DESCRIPTION
Demonstration Circuit 1073 is a
100kHz-500kHz programmable frequency,
high voltage, current-mode DC/DC
step-down converter featuring the
LT3845. The operating frequency can
be synchronized up to 600kHz. The
demo board is designed for 12V @
6.25A (75W) output from a 20V to 55V
input. A soft-start feature controls
the output voltage slew rate at
start-up, reducing current surges and
voltage overshoots. The modes of op-
eration (Burst Mode, Discontinuous
Current Mode and Continuous Current
Mode) are jumper selectable. Both
Burst Mode and Discontinuous Current
Mode increase the efficiency at light
loads. Continuous Current Mode will
maintain a constant switching fre-
quency regardless of the load cur-
rent.
An Optional Boost Bias circuit is
provided on the bottom side of the
board for back-driving the LT3845 in-
ternal regulator from the output
voltage. Customers may want to use
this optional circuit with modified
applications that have relatively
high input voltages and low (~ 3.3V)
output voltages. In such applica-
tions, the optional circuit can in-
crease the overall efficiency by re-
ducing the power loss in the LT3845.
The demonstration board has also been
laid out with the option of doubling
the switching MOSFETs to facilitate
higher output current. This board is
suitable for a wide range of indus-
trial control systems and particu-
larly suitable for 12V/42V automotive
applications and 48V Telecom power
supplies.
The LT3845 datasheet gives a complete
description of the part, operation
and application information. The
datasheet must be read in conjunction
with this quick start guide for demo
circuit 1073.
Note: It is best to ground the SYNC
pin if the SYNC function is not being
used.
Design files for this circuit board
are available. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technol-
ogy Corporation. ThinSOT and PowerPath are trademarks of Lin-
ear Technology Corporation.
Performance Summary ( T
A
= 25
o
C )
PARAMETER FOR STEP-DOWN CONVERTER CONDITION VALUE
Minimum input voltage 20V
Maximum input voltage 55V
Output voltage V
OUT
12V
+/- 4%
Maximum output current 6.25A
Typical switching frequency 300kHz
High Voltage Synchronous Buck Controller
with Programmable Frequency and Sync
