Datasheet
LTC3813
15
3813fb
increase in coulombs on the horizontal axis from a to b
while the curve is fl at) is specifi ed for a given V
DS
drain
voltage, but can be adjusted for different V
DS
voltages by
multiplying by the ratio of the application V
DS
to the curve
specifi ed V
DS
values. A way to estimate the C
MILLER
term
is to take the change in gate charge from points a and b
on a manufacturers data sheet and divide by the stated
V
DS
voltage specifi ed. C
MILLER
is the most important se-
lection criteria for determining the transition loss term in
the top MOSFET but is not directly specifi ed on MOSFET
data sheets. C
RSS
and C
OS
are specifi ed sometimes but
defi nitions of these parameters are not included.
When the controller is operating in continuous mode
the
duty cycles for the top and bottom MOSFETs are
given by:
Main Switch Duty Cycle =
V
OUT
V
IN
V
OUT
Synchronous Switch Duty Cycle =
V
IN
V
OUT
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
P
MAIN
= D
MAX
I
O(MAX)
1D
MAX
2
(
T
)R
DS(ON)
+
1
2
V
OUT
2
I
O(MAX)
1D
MAX
(R
DR
)(C
MILLER
)
•
1
DRV
CC
–V
TH(IL)
+
1
V
TH(IL)
(f)
P
SYNC
=
1
1D
MAX
(I
O(MAX)
)
2
(
T
)R
DS(0N)
where ρ
T
is the temperature dependency of R
DS(ON)
, R
DR
is the effective top driver resistance (approximately 2Ω at
V
GS
= V
MILLER
). V
TH(IL)
is the data sheet specifi ed typical
gate threshold voltage specifi ed in the power MOSFET data
sheet at the specifi ed drain current. C
MILLER
is the calculated
capacitance using the gate charge curve from the MOSFET
data sheet and the technique described above.
Both MOSFETs have I
2
R losses while the bottom N-channel
equation includes an additional term for transition losses.
Both top and bottom MOSFET I
2
R losses are greatest at
lowest V
IN
, and the top MOSFET I
2
R losses also peak
during an overcurrent condition when it is on close to
100% of the period. For most LTC3813 applications,
the transition loss and I
2
R loss terms in the bottom
MOSFET are comparable, so best effi ciency is obtained
by choosing a MOSFET that optimizes both R
DS(ON)
and
C
MILLER
. Since there is no transition loss term in the syn-
chronous MOSFET, however, optimal effi ciency is obtained
by minimizing R
DS(ON)
—by using larger MOSFETs or
paralleling multiple MOSFETs.
Multiple MOSFETs can be used in parallel to lower R
DS(ON)
and meet the current and thermal requirements if desired.
The LTC3813 contains large low impedance drivers capable
of driving large gate capacitances without signifi cantly
slowing transition times. In fact, when driving MOSFETs
with very low gate charge, it is sometimes helpful to
slow down the drivers by adding small gate resistors
(10Ω or less) to reduce noise and EMI caused by the
fast transitions.
APPLICATIONS INFORMATION