum
reverse voltage that the diode will see is V
OUT
. The average
diode current is equal to the maximum output load current,
I
OUT(MAX)
. A diode rated at 1.5 to 2 times the maximum
average diode current is recommended. Remember boost
converters are not short-circuit protected.
Boost Converter: Output Capacitor Selection
In boost mode, the output capacitor requirements are
more demanding due to the fact that the current waveform
is pulsed instead of continuous as in a buck converter.
The choice of component(s) is driven by the acceptable
ripple voltage which is affected by the ESR, ESL and bulk
capacitance. The total output ripple voltage is:
ΔVI
fC
ESR
DC
OUT OUT MAX
SW OUT MAX
=+
−
⎛
⎝
⎜
⎞
⎠
⎟
()
•
1
1
where the fi rst term is due to the bulk capacitance and the
second term due to the ESR.