LTC3814-5
13
38145fc
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
INTV
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 LTC3814-5 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 LTC3814-5 contains large low impedance
drivers capable of driving large gate capacitances without
signifi cantly slowing transition times. In fact, when driv-
ing 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.
Operating Frequency
The choice of operating frequency is a tradeoff between
effi ciency and component size. Low frequency operation
improves effi ciency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3814-5 applications is
determined implicitly by the one-shot timer that controls
the on-time t
OFF
of the synchronous MOSFET switch.
The on-time is set by the current into the I
OFF
pin and the
voltage at the V
OFF
pin according to:
t
OFF
=
V
VOFF
I
IOFF
76pF
()
Tying a resistor R
OFF
from V
OUT
to the I
OFF
pin yields a syn-
chronous MOSFET on-time inversely proportional to V
OUT
.
This results in the following operating frequency and also
keeps frequency constant as V
OUT
ramps up at start-up:
f =
V
IN
V
VOFF
•R
OFF
(76pF)
(Hz)
The V
OFF
pin can be connected to INTV
CC
or ground or
can be connected to a resistive divider from V
IN
. The V
OFF
pin has internal clamps that limit its input to the one-shot
timer. If the pin is tied below 0.7V, the input to the one-
shot is clamped at 0.7V. Similarly, if the pin is tied above
2.4V, the input is clamped at 2.4V. Note, however, that
if the V
OFF
pin is connected to a constant voltage, the
operating frequency will be proportional to the input
voltage V
IN
. Figures 4a and 4b illustrate how R
OFF
relates
to switching frequency as a function of the input voltage
and V
OFF
voltage. To hold frequency constant for input
APPLICATIONS INFORMATION
