LT3800
8
3800fc
Overview
The LT3800 is a high input voltage range step-down
synchronous DC/DC converter controller IC that uses a
200kHz constant frequency, current mode architecture
with external N-channel MOSFET switches.
The LT3800 has provisions for high effi ciency, low load
operation for battery-powered applications. Burst Mode
operation reduces total average input quiescent currents to
100µA 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.
The LT3800 also employs a reverse-current inhibit feature,
allowing increased effi ciencies 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 LT3800’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 LT3800 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, in turn, used as a
threshold for the current sense comparator.
During normal operation, the LT3800 internal oscilla-
tor runs at 200kHz. 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
450ns. This minimum off-time mode of operation assures
regeneration of the BOOST bootstrapped supply.
Power Requirements
The LT3800 is biased using a local linear regulator to
generate internal operational voltages from the V
IN
pin.
Virtually all of the circuitry in the LT3800 is biased via an
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 LT3800 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 maxi-
mizes conversion effi ciencies and is common practice. If
V
CC
is maintained above 6.5V using an external source,
the LT3800 can continue to operate with V
IN
as low as 4V.
The LT3800 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 LT3800 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.
APPLICATIONS INFORMATION
