Quick Start Procedure - Linear Technology LTC3725 Quick Start Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 888A-B
lates by observing the output voltage directly result-
ing in superior output voltage regulation and transient
OPERATING PRINCIPLES – POLYPHASE
The LTC3725 and LTC3706 allow the user to develop
modular power supply "building blocks" that can be
added as power/current requirements increase. Con-
necting two DC888A power supplies in a PolyPhase
configuration has several advantages. By distributing
power across multiple high power/current supplies,
heat is also distributed, reducing individual compo-
nent temperatures. Each parallel module develops
equal output currents so that electrical and thermal
stresses are shared, increasing reliability. Multi-phase
operation and Shared input and output filtering result
in fewer/smaller input/output capacitors and induc-
tors for a given voltage/current ripple or transient re-
sponse.
In PolyPhase systems, one power supply is config-
ured as a "master" and one as a "slave". The master
communicates switching frequency via the PT+ pin to
OPTIONAL POLYPHASE SETUP
Only minor modifications and minimal interconnec-
tions are needed to implement PolyPhase with the
DC888. See component changes list (Figure 18) and
schematics (Figures 21 and 22) for the required elec-
trical changes to master and slave units. After the
modifications are done, the boards are then stacked
one on top of another (Figure 23). J1 and P1 headers
interconnect small signals and E1, E2, E3, and E4
stand offs provide interconnection for the power sig-
nals.

QUICK START PROCEDURE

Demonstration Circuit 888A-B is easy to set up to
evaluate the performance of the LTC3725 and
LTC3706. Refer to Figure 1 for proper equipment
setup. Follow the procedure below:
36V-72VIN, ISOLATED SYNCHRONOUS FORWARD
response.
FS/SYNC pin of the slave (Figures 19 and 20). The
relative clock phase of each stage is determined by
the slave. The master's voltage error amplifier's out-
put (ITH pin) controls the output current of all the
phases via the ITH pin voltage which is distributed to
each slave's unity-gain differential amplifier.
Several of the signals that are shared between the
master and the slave are of a bidirectional nature. A
fault on either phase can be communicated to the op-
posite phase via the primary side SS/FLT pin inter-
connection or the secondary side RUN/SS intercon-
nection. Sharing Vcc on the secondary side ensures
the master, which initially develops this bias voltage,
and slave power up simultaneously. Each phase then
contributes to the shared Vcc bus. Finally, the input
voltage (Vin) and output voltage (Vout) busses are
interconnected to allow for load sharing.
The DC888 was designed primarily to demonstrate
the chipset's single phase operation and therefore be
further optimized for PolyPhase applications. A small
resistor can be placed between the R76/D27 junction
and C70/U2-16 junction to reduce already small PWM
jitter associated with separate master and slave
ground planes. Another optimization can result from
combining each individual phase's input/output filter
components into one shared input/output filter.
When measuring the input or output voltage
NOTE:
ripple, care must be taken to avoid a long ground lead
on the scope probe. Measure the output (or input)
voltage ripple by touching the probe tip and probe
2