1
DEMO MANUAL
Rev. 0
DESCRIPTION
LT8550
4-Phase DC/DC Expander with Internal
Gate Drivers for Buck Converters
Demonstration circuit 2331A-A/2331A-B is a 4-phase
DC/DC expander with internal gate drivers for buck con-
. The demonstration cir-
cuit expands (increases) the number of power sections
for a LT3763 60V step-down controller to provide higher
output power. The increased number of power sections
allowed by the LT8550 results in more available output
power without a corresponding increase in the difficulty
of the design or a sacrifice of LT3763 features. The demo
board uses the multi-chip (master-slave) feature of the
LT8550 so the total number of power sections can be
increased in four power section increments:
• DC2331A-A has one power section for the LT3763 pri-
mary controller and an additional four power sections
for its LT8550 which is configured as a master
• DC2331A-B does not have a primary controller, and
its LT8550 is configured as a slave with four power
sections
The DC2331A-A and DC2331A-B use the same PCB and
the PCB is an example of how ground plane layers should
be configured and traces routed for reliable performance.
The input voltage range of DC2331A-A/DC2331A-B is from
14V to 56V and the output is 12V. The maximum output
current is 14A per power section, so the maximum output
current of DC2331A-A is 70A and each slave DC2331A-B
adds an additional 56A of output current capability. The
switching frequency is 250kHz and at 24V input to 12V
OUT
at full load, the efficiency of DC2331A-A is 97.5%.
The DC2331A-A/DC2331A-B is designed so it is easy to
add power sections and increase output power. The mas-
ter and slave assemblies are programmed using jumpers
and adjustment of regulation loop compensation is usu-
ally not necessary as power sections are added. The con-
trol signal interface between master and slave assemblies
is a common ribbon cable. The power cabling between
master and slave assemblies is similarly simple.
All registered trademarks and trademarks are the property of their respective owners.
The DC2331A-A/DC2331A-B’s jumpers program the
phase relationship between the power sections and turn
phase shedding on and off. Jumper JP5 programs the
LT8550 SYNC pin for master or slave operation and allows
DC2331A-A to be synchronized to an external clock. The
VCC and RSYNC JP5 settings—and the DC2331A assem-
blies in general—are forward compatible with the next
generation of phase expander ICs.
A ribbon cable provided with DC2331A-B interfaces the
control logic between a master DC2331A-A and a sin-
gle or multiple slave DC2331A-B(s). Each DC2331A-A/
DC2331A-B has two identical ribbon cable headers, so
daisy-chain ribbon cables are not required, even for sys-
tems with multiple slave assemblies. The DC2331A-A/
DC2331A-B uses high current #10 bolt-on terminals for
the input supply and load connections; and a sufficient
quantity of #10 ring lugs is provided to parallel master and
slaves with minimal DC offset voltage between assem-
blies. VIN
–
, VIN
+
, VO
–
and VO
+
terminals allow convenient
monitoring of input and output voltages while minimizing
the measured I • R PCB voltage drops caused by high
currents.
The DC2331A-A/DC2331A-B has terminals for
SHDN
and SYNC inputs to the LT8550. There are also terminals
that make it easy to monitor the control logic between
master and slave DC2331s. Removal of jumper resis-
tors can cleanly separate the electrical interface between
the LT3763 primary controller and the remainder of
DC2331A-A, making it possible to connect an alternate
primary controller in place of the LT3763. Noise immune
signal and ground via pairs are available to re-establish
the connections between DC2331A-A and twisted wire
pairs sensing the new primary controller.
A combination of capacitors is used at the DC2331A-A/
DC2331A-B power input: 1210 ceramics, 2220 ceramics
in a metal frame, hybrids and bulk electrolytic capacitors.
