Linear Technology Analog Devices LT8708-1 Datasheet

RVS (0V)

DB2

80V Synchronous 4-Switch Buck-Boost DC/DC

BAT2

= 13.5V

BAT2

CHARGING CURRENT = 30A

BAT1

(V)

101214

169495

100

Efﬁciency

87081 TA01b

Slave Controller for LT8708 Multiphase System

FEATURES DESCRIPTION

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

High Voltage Buck-Boost Converters

Bidirectional Charging Systems

Automotive 48V Systems

All registered trademarks and trademarks are the property of their respective owners.

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 current 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 multiphase 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 VIN current and reverse VIN current) are available on each slave that can be set independently.

LT8708-1

TYPICAL APPLICATION

The LT8708-1 Two-Phase 12V Bidirectional Dual Battery System with FHCM and RHCM Efficiency

ICP ICN

BAT1

TO DIODE

DB1

TG1 BOOST1 SW1 BG1 CSP CSN

CSNIN

CSPIN V

INCHIP

SHDN

SWEN

RVSOFF

SYNC ICP ICN

VINHIMON

DIR

FBIN

LD033

MASTER

*REFER TO LT8708 DATA SHEET FOR MASTER SETUP

FWD (3V)

POWER TRANSFER

DECISION LOGIC

10V TO 16V

BATTERY

LT8708*

RVSOFF

CLKOUT

DIR

LD033

SWEN

LD033

MODE

GND BG2 SW2 BOOST2 TG2

LT8708-1

SLAVE

SSRT

TO DIODE

CLKOUT

120kHz

DB2

CSPOUT

CSNOUT

EXTV

VOUTLOMON

INTV

GATEV

IMON_OP

IMON_ON IMON_INP IMON_INN

FBOUT

BAT2

10V TO 16V BATTERY

LD033

DB1

BOOST1TOBOOST2

87081 TA01a

Rev 0

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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)

Monitoring and Limiting: I

.........................................21

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

CIN and C

Selection ...........................................24

OUT

Selection: VIN Capacitance .............. 24

OUT

Selection: V

OUT

Capacitance ........... 25

OUT

VINHIMON, VOUTLOMON and RVSOFF .................. 25

Configuring the I Regulating I

OUT(SLAVE)

: Circuit Description ............................. 26

: Configuration .................................... 28

IN(SLAVE)

Current Limits ................ 26

............................................ 26

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

Rev 0

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(Note 1)

CSP

CSPOUT

– V

CSN

, V

– V

CSPIN CSNOUT

CSNIN

............................... –0.3V to 0.3V

CSP, CSN Voltage ......................................... –0.3V to 3V

VC 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

INTVCC, GATEVCC Voltage ............................ –0.3V to 7V

BOOST1

– V

SW1

, V

BOOST2

– V

................ –0.3V to 7V

SW2

SWEN, RVSOFF Voltage ............................... –0.3V to 7V

SWEN Current .......................................................0.5mA

RVSO FF 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

, EXTVCC Voltage ............................ –0.3V to 80V

INCHIP

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

LT8708-1

PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS

TOP VIEW

LDO33

IMON_ON

IMON_OP

MODE

SWEN

40 39 38 37 36 35 34

1CLKOUT

SHDN

CSN

CSP

ICN

DIR

FBIN

FBOUT

IMON_INP

IMON_INN

SYNC

15 16 17 18

GND

40-LEAD (5mm × 8mm) PLASTIC QFN

= 150°C, θJA = 36°C/W, θJC = 38°C/W

JMAX

EXPOSED PAD (PIN 41) IS GND, MUST BE SOLDERED TO PCB

GND

BG1

BG2

GATEV

UHG PACKAGE

19 20 21

INTVCC

TG2

BOOST2

INCHIP

SW2

CSPIN

CSNIN

CSNOUT

CSPOUT

EXTV

ICP

VINHIMON

VOUTLOMON

RVSOFF

BOOST1

TG1

SW1

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LT8708EUHG-1#PBF LT8708EUHG-1#TRPBF 87081 40-Lead (5mm × 8mm) Plastic QFN –40°C to 125°C

LT8708IUHG-1#PBF LT8708IUHG-1#TRPBF 87081 40-Lead (5mm × 8mm) Plastic QFN –40°C to 125°C

LT8708HUHG-1#PBF LT8708HUHG-1#TRPBF 87081 40-Lead (5mm × 8mm) Plastic QFN –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.

Rev 0

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LT8708-1

ELECTRICAL CHARACTERISTICS

The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Voltage Supplies and Regulators

Operating Voltage Range EXTVCC = 0V

INCHIP

Quiescent Current Not Switching, V

INCHIP

Quiescent Current in Shutdown V

INCHIP

EXTVCC Switchover Voltage I

EXTVCC Switchover Hysteresis 0.2 V

INTVCC Current Limit Max Current Draw from INTVCC and LDO33 Pins

INTVCC Voltage Regulated from V

INTVCC Load Regulation I

INTVCC, GATEVCC Undervoltage Lockout INTVCC Falling, GATEVCC Connected to INTV

INTVCC, GATEVCC Undervoltage Lockout Hysteresis GATEVCC Connected to INTV

INTVCC Regulator Dropout Voltage V

LDO33 Pin Voltage 5mA from LDO33 Pin

LDO33 Pin Load Regulation I

LDO33 Pin Current Limit SYNC = 3V

LDO33 Pin Undervoltage Lockout LDO33 Falling 2.96 3.04 3.12 V

LDO33 Pin Undervoltage Lockout Hysteresis 35 mV

Switching Regulator Control

Maximum Current Sense Threshold (V

CSP

CSN

CSP

– V

) Boost Mode, Minimum M3 Switch Duty Cycle

CSN

– V

) Buck Mode, Minimum M2 Switch Duty Cycle

CSP

– V

) Boost Mode, Minimum M3 Switch Duty Cycle

CSP

– V

) Buck Mode, Minimum M2 Switch Duty Cycle

CSN

Gain from VC to Max Current Sense Voltage (V

– V

CSP

) (A5 in the Block Diagram)

CSN

SHDN Input Voltage High SHDN Rising to Enable the Device SHDN Input Voltage High Hysteresis 40 mV SHDN Input Voltage Low Device Disabled, Low Quiescent Current

SHDN Pin Bias Current V

SWEN Rising Threshold Voltage

SWEN Threshold Voltage Hysteresis 22 mV

SWEN Output Voltage Low I

SWEN Internal Pull-Down Release Voltage SHDN = 3V

EXTVCC = 7.5V

SWEN = 3.3V SWEN = 0V

= 0V 0 1 µA

SHDN

= –20mA, V

INTVCC

Combined. Regulated from V INTVCC = 5.25V INTVCC = 4.4V

Regulated from EXTVCC (12V), I

= 0mA to 50mA –0.5 –1.5 %

INTVCC

– V

INCHIP

= 0.1mA to 5mA –0.25 –1 %

LDO33

Boost Mode Buck Mode

(LT8708E-1, LT8708I-1) (LT8708H-1)

= 3V

SHDN

= 12V

SHDN

= 200µA

SWEN

SHDN = 0V or V SHDN = 3V

=12V, SHDN = 3V, DIR = 3.3V unless otherwise noted. (Note 3).

INCHIP

5.5

2.8

EXTVCC

= 0

4.7

2.45

6.15 6.4 6.6 V

127

6.1

6.3

6.1

6.3

4.45 4.65 4.85 V

160 mV

3.23 3.295 3.35 V

12 17.25 22 mA

76 93 110 mV

68 82 97 mV

79 93 108 mV

72 84 96 mV

INTVCC

Rising

EXTVCC

or EXTVCC (12V)

INCHIP

, I

INCHIP

, I

INTVCC

= 20mA

INTVCC

= 20mA

= 20mA 245 mV

135

–135

1.175 1.221 1.275 V

l l

0.35

141 22

1.156 1.208 1.256 V

INCHIP

= 0V

l l

0.75 0.8 V

0.9

0.2

80 80

7.5

4.5

165

6.5

0.3

1.1

0.5

mA mA

mV/V mV/V

µA µA

V V

Rev 0

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LT8708-1

ELECTRICAL CHARACTERISTICS

The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. V

PARAMETER CONDITIONS MIN TYP MAX UNITS

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 I RVSOFF Falling Threshold Voltage RVSOFF Threshold Voltage Hysteresis 165 mV

Soft-Start Charging Current VSS = 0V

IMON_ON Rising Threshold for FDCM Operation MODE = 1V (HCM), DIR = 3.3V

IMON_ON Falling Threshold for CCM Operation MODE = 1V (HCM), DIR = 3.3V

IMON_INP Rising Threshold for RDCM Operation MODE = 1V (HCM), DIR = 0V

IMON_INP Falling Threshold for CCM Operation MODE = 1V (HCM), DIR = 0V

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 Regulate VC to 1.2V

Regulation Voltage for FBIN Regulate VC to 1.2V

Line Regulation for FBOUT and FBIN Error Amp ReferenceVoltage V

FBOUT Pin Bias Current Current Out of Pin 15 nA

FBOUT Error Amp EA4 g

FBOUT Error Amp EA4 Voltage Gain 245 V/V

VOUTLOMON Voltage Activation Threshold Falling

VOUTLOMON Threshold Voltage Hysteresis 24 mV

VOUTLOMON Pin Bias Current V

FBIN Pin Bias Current Current Out of Pin 10 nA

FBIN Error Amp EA3 g

FBIN Error Amp EA3 Voltage Gain 150 V/V

VINHIMON Voltage Activation Threshold Rising

VINHIMON Threshold Voltage Hysteresis 24 mV

VINHIMON Pin Bias Current V

= 200µA

RVSOFF

VSS = 0.5V

= 12V to 80V, Not Switching 0.002 0.005 %/V

INCHIP

VOUTLOMON

VINHIMON

=12V, SHDN = 3V, DIR = 3.3V unless otherwise noted. (Note 3).

INCHIP

0.4 V

0.8 1.2 V

1.6 2.0 V

1.6 V

= 1.24V, Current Into Pin = 1.17V, Current Into Pin

= 1.17V, Current Into Pin = 1.24V, Current Out of Pin

0.08 0.5 V

1.155 1.209 1.275 V

13 2119 3125

235 255 280 mV

185 205 235 mV

235 255 280 mV

185 205 235 mV

485 510 536 mV

680 704 730 mV

500 530 560 mV

680 704 730 mV

500 530 560 mV

1.193 1.207 1.222 V

1.184 1.205 1.226 V

345 µmho

1.185 1.207 1.225 V

0.01 1

0.8

235 µmho

1.185 1.207 1.23 V

0.01 1

0.8

2.4 V

1.2 V

µA

1.2

µA

µA µA

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Rev 0

LT8708-1

ELECTRICAL CHARACTERISTICS

The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Current Regulation Loops (Refer to Block Diagram to Locate Amplifiers)

Regulation Voltages for IMON_INP and IMON_OP VC = 1.2V

Regulation Voltages for IMON_INN VC = 1.2V

Line Regulation for IMON_INP, IMON_INN and IMON_OP Error Amp Reference Voltage

CSPIN Bias Current V

CSNIN Bias Current BOOST Capacitor Charge Control Block Not Active

CSPIN, CSNIN Common Mode Operating Voltage Range

CSPIN, CSNIN Differential Mode Operating Voltage Range

IMON_INP Output Current V

IMON_INN Output Current V

IMON_INP and IMON_INN Max Output Current

IMON_INP Error Amp EA5 g

IMON_INP Error Amp EA5 Voltage Gain 130 V/V

IMON_INN Error Amp EA1 g

IMON_INN Error Amp EA1 Voltage Gain FBIN = 0V, FBOUT = 3.3V 130 V/V

CSPOUT Bias Current V

CSNOUT Bias Current BOOST Capacitor Charge Control Block Not Active

CSPOUT, CSNOUT Common Mode Operating Voltage Range

CSPOUT, CSNOUT Differential Mode Operating Voltage Range

IMON_ON Output Current V

IMON_ON Max Output Current

CSPOUT–CSNOUT Regulation Voltage Regulate VC to 1.2V

= 12V to 80V 0.002 0.005 %/V

INCHIP

= 12V

CSPIN

= 1.5V

CSPIN

= 3.3V, V

SWEN

= 3.3V, V

SWEN

= 0V

SWEN

– V

CSPIN

– V

CSPIN

– V

CSPIN

– V

CSPIN

– V

CSNIN

– V

CSNIN

– V

CSNIN

– V

CSNIN

FBIN = 0V, FBOUT = 3.3V 190 µmho

= 12V

CSPOUT

= 1.5V

CSPOUT

= 3.3V, V

SWEN

= 3.3V, V

SWEN

= 0V

SWEN

– V

CSNOUT

– V

CSNOUT

– V

CSNOUT

– V

CSNOUT

– V

CSNOUT

– V

CSNOUT

IMON_OP

= 12V

CSNOUT

=12V, SHDN = 3V, DIR = 3.3V unless otherwise noted. (Note 3).

INCHIP

1.185 1.209 1.231 V

1.185 1.21 1.24 V

0.01

CSPIN CSPIN

= V = V

CSNIN CSNIN

= 12V = 1.5V

4.25

0.01

0 80 V

–100 100 mV

CSNIN CSNIN CSNIN CSNIN

CSPIN CSPIN CSPIN CSPIN

= 50mV, V = 50mV, V = 5mV, V = 5mV, V

CSNIN

CSNIN CSNIN CSNIN

CSNIN

CSNIN CSNIN CSNIN

= 5V

= 5V = 5V = 5V

= 5V

= 5V = 5V = 5V

64.5

22.5

70 25

120 µA

190 µmho

0.01

CSPOUT CSPOUT

= V = V

CSNOUT CSNOUT

= 12V = 1.5V

4.25

0.01

0 80 V

–100 100 mV

CSPOUT CSPOUT CSPOUT CSPOUT CSPOUT CSPOUT

= 17.4kΩ

= 50mV, V = 50mV, V = 5mV, V = 5mV, V = −5mV, V = −5mV, V

ICP = 1.218V, ICN = 0V

ICP = 0V, ICN = 1.218V

ICP = ICN = 0.348V

CSNOUT

CSNOUT CSNOUT CSNOUT

CSNOUT

= 5V

= 5V = 5V = 5V

= 5V

22.5

20.5

12.5

10.5

25 25 15 15

120 µA

43 50 55 mV

–55 –50 –44 mV

–6 0 6 mV

75.5

27.5 30

74 75

30.5 32

73 75

27.5 29

17.5

19.5

µA µA

µA µA µA

µA µA µA µA

µA µA

µA µA µA

µA µA µA µA µA µA

Rev 0

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LT8708-1

ELECTRICAL CHARACTERISTICS

The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. V

PARAMETER CONDITIONS MIN TYP MAX UNITS

NMOS Gate Drivers

TG1, TG2 Rise Time C

TG1, TG2 Fall Time C

BG1, BG2 Rise Time C

BG1, BG2 Fall Time C

TG1 Off to BG1 On Delay C

BG1 Off to TG1 On Delay C

TG2 Off to BG2 On Delay C

BG2 Off to TG2 On Delay C

Min On-Time for Main Switch in Boost Operation (t

ON(M3,MIN)

Min On-Time for Synchronous Switch in Buck Operation (t

) Switch M3, C

ON(M2,MIN)

Min Off-Time for Main Switch in Steady-State Boost Operation Switch M3, C

Min Off-Time for Synchronous Switch in Steady-State Buck Operation Switch M2, C

Oscillator

Switch Frequency Range SYNCing or Free Running 100 400 kHz

Switching Frequency, f

OSC

SYNC High Level for Synchronization

SYNC Low Level for Synchronization

SYNC Clock Pulse Duty Cycle V

Recommended Min SYNC Ratio f

SYNC/fOSC

CLKOUT Output Voltage High V

CLKOUT Output Voltage Low 1mA Into CLKOUT Pin 25 100 mV

CLKOUT Duty Cycle TJ = –40°C

CLKOUT Rise Time C

CLKOUT Fall Time C

CLKOUT Phase Delay SYNC Rising to CLKOUT Rising, f

= 3300pF (Note 4) 20 ns

LOAD

= 3300pF (Note 4) 20 ns

LOAD

= 3300pF (Note 4) 20 ns

LOAD

= 3300pF (Note 4) 20 ns

LOAD

= 3300pF Each Driver 90 ns

LOAD

= 3300pF Each Driver 80 ns

LOAD

= 3300pF Each Driver 90 ns

LOAD

= 3300pF Each Driver 80 ns

LOAD

) Switch M2, C

RT = 365k RT = 215k RT = 124k

= 0V to 2V 20 80 %

SYNC

– V

LDO33

= 0µA

LDO33

TJ = 25°C TJ = 125°C

= 200pF 20 ns

LOAD

= 200pF 20 ns

LOAD

=12V, SHDN = 3V, DIR = 3.3V unless otherwise noted. (Note 3).

INCHIP

= 3300pF 200 ns

LOAD

= 3300pF 200 ns

LOAD

= 3300pF 230 ns

LOAD

= 3300pF 230 ns

LOAD

102

l l

120

170

202

310

350

1.3 V

3/4

, 1mA Out of CLKOUT Pin,

CLKOUT

100 250 mV

22.7

44.1 77

= 100kHz

OSC

160 180 200 Degree

142 235 400

kHz kHz kHz

0.5 V

% % %

Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.

Note 2: Do not force voltage on the VC pin. Note 3: The LT8708E-1 is guaranteed to meet performance specifications

from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT8708I-1 is guaranteed over the full –40°C to 125°C junction temperature range. The LT8708H-1 is guaranteed over the full –40°C to 150°C operating junction temperature range.

Note 4: Rise and fall times are measured using 10% and 90% levels. Delay times are measured using 50% levels.

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Note 5: Do not apply a voltage or current source to these pins. They must be connected to capacitive loads only, otherwise permanent damage may occur.

Note 6: Negative voltages on the SW1 and SW2 pins are limited, in an application, by the body diodes of the external NMOS devices, M2 and M3, or parallel Schottky diodes when present. The SW1 and SW2 pins are tolerant of these negative voltages in excess of one diode drop below ground, guaranteed by design.

Note 7: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability.

Note 8: Do not force voltage or current into these pins.

Rev 0

LT8708-1

OUT

(A)

0.01

0.1110

010203040

708090

100

EFFICIENCY (%)

87081 G01

VIN = 11.5V

OUT

= 14.5V

HCM

DCM

CCM

OUT

(A)

0.01

0.1110

3001020304050

60708090100

EFFICIENCY (%)

87081 G02

VIN = 16V

OUT

= 12V

HCM

DCM

CCM

VIN = 14.5V

OUT

= 14.5V

HCM

DCM

CCM

OUT

(A)

0.01

0.1110

300102030

405060708090100

EFFICIENCY (%)

87081 G03

VC ≅ 1.3V

TEMPERATURE (°C)

–50

–30

–101030507090110

130

1502030

4050607080

87081 G04

ICP = ICN = 0.348V

CSPOUT–CSNOUT (mV)

–150

–75075

150

–103070

110

150

190

230

87081 G06

TJ = 25°C

MAXIMUM V

MINIMUM V

ICP/ICN VOLTAGE (V)

0.25

0.50

0.7511.25

0.5

1.0

1.5

2.0

2.5

(V)

87081 G07

TJ = 25°C

MAXIMUM V

MINIMUM V

SS (V)00.511.5

0.5

1.0

1.5

2.0

2.5

(V)

87081 G08

TJ = 25°C

MAXIMUM V

MINIMUM V

SS (V)

0.25

0.50

0.7510

0.5

1.0

1.5

2.0

2.5

(V)

87081 G09

VC = ~1.3V

TEMPERATURE (°C)

–50

–30

–101030507090110

130

15020304050

607080

87081 G05

TYPICAL PERFORMANCE CHARACTERISTICS

Efficiency vs Output Current (Boost Region – Page 32)

CSPOUT – CSNOUT Voltages (ICP=1.218V, ICN = 0V) (Five Parts)

Efficiency vs Output Current (Buck Region – Page 32)

CSNOUT – CSPOUT Voltages (ICP=1.218V, ICN = 0V) (Five Parts)

TA = 25°C, unless otherwise noted.

Efficiency vs Output Current (Buck-Boost Region – Page 32)

IMON_OP Output Current

Maximum and Minimum VC vs ICP_ICN (SS = 0)

Maximum and Minimum VC vs SS (ICP = ICN =0.348V)

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Maximum and Minimum VC vs SS (ICP or ICN = 1V)

Rev 0

LT8708-1

87081 G10

LT8708-1 I

BATTERY DISCONNECTED

LT8708-1 I

BATTERY DISCONNECTED

LT8708-1 I

BATTERY DISCONNECTED

LT8708-1 I

BATTERY DISCONNECTED

TYPICAL PERFORMANCE CHARACTERISTICS

Load Step (Page 32) VIN = 12V V

= 14.5V

OUT

LT8708 I

10A/DIV

= 12V, V

BAT1

LOAD STEP = 10A TO 25A LOAD APPLIED AT V

BAT2

Load Step (Page 32) VIN = 16V V

= 14.5V

OUT

LT8708 I

10A/DIV

500μs/DIV

REGULATED TO 14.5V

WITH

BAT2

Load Step (Page 32) VIN = 14.5V V

OUT

LT8708 I

10A/DIV

BAT1

LOAD STEP = 10A TO 25A LOAD APPLIED AT V

Load Step (Page 33) VIN = 48V V

OUT

LT8708 I

20A/DIV

TA = 25°C, unless otherwise noted.

= 14.5V

87081 G11

= 14.5V, V

500μs/DIV

REGULATED TO 14.5V

BAT2

WITH

BAT2

= 14.5V

PHASE 1

PHASE 2

PHASE 3

PHASE 4

500μs/DIV

= 16V, V

BAT1

LOAD STEP = 10A TO 25A LOAD APPLIED AT V

REGULATED TO 14.5V

BAT2

WITH

87081 G12

500μs/DIV

= 48V, V

BAT1

LOAD STEP = 20A TO 55A LOAD APPLIED AT V

REGULATED TO 14.5V

BAT2

87081 G13

WITH

Rev 0

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LT8708-1

PIN FUNCTIONS

CLKOUT (Pin 1): Clock Output Pin. Use this pin to synchronize one or more compatible switching regulator ICs. CLKOUT toggles at the same frequency as the internal oscillator or as the SYNC pin, but is approximately 180° out of phase. CLKOUT may also be used as a temperature monitor since the CLKOUT duty cycle varies linearly with the part’s junction temperature. The CLKOUT pin can drive capacitive loads up to 200pF.

SS (Pin 2): Soft-Start Pin. Place a capacitor from this pin to ground. A capacitor identical to the SS pin capacitor used on the master LT8708 is recommended. Upon startup, this pin will be charged by an internal resistor to 3.3V.

SHDN (Pin 3): Shutdown Pin. Tie high to enable chip. Ground to shut down and reduce quiescent current to a minimum. Do not float this pin.

CSN (Pin 4): The (–) Input to the Inductor Current Sense and DCM Detect Comparator.

CSP (Pin 5): The (+) Input to the Inductor Current Sense and DCM Detect Comparator. The VC pin voltage and builtin offsets between CSP and CSN pins, in conjunction with the R It is recommended to use the same value R master LT8708.

ICN (Pin 6): Negative V voltage on this pin determines the negative V for LT8708-1 to regulate to. Connect this pin to the master LT8708’s ICN pin. See the Applications Information section for more information.

value, set the inductor current trip threshold.

SENSE

Current Command Pin. The

OUT

as the

current

FBOUT (Pin 9): V to the input of error amplifier EA4. Typically, connect this pin to GND to disable the EA4.

VC (Pin 10): Error Amplifier Output Pin. Tie external compensation network to this pin.

IMON_INP (Pin 11): Positive VIN Current Monitor and Limit Pin. The current out of this pin is 20µA plus a current proportional to the positive average VIN current. IMON_INP also connects to error amplifier EA5 and can be used to limit the maximum positive VIN current. See the Applications Information section for more information.

IMON_INN (Pin 12): Negative VIN Current Monitor and Limit Pin. The current out of this pin is 20µA plus a current proportional to the negative average VIN current. IMON_INN also connects to error amplifier EA1 and can be used to limit the maximum negative VIN current. See the Applications Information section for more information.

RT (Pin 13): Timing Resistor Pin. Adjusts the switching frequency. Place a resistor from this pin to ground to set the frequency. It is recommended to use the same value RT resistor as the master LT8708. Do not float this pin.

SYNC (Pin 14): To synchronize the switching frequency to an outside clock, simply drive this pin with a clock. The high voltage level of the clock needs to exceed 1.3V, and the low level should be less than 0.5V. In a two-phase system, connect this pin to the master LT8708’s CLKOUT pin to have a 180° phase shift. See the Applications Information section for more information.

Feedback Pin. This pin is connected

OUT

DIR (Pin 7): Direction pin when MODE is set for DCM (discontinuous conduction mode) or HCM (hybrid conduction mode) operation. Otherwise this pin is ignored. Connect the pin to GND to process power from the V to VIN. Connect the pin to LDO33 to process power from the VIN to V signal, or connect this pin to the same voltages as the master LT8708.

FBIN (Pin 8): VIN Feedback Pin. This pin is connected to the input of error amplifier EA3. Typically, connect this pin to LDO33 to disable the EA3.

. Drive this pin with the same control

OUT

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OUT

BG1, BG2 (Pin 16, Pin 18): Bottom Gate Drive. Drives the gate of the bottom N-channel MOSFETs between ground and GATEVCC.

GATEVCC (Pin 17): Power supply for bottom gate drivers. Must be connected to the INTVCC pin. Do not power from any other supply. Locally bypass to GND. It is recommended to use the same value bypass cap as the master LT8708.

Rev 0

PIN FUNCTIONS

LT8708-1

BOOST1, BOOST2 (Pin 24, Pin 19): Boosted Floating Driver Supply. The (+) terminal of the bootstrap capacitor connects here. The BOOST1 pin swings from a diode voltage below GATEVCC up to V

+ GATEVCC. The BOOST2

pin swings from a diode voltage below GATEVCC up to V

+ GATEVCC.

OUT

TG1, TG2 (Pin 23, Pin 20): Top Gate Drive. Drives the top N-channel MOSFETs with voltage swings equal to GATEVCC superimposed on the switch node voltages.

SW1, SW2 (Pin 22, Pin 21): Switch Nodes. The (–) terminals of the bootstrap capacitors connect here.

RVSOFF (Pin 25): Reverse Conduction Disable Pin. This is an input/output open-drain pin that requires a pull-up resistor. Pulling this pin low disables reverse current operation. Typically, connect this pin to the LT8708’s RVSOFF pin. See the Unidirectional and Bidirectional Conduction section for more information.

VOUTLOMON (Pin 26): V Pin. Connect a ±1% resistor divider between V

Low Voltage Monitor

OUT

, VOUTLOMON and GND to set an undervoltage level on V

. When V

OUT

tion is disabled to prevent drawing current from V

is lower than this level, reverse conduc-

OUT

. See

the Applications Information section for more information.

CSPOUT (Pin 30): The (+) Input to the V

Current

OUT

Monitor Amplifier. This pin and the CSNOUT pin measure the voltage across the sense resistor, R the V same value R

current signals. It is recommended to use the

OUT

SENSE2

between the CSPOUT and CSNOUT

SENSE2

, to provide

pins as the master LT8708. See Applications Information section for proper use of this pin.

CSNOUT (Pin 31): The (–) Input to the V

Current

OUT

Monitor Amplifier. See Applications Information section for proper use of this pin.

CSNIN (Pin 32): The (–) Input to the VIN Current Monitor Amplifier. This pin and the CSPIN pin measure the voltage across the sense resistor, R

SENSE1

, to provide the VIN current signals. Connect this pin to VIN when not in use. See Applications Information section for proper use of this pin.

CSPIN (Pin 33): The (+) Input to the VIN Current Monitor Amplifier. Connect this pin to VIN when not in use. See Applications Information section for proper use of this pin.

(Pin 34): Main Input Supply Pin for the LT8708-1.

INCHIP

It must be locally bypassed to ground. It is recommended to use the same value bypass cap as the master LT8708.

VINHIMON (Pin 27): VIN High Voltage Monitor Pin. Connect a ±1% resistor divider between VIN, VINHIMON and GND in order to set an overvoltage level on VIN. When VIN is higher than this level, reverse conduction is disabled to prevent current flow into VIN. See the Applications Information section for more information.

ICP (Pin 28): Positive V The voltage on this pin determines the positive V

Current Command Pin.

OUT

current for LT8708-1 to regulate to. Connect this pin to LT8708’s ICP pin. See the Applications Information section for more information.

EXTVCC (Pin 29): External VCC Input. When EXTVCC exceeds 6.4V (typical), INTVCC will be powered from this pin. When EXTVCC is lower than 6.4V, the INTVCC will be powered from V

. It is recommended to use the same

INCHIP

value bypass cap as the master LT8708.

INTVCC (Pin 35): 6.35V Regulator Output. Must be connected to the GATEVCC pin. INTVCC is powered from EXTVCC when the EXTVCC voltage is higher than 6.4V, otherwise INTVCC is powered from V

Bypass this

INCHIP.

pin to ground with a minimum 4.7µF ceramic capacitor. It is recommended to use the same value bypass cap as the master LT8708.

SWEN (Pin 36): Switching Regulator Enable Pin. Tie high through a resistor to enable the switching. Ground to disable switching. This pin is pulled down during shutdown, a thermal lockout or when an internal UVLO (Under Voltage Lockout) is detected. Do not float this pin. Connect this pin to the LT8708’s SWEN pin for synchronized start-up. See the Start-Up: SWEN Pin section for more details.

Rev 0

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+ 25 hidden pages

Linear Technology Analog Devices LT8708-1 Datasheet

Specifications and Main Features

Frequently Asked Questions

User Manual

FEATURES DESCRIPTION

APPLICATIONS

TYPICAL APPLICATION

PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS

ORDER INFORMATION

ELECTRICAL CHARACTERISTICS

TYPICAL PERFORMANCE CHARACTERISTICS

PIN FUNCTIONS