LT3845
10
3845fd
APPLICATIONS INFORMATION
Overview
The LT3845 is a high input voltage range step-down
synchronous DC/DC converter controller IC that uses a
programmable constant frequency, current mode archi-
tecture with external N-channel MOSFET switches.
The LT3845 has provisions for high effi ciency, low load
operation for battery-powered applications. Burst Mode
operation reduces total average input quiescent currents to
120μA during no load conditions. A low current shutdown
mode can also be activated, reducing quiescent current to
10μA. Burst Mode operation can be disabled if desired.
A reverse-current inhibit feature allows increased effi cien-
cies during light loads through nonsynchronous operation.
This feature disables the synchronous switch if inductor
current approaches zero. If full time synchronous opera-
tion is desired, this feature can be disabled.
Much of the IC’s internal circuitry is biased from an
internal linear regulator. The output of this regulator is
the V
CC
pin, allowing bypassing of the internal regulator.
The associated internal circuitry can be powered from
the output of the converter, increasing overall converter
effi ciency. Using externally derived power also eliminates
the IC’s power dissipation associated with the internal V
IN
to V
CC
regulator.
Theory of Operation (See Block Diagram)
The LT3845 senses converter output voltage via the V
FB
pin. The difference between the voltage on this pin and
an internal 1.231V reference is amplifi ed to generate an
error voltage on the V
C
pin which is used as a threshold
for the current sense comparator.
During normal operation, the LT3845 internal oscillator
runs at the programmed frequency. At the beginning of
each oscillator cycle, the switch drive is enabled. The
switch drive stays enabled until the sensed switch current
exceeds the V
C
derived threshold for the current sense
comparator and, in turn, disables the switch driver. If
the current comparator threshold is not obtained for the
entire oscillator cycle, the switch driver is disabled at the
end of the cycle for 350ns, typical. This minimum off-time
mode of operation assures regeneration of the BOOST
bootstrapped supply.
Power Requirements
The LT3845 is biased using an internal linear regulator to
generate operational voltages from the V
IN
pin. Virtually
all of the circuitry in the LT3845 is biased via this internal
linear regulator output (V
CC
). This pin is decoupled with
a low ESR, 1μF capacitor to PGND.
The V
CC
regulator generates an 8V output provided there
is ample voltage on the V
IN
pin. The V
CC
regulator has
approximately 1V of dropout, and will follow the V
IN
pin
with voltages below the dropout threshold.
The LT3845 has a start-up requirement of V
IN
> 7.5V. This
assures that the onboard regulator has ample headroom
to bring the V
CC
pin above its UVLO threshold. The V
CC
regulator can only source current, so forcing the V
CC
pin
above its 8V regulated voltage allows use of externally
derived power for the IC, minimizing power dissipation
in the IC. Using the onboard regulator for start-up, then
deriving power for V
CC
from the converter output maximizes
conversion effi ciencies and is common practice. If V
CC
is
maintained above 6.5V using an external source, the LT3845
can continue to operate with V
IN
as low as 4V.
The LT3845 operates with 3mA quiescent current from
the V
CC
supply. This current is a fraction of the actual V
CC
quiescent currents during normal operation. Additional
current is produced from the MOSFET switching currents
for both the boosted and synchronous switches and are
typically derived from the V
CC
supply.
Because the LT3845 uses a linear regulator to generate
V
CC
, power dissipation can become a concern with high
V
IN
voltages. Gate drive currents are typically in the range
of 5mA to 15mA per MOSFET, so gate drive currents can
create substantial power dissipation. It is advisable to
derive V
CC
and V
BOOST
power from an external source
whenever possible.
The onboard V
CC
regulator will provide gate drive power
for start-up under all conditions with total MOSFET gate
charge loads up to 180nC. The regulator can operate the
LT3845 continuously, provided the power dissipation of the
regulator does not exceed 250mW. The power dissipation
of the regulator is calculated as follows:
P
D(REG)
= (V
IN
– 8V) • (f
SW
• Q
G(TOTAL)
+ 3mA)
