Datasheet

LTC3860
25
3860fc
APPLICATIONS INFORMATION
The curve is generated by forcing a constant input cur-
rent into the gate of a common source, current source
loaded stage and then plotting the gate voltage versus
time. The initial slope is the effect of the gate-to-source
and the gate-to-drain capacitance. The fl at portion of the
curve is the result of the Miller multiplication effect of the
drain-to-gate capacitance as the drain drops the voltage
across the current source load. The upper sloping line
is due to the drain-to-gate accumulation capacitance
and the gate-to-source capacitance. The Miller charge
(the 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 selection 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
Synchronous Switch Duty Cycle =
V
IN
–V
OUT
V
IN
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
P
MAIN
=
V
OUT
V
IN
I
MAX
()
2
(1+δ)R
DS(ON)
+
V
IN
2
I
MAX
2
(R
DR
)(C
MILLER
)•
1
V
CC
–V
TH(IL)
+
1
V
TH(IL)
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
(f)
P
SYNC
=
V
IN
− V
OUT
V
IN
(I
MAX
)
2
(1+δ)R
DS(0N)
where δ is the temperature dependency of R
DS(ON)
, R
DR
is the effective top driver resistance, V
IN
is the drain po-
tential and the change in drain potential in the particular
application. 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 previously described.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs temperature curve. Typical
values for δ range from 0.005/°C to 0.01/°C depending
on the particular MOSFET used.
Multiple MOSFETs can be used in parallel to lower R
DS(ON)
and meet the current and thermal requirements if desired.
Suitable drivers such as the LTC4449 are capable of driv-
ing 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 (5Ω or less) to
reduce noise and EMI caused by the fast transitions
MOSFET Driver Selection
Gate driver ICs, DRMOSs and power blocks with an interface
compatible with the LTC3860’s three-state PWM outputs
or the LTC3860’s PWM/PWMEN outputs can be used.