Datasheet

LTC3812-5
16
38125fc
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 topside N-chan-
nel equation incudes an additional term for transition
losses, which peak at the highest input voltage. For high
input voltage low duty cycle applications that are typical
for the LTC3812-5, transition losses are the dominate
loss term and therefore using higher R
DS(ON)
device with
lower C
MILLER
usually provides the highest effi ciency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during a
short-circuit when the synchronous switch is on close to
100% of the period. Since there is no transition loss term
in the synchronous MOSFET, 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 LTC3812-5 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 LTC3812-5 applications is
determined implicitly by the one-shot timer that controls
the on-time t
ON
of the top MOSFET switch. The on-time
is set by the current out of the I
ON
pin and the voltage at
the V
ON
pin according to:
t
ON
=
2.4V
I
ION
(76pF)
T ying a resistor R
ON
from V
IN
to the I
ON
pin yields an
on-time inversely proportional to V
IN
. For a step-down
converter, this results in approximately constant frequency
operation as the input supply varies:
f =
V
OUT
2.4V • R
ON
(76pF)
[Hz]
Figure 7 shows how R
ON
relates to switching frequency
for several common output voltages.
Figure 7. Switching Frequency vs R
ON
APPLICATIONS INFORMATION
R
ON
(kΩ)
10
100
SWITCHING FREQUENCY (kHz)
1000
100 1000
38112 F07
V
OUT
= 3.3V
V
OUT
= 12V
V
OUT
= 5V