LT8708-1
80V Synchronous 4-Switch Buck-Boost DC/DC
Slave Controller for LT8708 Multiphase System
FEATURES
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DESCRIPTION
The
LT
®
8708-1
is a high performance buck-boost
switching regulator controller that is paralleled with the
LT8708 to add power and phases to an LT8708 system.
The LT8708-1 always operates as a slave to the master
LT8708 and has the capability of delivering as much cur-
rent or power as the master. One or more slaves can be
connected to a single master, proportionally increasing
power and current capability of the system.
The LT8708-1 has the same conduction modes as LT8708,
allowing the LT8708-1 to conduct current and power in
the same direction(s) as the master. The master controls
the overall current and voltage limits for an LT8708 mul-
tiphase system, and the slaves comply with these limits.
LT8708-1s can be easily paralleled with the LT8708 by
connecting four signals together. Two additional current
limits (forward V
IN
current and reverse V
IN
current) are
available on each slave that can be set independently.
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Slave Chip of LT8708 to Deliver Additional Power
Good Current Matching to the Average Output
Current of LT8708 Through Current Regulation
Easily Paralleled with LT8708 Through Four Pins
Synchronized Start-Up with LT8708
Same Conduction Modes as LT8708
Synchronous Rectification: Up to 98% Efficiency
Frequency Range: 100kHz to 400kHz
Available in 40-Lead (5mm × 8mm) QFN with High
Voltage Pin Spacing
APPLICATIONS
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High Voltage Buck-Boost Converters
Bidirectional Charging Systems
Automotive 48V Systems
All registered trademarks and trademarks are the property of their respective owners.
TYPICAL APPLICATION
The LT8708-1 Two-Phase 12V Bidirectional Dual Battery System with FHCM and RHCM
10V TO 16V
BATTERY
V
BAT1
V
BAT2
100
10V TO
16V
BATTERY
EFFICIENCY (%)
99
98
97
96
95
94
LD033
Efficiency
Efficiency
TO DIODE
DB1
TO DIODE
DB2
MASTER
TG1 BOOST1 SW1 BG1 CSP CSN
CSNIN
LD033
LT8708*
SWEN
RVSOFF
CLKOUT
ICP
DIR ICN
CSPIN
V
INCHIP
SHDN
SWEN
RVSOFF
SYNC
ICP
ICN
VINHIMON
DIR
FBIN
LD033
MODE V
C
LT8708-1
SLAVE
GND BG2 SW2 BOOST2 TG2
CSPOUT
CSNOUT
EXTV
CC
VOUTLOMON
INTV
CC
GATEV
CC
IMON_OP
IMON_ON
IMON_INP
IMON_INN
SS
CLKOUT FBOUT
120kHz
87081 TA01a
V
BAT2
= 13.5V
V
BAT2
CHARGING CURRENT = 30A
10
12
V
BAT1
(V)
14
16
87081 TA01b
*REFER TO
LT8708 DATA
SHEET FOR
MASTER SETUP
RVS (0V) FWD (3V)
POWER TRANSFER
DECISION LOGIC
DB1
DB2
RT
TO
TO
BOOST1 BOOST2
LD033
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1
Rev 0
LT8708-1
TABLE OF CONTENTS
Features ..................................................... 1
Applications ................................................ 1
Typical Application ........................................ 1
Description.................................................. 1
Absolute Maximum Ratings .............................. 3
Order Information .......................................... 3
Pin Configuration .......................................... 3
Electrical Characteristics ................................. 4
Typical Performance Characteristics ................... 8
Pin Functions .............................................. 10
Block Diagram............................................. 13
Operation................................................... 14
Common LT8708-1 and LT8708 Features ............... 14
Adding Phases to an LT8708 Application................ 14
Adding Phases: The Master LT8708 .................... 14
Adding Phases: The Slave LT8708-1 .................... 16
Start-Up .................................................................. 17
Start-Up: SWEN Pin .............................................. 17
Start-Up: Soft-Start of Switching Regulator ......... 18
Control Overview .................................................... 18
Power Switch Control ............................................ 19
Unidirectional and Bidirectional Conduction ........... 19
Error Amplifiers ...................................................... 19
Transfer Function: I
OUT(SLAVE)
Vs I
OUT(MASTER)
......20
Transfer Function: CCM ........................................ 21
Transfer Function: DCM, HCM and Burst Mode
Operation ............................................................ 21
Current Monitoring and Limiting............................. 21
Monitoring: I
OUT(SLAVE)
......................................... 21
Monitoring and Limiting: I
IN(SLAVE)
...................... 21
Multiphase Clocking ...............................................22
Applications Information ................................ 23
Quick-Start Multiphase Setup .................................23
Quick Setup: Design the Master Phase .................23
Quick Setup: Design the Slave Phase(s) ...............23
Quick Setup: Evaluation ........................................23
Choosing the Total Number of Phases ....................23
Operating Frequency Selection ............................... 24
C
IN
and C
OUT
Selection ........................................... 24
C
IN
and C
OUT
Selection: V
IN
Capacitance .............. 24
C
IN
and C
OUT
Selection: V
OUT
Capacitance ...........25
VINHIMON, VOUTLOMON and
RVSOFF
..................25
Configuring the I
IN(SLAVE)
Current Limits ................26
Regulating I
OUT(SLAVE)
............................................26
I
OUT(SLAVE)
: Circuit Description.............................26
I
OUT(SLAVE)
: Configuration .................................... 28
Loop Compensation................................................ 28
Voltage Lockouts ....................................................29
Circuit Board Layout Checklist ...............................29
Design Example .....................................................29
Typical Applications ...................................... 31
Package Description ..................................... 35
Typical Application ....................................... 36
Related Parts .............................................. 36
2
Rev 0
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LT8708-1
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
IMON_ON
IMON_OP
V
INCHIP
34
33 CSPIN
32 CSNIN
31 CSNOUT
30 CSPOUT
29 EXTV
CC
41
GND
INTV
CC
LDO33
MODE
SWEN
V
CSP
– V
CSN
, V
CSPIN
– V
CSNIN
,
V
CSPOUT
– V
CSNOUT
............................... –0.3V to 0.3V
CSP, CSN Voltage ......................................... –0.3V to 3V
V
C
Voltage (Note 2) ................................... –0.3V to 2.2V
RT, FBOUT, SS Voltage ................................ –0.3V to 5V
IMON_INP, IMON_INN, IMON_OP,
IMON_ON, ICP, ICN Voltage ..................... –0.3V to 5V
SYNC Voltage............................................ –0.3V to 5.5V
INTV
CC
, GATEV
CC
Voltage ............................ –0.3V to 7V
V
BOOST1
– V
SW1
, V
BOOST2
– V
SW2
................ –0.3V to 7V
SWEN,
RVSOFF
Voltage ............................... –0.3V to 7V
SWEN Current .......................................................0.5mA
RVSOFF
Current .......................................................1mA
FBIN,
SHDN
Voltage .................................. –0.3V to 30V
VINHIMON Voltage..................................... –0.3V to 30V
VOUTLOMON Voltage .................................. –0.3V to 5V
DIR, MODE Voltage ...................................... –0.3V to 5V
CSNIN, CSPIN, CSPOUT, CSNOUT Voltage ...–0.3V to 80V
V
INCHIP
, EXTV
CC
Voltage ............................ –0.3V to 80V
SW1, SW2 Voltage ..................................... 81V (Note 6)
BOOST1, BOOST2 Voltage ......................... –0.3V to 87V
BG1, BG2, TG1, TG2 ........................................... (Note 5)
LDO33, CLKOUT ................................................ (Note 8)
Operating Junction Temperature Range
LT8708-1E (Notes 3, 8) .......................–40°C to 125
°
C
LT8708-1I (Notes 3, 8) ........................–40
°
C to 125
°
C
LT8708-1H (Notes 3, 8).......................–40
°
C to 150
°
C
Storage Temperature Range ...................–65
°
C to 150
°
C
40 39 38 37 36 35
CLKOUT 1
SS 2
SHDN
3
CSN 4
CSP 5
ICN 6
DIR 7
FBIN 8
FBOUT 9
V
C
10
IMON_INP 11
IMON_INN 12
RT 13
SYNC 14
15 16 17 18
GND
BG1
GATEV
CC
BG2
28
ICP
27
VINHIMON
26
VOUTLOMON
25
RVSOFF
24 BOOST1
23 TG1
22 SW1
19 20 21
BOOST2
TG2
SW2
UHG PACKAGE
40-LEAD (5mm × 8mm) PLASTIC QFN
T
JMAX
= 150°C,
θ
JA
= 36°C/W,
θ
JC
= 38°C/W
EXPOSED PAD (PIN 41) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT8708EUHG-1#PBF
LT8708IUHG-1#PBF
LT8708HUHG-1#PBF
TAPE AND REEL
LT8708EUHG-1#TRPBF
LT8708IUHG-1#TRPBF
LT8708HUHG-1#TRPBF
PART MARKING*
87081
87081
87081
PACKAGE DESCRIPTION
40-Lead (5mm × 8mm) Plastic QFN
40-Lead (5mm × 8mm) Plastic QFN
40-Lead (5mm × 8mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
Consult ADI Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Tape and reel specifications.
Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
For more information
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3
Rev 0
LT8708-1
ELECTRICAL CHARACTERISTICS
PARAMETER
Voltage Supplies and Regulators
V
INCHIP
Operating Voltage Range
V
INCHIP
Quiescent Current
V
INCHIP
Quiescent Current in Shutdown
EXTV
CC
Switchover Voltage
EXTV
CC
Switchover Hysteresis
INTV
CC
Current Limit
Max Current Draw from INTV
CC
and LDO33 Pins
Combined. Regulated from V
INCHIP
or EXTV
CC
(12V)
INTV
CC
= 5.25V
l
INTV
CC
= 4.4V
l
Regulated from V
INCHIP
, I
INTVCC
= 20mA
Regulated from EXTV
CC
(12V), I
INTVCC
= 20mA
I
INTVCC
= 0mA to 50mA
INTV
CC
Falling, GATEV
CC
Connected to INTV
CC
GATEV
CC
Connected to INTV
CC
V
INCHIP
– V
INTVCC
, I
INTVCC
= 20mA
5mA from LDO33 Pin
I
LDO33
= 0.1mA to 5mA
SYNC = 3V
LDO33 Falling
l
l
l
l
l
The
l
denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at T
A
= 25°C. V
INCHIP
=12V,
SHDN
= 3V, DIR = 3.3V unless otherwise noted. (Note 3).
CONDITIONS
EXTV
CC
= 0V
EXTV
CC
= 7.5V
Not Switching, V
EXTVCC
= 0
SWEN = 3.3V
SWEN = 0V
V
SHDN
= 0V
I
INTVCC
= –20mA, V
EXTVCC
Rising
l
l
l
MIN
5.5
2.8
TYP
MAX
80
80
UNITS
V
V
mA
mA
µA
V
V
4.7
2.45
0
6.15
6.4
0.2
7.5
4.5
1
6.6
90
28
6.1
6.1
4.45
127
42
6.3
6.3
–0.5
4.65
160
245
165
55
6.5
6.5
–1.5
4.85
mA
mA
V
V
%
V
mV
mV
INTV
CC
Voltage
INTV
CC
Load Regulation
INTV
CC
, GATEV
CC
Undervoltage Lockout
INTV
CC
, GATEV
CC
Undervoltage Lockout Hysteresis
INTV
CC
Regulator Dropout Voltage
LDO33 Pin Voltage
LDO33 Pin Load Regulation
LDO33 Pin Current Limit
LDO33 Pin Undervoltage Lockout
LDO33 Pin Undervoltage Lockout Hysteresis
Switching Regulator Control
Maximum Current Sense Threshold (V
CSP
– V
CSN
)
Maximum Current Sense Threshold (V
CSN
– V
CSP
)
Maximum Current Sense Threshold (V
CSN
– V
CSP
)
Maximum Current Sense Threshold (V
CSP
– V
CSN
)
Gain from V
C
to Max Current Sense Voltage
(V
CSP
– V
CSN
) (A5 in the Block Diagram)
SHDN
Input Voltage High
SHDN
Input Voltage High Hysteresis
SHDN
Input Voltage Low
3.23
12
2.96
3.295
–0.25
17.25
3.04
35
3.35
–1
22
3.12
V
%
mA
V
mV
Boost Mode, Minimum M3 Switch Duty Cycle
Buck Mode, Minimum M2 Switch Duty Cycle
Boost Mode, Minimum M3 Switch Duty Cycle
Buck Mode, Minimum M2 Switch Duty Cycle
Boost Mode
Buck Mode
SHDN
Rising to Enable the Device
Device Disabled, Low Quiescent Current
(LT8708E-1, LT8708I-1)
(LT8708H-1)
V
SHDN
= 3V
V
SHDN
= 12V
l
l
l
l
76
68
79
72
93
82
93
84
135
–135
110
97
108
96
mV
mV
mV
mV
mV/V
mV/V
V
mV
l
1.175
1.221 1.275
40
l
l
0.35
0.3
0
14
1
22
V
V
µA
µA
V
mV
SHDN
Pin Bias Current
SWEN Rising Threshold Voltage
SWEN Threshold Voltage Hysteresis
SWEN Output Voltage Low
l
1.156
1.208 1.256
22
I
SWEN
= 200µA
SHDN
= 0V or V
INCHIP
= 0V
SHDN
= 3V
SHDN
= 3V
l
l
l
0.9
0.2
0.75
0.8
1.1
0.5
V
V
V
SWEN Internal Pull-Down Release Voltage
4
Rev 0
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LT8708-1
ELECTRICAL CHARACTERISTICS
PARAMETER
MODE Pin Continuous Conduction Mode (CCM) Threshold
MODE Pin Hybrid DCM/CCM Mode (HCM) Range
MODE Pin Discontinuous Conduction Mode (DCM) Range
MODE Pin Burst Mode Operation Threshold
DIR Pin Forward Operation Threshold
DIR Pin Reverse Operation Threshold
RVSOFF
Output Voltage Low
RVSOFF
Falling Threshold Voltage
RVSOFF
Threshold Voltage Hysteresis
Soft-Start Charging Current
IMON_ON Rising Threshold for FDCM Operation
IMON_ON Falling Threshold for CCM Operation
IMON_INP Rising Threshold for RDCM Operation
IMON_INP Falling Threshold for CCM Operation
ICP Rising Threshold for Start Switching
ICN Rising Threshold for Start Switching
ICP Rising Threshold for Enabling Non-CCM Offset Current
ICP Falling Threshold for Disabling Non-CCM Offset Current
ICN Rising Threshold for Enabling Non-CCM Offset Current
ICN Falling Threshold for Disabling Non-CCM Offset Current
Voltage Regulation Loops (Refer to Block Diagram to Locate Amplifiers)
Regulation Voltage for FBOUT
Regulation Voltage for FBIN
FBOUT Pin Bias Current
FBOUT Error Amp EA4 g
m
FBOUT Error Amp EA4 Voltage Gain
VOUTLOMON Voltage Activation Threshold
VOUTLOMON Threshold Voltage Hysteresis
VOUTLOMON Pin Bias Current
FBIN Pin Bias Current
FBIN Error Amp EA3 g
m
FBIN Error Amp EA3 Voltage Gain
VINHIMON Voltage Activation Threshold
VINHIMON Threshold Voltage Hysteresis
VINHIMON Pin Bias Current
V
VINHIMON
= 1.17V, Current Into Pin
V
VINHIMON
= 1.24V, Current Out of Pin
l
The
l
denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at T
A
= 25°C. V
INCHIP
=12V,
SHDN
= 3V, DIR = 3.3V unless otherwise noted. (Note 3).
CONDITIONS
l
l
l
l
l
l
MIN
0.4
0.8
1.6
1.6
TYP
MAX
1.2
2.0
2.4
1.2
UNITS
V
V
V
V
V
V
V
V
mV
µA
µA
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
V
V
%/V
nA
µmho
V/V
V
mV
µA
µA
nA
µmho
V/V
I
RVSOFF
= 200µA
l
l
1.155
0.08
165
0.5
1.209 1.275
19
31
255
205
255
205
510
510
704
530
704
530
25
41
280
235
280
235
536
536
730
560
730
560
V
SS
= 0V
V
SS
= 0.5V
MODE = 1V (HCM), DIR = 3.3V
MODE = 1V (HCM), DIR = 3.3V
MODE = 1V (HCM), DIR = 0V
MODE = 1V (HCM), DIR = 0V
l
l
l
l
l
l
l
l
l
l
13
21
235
185
235
185
485
485
680
500
680
500
Regulate V
C
to 1.2V
Regulate V
C
to 1.2V
Current Out of Pin
l
1.193
l
1.184
1.207 1.222
1.205 1.226
0.002 0.005
15
345
245
Line Regulation for FBOUT and FBIN Error Amp Reference Voltage V
INCHIP
= 12V to 80V, Not Switching
Falling
V
VOUTLOMON
= 1.24V, Current Into Pin
V
VOUTLOMON
= 1.17V, Current Into Pin
Current Out of Pin
l
1.185
1.207 1.225
24
0.01
1
10
235
150
l
0.8
1.2
Rising
l
1.185
1.207
24
0.01
1
1.23
V
mV
µA
µA
0.8
1.2
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5
Rev 0
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