Datasheet

LTC3766
47
3766fa
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applicaTions inForMaTion
switched once. The SSFLT pin currents then decrease to
their nominal values. This ensures that all phases begin their
asynchronous, open-loop start-up at nearly the same time.
On the secondary side, the SS pins from all phases are
interconnected as well. This prevents any one phase from
starting until all phases have adequate bias voltage and
have detected switching on their respective SW pins.
Once this condition is met, the master will advance the
soft-start voltage to match the V
OUT
of the converter, and
switching begins on the secondary side on all phases. After
a brief lock sequence, all phases will transfer control to
the secondary. The ITH pins are interconnected between
the phases so that current is shared evenly between the
master (which controls the ITH pin to regulate V
OUT
) and
the slaves.
The LTC3765 SSFLT connection is also used to com
-
municate faults. If one phase has a primary-side fault
(undervoltage, overcurrent, overtemperature, or com-
munication loss), it immediately stops switching and
rapidly pulls SSFL
T to 6V
. The other phases will detect
that SSFLT is above 5V and will also stop switching. On
the secondary side, the LTC3766s detect that switching
has stopped and also fault, which is communicated to all
phases through the common SS connection. The voltage
on the primary-side SSFLT node then slowly decreases
and a restart begins. Likewise, if a fault originates on the
secondary side on a give phase, this fault is communicated
to the other LTC3766s so that all phases stop switching.
This will cause a communication fault on the primary side
followed by a restart attempt.
For the LTC3765 on the primary side, choose components
based on a single-phase design. Duplicate the single phase
to the desired number of phases, up to the maximum of
four, with the following modifications:
1. Connect the SSFLT pins together. Instead of having
multiple capacitors from the SSFLT node to ground,
the capacitors can be consolidated into one capacitor
with a value equal to N • C
SSFLT
, where N is the number
of phases.
2. If desired, the phases can share the linear regulator of
one phase by shorting their V
CC
and NDRV pins to the
linear regulator output; however, be aware that the linear
regulator pass device will be dissipating more power
and may require a larger and more thermally conduc
-
tive package. The design and PCB layout are generally
simplified if each phase uses its own linear regulator.
The secondar
y side follows a similar procedure; however,
there is more differentiation between the master phase
and the slave phases. For the master, choose components
based on the above design equations. Be aware that each
phase should have its own linear regulator pass device to
distribute the power dissipation. Duplicate the components
for each slave, with the following exceptions:
1. Connect all of the SS pins together. Instead of having
multiple capacitors from the SS node to ground, the
capacitors can be consolidated into one capacitor. Note
that only the master charges and discharges the soft-
start capacitor.
2. Connect the FB pin of the slaves to V
CC
. This connection
puts the LTC3766 into slave mode. In this mode, the ITH
pin becomes a high impedance input and the SS pin is
only used for fault communication. An LTC3766 slave
will not perform a pre-set of the soft-start capacitor, nor
will it charge or discharge it. A slave can only force the
SS pin high to indicate a fault, and it also monitors the
SS pin to respond to a fault in another phase.
3. For each slave, the integrated unity-gain differential
amplifier is used to sense the voltage on the ITH pin
of the master. Connect the V
S
+
/V
S
–
inputs of each slave
between the ITH and signal GND pins of the master.
Connect the V
SOUT
pin on each slave to its own ITH pin.
4. Connect the FS/SYNC pins of each slave to the PT
–
pin
of the master. The PT
–
pin of the master contains the
clock signal used to synchronize the slaves and master
together.
For each slave, set the relative phase using the PHASE pin.
Note that ripple current in the input capacitor is minimized
by operating the controllers out of phase. For a 2-phase
system, set the slave at 180°. For a 3-phase system, set
the slaves at 120° and 240°. For a 4-phase system, set
the slaves at 90°, 180°, and 270°. Refer to Setting the
Switching Frequency and Synchronization for details on
setting the PHASE pin.