LT3845A
10
3845afa
APPLICATIONS INFORMATION
Overview
The LT3845A is a high input voltage range step-down
synchronous DC/DC converter controller IC that uses
a programmable constant frequency, current mode
architecture with external N-channel MOSFET switches.
The LT3845A has provisions for high efficiency, 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
efficiencies during light loads through nonsynchronous
operation. This feature disables the synchronous switch if
inductor current approaches zero. If full time synchronous
operation 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
efficiency. 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 LT3845A senses converter output voltage via the V
FB
pin. The difference between the voltage on this pin and
an internal 1.231V reference is amplified 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 LT3845A 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 LT3845A is biased using an internal linear regulator to
generate operational voltages from the V
IN
pin. Virtually all
of the circuitry in the LT3845A 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 LT3845A 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 efficiencies and is common practice. If V
CC
is maintained above 6.5V using an external source, the
LT3845A can continue to operate with V
IN
as low as 4V.
The LT3845A 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.
