LT8708 80V Synchronous 4-Switch Buck-Boost DC/DC Controller with Bidirectional Capability FEATURES DESCRIPTION n Single Inductor Allows V Above, Below, or The LT 8708 is a high performance buck-boost switching IN Equalto V regulator controller that operates from an input voltage OUT n Six Independent Forms of Regulation that can be above, below or equal to the output voltage. n V Current (Forward and Reverse) Features are included to simplify bidirectional power con- IN n V Current (Forward and Reverse) version in battery/capacitor backup systems and other OUT n V and V Voltage applications that may need regulation of V , V , I , IN OUT OUT IN OUT n Forward and Reverse Discontinuous Conduction and/or I . Forward and reverse current can be monitored IN Mode Supported and limited for the input and output sides of the converter. n Supports MODE and DIR Pin Changes While Switching All four current limits (forward input, reverse input, for- n V Range 2.8V (Need EXTV > 6.4V) to 80V ward output and reverse output) can be set independently INCHIP CC n V Range: 1.3V to 80V using four resistors on the PCB. OUT n Synchronous Rectification: Up to 99% Efficiency The MODE pin can select between discontinuous conduction n Available in 40-Lead (5mm 8mm) QFN with High mode (DCM), continuous conduction mode (CCM), hybrid Voltage Pin Spacing and 64-Lead (10mm 10mm) conduction mode (HCM) and Burst Mode operation. In eLQFP combination with the DIR (direction) pin, the chip can be n AEC-Q100 in Progress configured to process power only from V to V or only IN OUT from V to V . With a wide 2.8V to 80V input and 1.3V OUT IN APPLICATIONS to 80V output range, the LT8708 is compatible with most solar, automotive, telecom and battery-powered systems. n High Voltage Buck-Boost Converters n All registered trademarks and trademarks are the property of their respective owners. Bidirectional Charging System n Automotive 48V Systems TYPICAL APPLICATION 12V Bidirectional Dual Battery System with FHCM and RHCM I I LIM LIM V V BAT1 BAT2 + + 10V + 10V + Efficiency TO 16V TO 16V BATTERY TO DIODE TO DIODE BATTERY D D 100 B1 B2 99 TG1 BOOST1 SW1 BG1 CSP CSN GND BG2 SW2 BOOST2 TG2 98 CSNIN CSPOUT POWER TRANSFER CSNOUT CSPIN DECISION LOGIC EXTV CC V 97 INCHIP VOUTLOMON SHDN FBOUT FBIN INTV CC LT8708 VINHIMON 96 GATEV CC RVS (0V) FWD (3V) DIR IMON OP IMON ON LD033 SWEN ICN 95 ICP V = 13.5V BAT2 LDO33 IMON INP D D B1 B2 I = 15A OUT RVSOFF IMON INN V MODE C RT SS SYNC CLKOUT 94 TO TO 8708 TA01a 10 12 14 16 BOOST1 BOOST2 126kHz V (V) BAT1 8708 TA01b Rev. B 1 Document Feedback For more information www.analog.com EFFICIENCY (%)LT8708 TABLE OF CONTENTS Features ..................................................... 1 Internal Oscillator ...................................................27 Applications ................................................ 1 SYNC Pin and Clock Synchronization .....................27 Typical Application ........................................ 1 CLKOUT Pin and Clock Synchronization .................27 Description.................................................. 1 Inductor Current Sensing and Slope Absolute Maximum Ratings .............................. 3 Compensation .........................................................27 Pin Configuration .......................................... 3 R Selection and Maximum Current ................28 SENSE Order Information .......................................... 4 R Selection: Max R in the Boost SENSE SENSE Electrical Characteristics ................................. 4 Region ................................................................28 Typical Performance Characteristics ................... 9 R Selection: Max R in the Buck SENSE SENSE Pin Functions .............................................. 13 Region ................................................................30 Block Diagram ............................................. 15 R Selection: Final R Value .................31 SENSE SENSE Operation................................................... 16 R Filtering ......................................................32 SENSE Typographical Conventions ..................................... 16 Inductor (L) Selection ............................................32 Start-Up .................................................................. 16 L Selection: Load Current in Buck and Boost Start-Up: SHDN Pin ............................................ 16 Regions ..............................................................32 Start-Up: SWEN Pin ............................................ 16 L Selection: Subharmonic Oscillations ...............33 Start-Up: Soft-Start of Switching Regulator ....... 17 L Selection: Maximum Current Rating ................33 Control Overview .................................................... 17 Power MOSFET Selection .......................................34 Power Switch Control ............................................ 18 C and C Selection ...........................................37 IN OUT Switch Control: Buck Region (V >> V ) ....... 19 Schottky Diode (D1, D2, D3, D4) Selection .............38 IN OUT Switch Control: Buck-Boost (V V ) ......... 19 Topside MOSFET Driver Supply IN OUT Switch Control: Boost Region (V << V ) ......20 (C ,D ,C ,D ) ................................................39 IN OUT B1 B1 B2 B2 Switch Control: Soft-Start ..................................20 Top Driver: Boost Cap Charge Control Block ......39 Uni and Bidirectional Conduction ............................20 Top Driver: Boost Diodes D and D B1 B2 ....................39 Bidirectional Conduction: CCM ..........................21 VINHIMON, VOUTLOMON and RVSOFF .................39 Unidirectional Conduction: DCM .........................21 I and I Current Monitoring and Limiting .........42 IN OUT Unidirectional Conduction: HCM .........................22 Loop Compensation ................................................44 Unidirectional Conduction: Burst Mode ..............22 INTV Regulators and EXTV Connection ...........45 CC CC Error Amplifiers ......................................................22 LDO33 Regulator ....................................................45 V Regulation and Sensing .................................23 Voltage Lockouts ....................................................46 OUT V : Regulation .................................................23 Junction Temperature Measurement ......................46 OUT V : Above Regulation ......................................23 Thermal Shutdown ................................................. 47 OUT V : Below Regulation and Undervoltage .........23 Efficiency Considerations ...................................... 47 OUT V Regulation and Sensing .................................... 24 Circuit Board Layout Checklist ............................... 47 IN V : Regulation ................................................... 24 Hot Plugging Considerations .................................49 IN V : Above Regulation and Overvoltage .............. 24 Design Example ......................................................49 IN V : Below Regulation ......................................... 24 Typical Applications ...................................... 55 IN Current Monitoring and Limiting ............................. 24 12V Bidirectional Dual Battery System with FHCM Monitoring and Limiting: IMON Pins .................. 24 & RHCM Details .................................................56 Monitoring: ICP and ICN Pins .............................25 48V to 14V Bidirectional Dual Battery System with INTV /EXTV /GATEV /LDO33 Power ...............25 FHCM & RHCM Details .......................................58 CC CC CC CLKOUT and Temperature Sensing .........................25 Package Description ..................................... 63 Applications Information ................................ 26 Revision History .......................................... 65 Verify the Power Flow Conditions ...........................26 Typical Application ....................................... 66 Operating Frequency Selection ...............................26 Related Parts .............................................. 66 Rev. 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