Linear Technology LT8705 Demo Manual

DESCRIPTION
Demonstration circuit 1924A is a high performance buck-
boost converter featuring the
from input voltages above, below or equal to the output
voltage. The demo board input range is 36V to 80V. The
output is optimized for 48V at 5A, with the output current
limit set at 7A. (The circuit will operate with lower input
voltage than 36V if load current is reduced).
The controller has integrated input current, input voltage,
output current and output voltage regulators. The one
regulator that wants to decrease current gets control over
the compensation pin VC. The inductor current is controlled
by the VC signal that is fed into a current comparator
together with a ramp compensation signal.
While the current mode control limits the inductor current
both in normal and in reverse direction, these current
limits have some variation as input voltage changes. The
input and output current regulators offer more accurate
current limits.
The input voltage regulator is typically used in applica-
tions with solar panels or other high impedance power
sources, and will reduce the current if the input voltage
drops below the set point.
The operating mode of the controller is determined
through the MODE pin (jumper JP1) and can be set to
discontinuous mode, forced continuous mode and Burst
Mode operation.
®
The LT8705 is capable of bidirectional operation when
operating in forced continuous mode. Additional circuitry
may be needed depending on the application.
Arrow.com.
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80V V
LT 8705 that can operate
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DEMO MANUAL DC1924A
and V
IN OUT
4-Switch Buck-Boost DC/DC
The CLKOUT output and the SYNC input can be used
to synchronize two DC1924A circuits with 180 degree
phase shift.
By feeding the LT8705 from a separate low voltage sup-
ply, the power dissipation can be reduced. To supply the
LT8705 chip from an external voltage supply (> 6.4V), cut
the trace as marked on the board to disconnect the EXTVCC
pin from V . The LT8705 will start when voltage is ap-
OUT
plied at the input (VIN pin), and when it is running it will
draw current from the EXTVCC pin if the voltage is > 6.4V.
Typical efficiency with 5A load is above 97% across a 36V
to 72V input range using the supplied inductor. Lower
core loss can be achieved by using a ferrite core inductor.
The LT8705 data sheet gives a complete description of
the part, operation and application information. The data
sheet should be read in conjunction with this quick start
guide for demo circuit 1924A. The input voltage range of
the LT8705 itself is 2.8V (need EXTVCC > 6.4V) to 80V
and the output range is 1.3V to 80V.
The LT8705EUHF is assembled in a 38-Lead (5mm ×
7mm) plastic QFN package with a thermal pad underneath
the chip. Proper board layout is essential for maximum
thermal and electrical performance. See the data sheet
section Circuit Board Layout Checklist.
Design files for this circuit board are available at
http://www.linear.com/demo/DC1924A
L, LT, LTC, LTM, Linear Technology Burst Mode and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
LT8705
Synchronous
Controller
dc1924afa
1