LT8708-1
14
Rev 0
For more information
OPERATION
The LT8708-1 is a high performance 4-switch buck-boost
slave controller that is paralleled with the master LT8708
to increase power capability. Using LT8708-1(s) with the
LT8708, an application can command power to be deliv-
ered from V
IN
to V
OUT
or from V
OUT
to V
IN
as needed.
COMMON LT8708-1 AND LT8708 FEATURES
The LT8708-1 and LT8708 share many common func-
tions and features that are already documented in the
LT8708 data sheet. Table 1 lists the LT8708 data sheet
sections that also apply to the LT8708-1. For some of
these features, additional LT8708-1 specific information
is provided in this data sheet, as indicated in Table 1.
The focus of this data sheet is on how to use the
LT8708-1 to increase the number of switching phases
in an LT8708-based application. As such, functionality
that is identical in both the LT8708 and LT8708-1 will not
necessarily be repeated here. It is assumed that readers
of this data sheet are already familiar with the LT8708.
ADDING PHASES TO AN LT8708 APPLICATION
In a multiphase LT8708 application, a single LT8708 is the
master of the system. One or more LT8708-1s are slaves
that provide additional current as needed. As the master
of the multiphase system, the LT8708 and its respec-
tive error amplifiers, determine the current necessary to
regulate the V
IN
voltage, V
OUT
voltage, V
IN
current and
V
OUT
current. The slave LT8708-1 operates by sensing
the I
OUT(MASTER)
(see Figure 2) and delivering a propor-
tional amount of I
OUT(SLAVE)
. Again, since I
OUT(SLAVE)
is
proportional to I
OUT(MASTER)
, the master LT8708 is in
control of setting regulation voltages and current limits
to the system.
Each LT8708 and LT8708-1, connected in parallel, is
hereon referred to as a phase, the master and slave
V
IN
current is referred to as I
IN(MASTER)
and I
IN(SLAVE)
,
respectively. For multiphase operation, the LT8708
should be configured according to the LT8708 data sheet.
Configuration of LT8708-1s should follow instructions
in this data sheet. Figure 2 shows a simplified drawing
of a multiphase system with one LT8708 and multiple
Table 1. LT8708 Data Sheet Sections that Apply to the LT8708-1
LT8708 DATA SHEET SECTION
ADDITIONAL INFORMATION
IN THIS DATA SHEET
Operation
Start-Up:
SHDN
Pin
Power Switch Control
Unidirectional and Bidirectional Conduction
Yes
INTV
CC
/EXTV
CC
/GATEV
CC
/LDO33 Power
CLKOUT and Temperature Sensing
Applications Information
Internal Oscillator
SYNC Pin and Clock Synchronization
CLKOUT Pin and Clock Synchronization
Inductor Current Sensing and Slope
Compensation
R
SENSE
Selection and Maximum Current
R
SENSE
Filtering
Inductor (L) Selection
Power MOSFET Selection
Schottky Diode (D1, D2, D3, D4) Selection
Topside MOSFET Driver Supply
(C
B1
, D
B1
, C
B2
, D
B2
)
VINHIMON, VOUTLOMON and
RVSOFF
Yes
INTV
CC
Regulators and EXTV
CC
Connection
LDO33 Regulator
Voltage Lockouts
Yes
Junction Temperature Measurement
Thermal Shutdown
Efficiency Considerations
Circuit Board Layout Checklist
Yes
LT8708-1s. It illustrates the basic connections needed to
add LT8708-1s to a multiphase system.
Adding Phases: The Master LT8708
The master controls the overall current delivered by the
multiphase system. For example, the LT8708 controls the
V
IN
and V
OUT
regulation voltages through its FBIN and
FBOUT pins. Since the slaves primarily duplicate the mas-
ter’s I
OUT(MASTER)
current, the slave’s FBIN and FBOUT
pins and related circuitry are typically not used. See the
Error Amplifiers section on how they can affect V
C
and
how to disable them.
