LT8708-1
23
Rev 0
APPLICATIONS INFORMATION
This Applications Information section provides additional
details for setting up a multiphase application using the
LT8708-1(s) and LT8708. Topics include quick multi-
phase setup guidelines, choosing the total number of
phases, clock synchronization, and selection of various
external components. In addition, more information is
provided about voltage lockouts, current monitoring,
PCB layout considerations. This section wraps up with
a design example.
QUICK-START MULTIPHASE SETUP
This section provides a step-by-step summary on how
to setup a multiphase system using the LT8708-1(s) and
LT8708.
Quick Setup: Design the Master Phase
Design the LT8708 application circuit according to the
LT8708 data sheet. Make sure the maximum CSPOUT–
CSNOUT current sense voltage is limited to ±50mV by
setting the IMON_OP and IMON_ON resistor values equal
to or higher than 17.4k. This is the master phase.
Quick Setup: Design the Slave Phase(s)
Step 1 – Power Stage: Apply the same power stage design
from the LT8708 application circuit to the LT8708-1 cir-
cuit. This includes the inductor, power MOSFETs and
their gate resistors, R
SENSE
, R
SENSE
filtering, R
SENSE1
,
R
SENSE2
, CSPIN–CSNIN filtering, CSPOUT–CSNOUT fil-
tering, topside MOSFET driver supply (C
B1
, D
B1
, C
B2
, D
B2
)
and Schottky diodes D1, D2, D3, D4 (if used). See the
Selection section on how to optimize the
capacitor values.
Step 2 – Peripheral Pins: The following components
should be identical on the LT8708 as the LT8708-1:
• R
T
resistor
• SS pin capacitor
• INTV
CC
, GATEV
CC
, V
INCHIP
and LDO33 pin bypass
capacitors
Connect identical resistor divider networks on
SHDN
as
well as on VINHIMON and VOUTLOMON (if used). If not
used, connect VINHIMON to GND and/or VOUTLOMON to
the LT8708-1’s LDO33. Connect the LT8708-1’s FBOUT
pin to GND and the FBIN pin to the LT8708-1’s LDO33
node.
Step 3 – Interconnect: Connect the LT8708-1’s ICP, ICN,
EXTV
CC
, SWEN and
RVSOFF
pins to their counterparts on
the LT8708. Connect the same control signals, or con-
nect the same value resistor dividers or voltages to the
MODE pins and the DIR pins of the LT8708 and LT8708-1,
respectively. Connect the LT8708’s CLKOUT signal or
a clock chip’s phase shifted clock to the LT8708-1’s
SYNC pin.
Step 4 – Regulation and Limiting: Connect a 17.4k
resistor in parallel with a compensation network from
IMON_OP to GND. Connect a resistor in parallel with a
filter capacitor from IMON_ON to GND for current moni-
toring. Connect resistors in parallel with filter capacitors
from IMON_INP and IMON_INN to GND, respectively, to
set the magnitudes of the I
IN(SLAVE)
current limits to be
equal to or higher than their counterparts on the LT8708.
Step 5: Repeat steps 1 through 4 to add any additional
LT8708-1 phases.
Quick Setup: Evaluation
Test and optimize the stability of the multiphase system.
See the Loop Compensation section for more details.
CHOOSING THE TOTAL NUMBER OF PHASES
In general, the number of phases needed is selected to
meet a multiphase system’s total power requirement as
well as each phase’s thermal requirement. In general, for
a given application, the more phases that the system has,
the less power that each phase needs to deliver, and the
better the thermal performance that each phase has. In
many cases, the total number of phases is selected to
optimize the total input or output RMS current ripple.
See the C
