80V Synchronous 4-Switch Buck-Boost Controller Delivers Hundreds of Watts with 99% Efficiency – Design Note 521 Keith Szolusha and Tage Bjorklund Introduction A DC/DC converter’s efficiency and component temperature are important considerations in high power applications where high current could overheat the catch diode used in an asynchronous buck or boost topology. Replacing the catch diode with a synchronous switch can significantly improve overall converter efficiency and eliminate much of the heat that would be otherwise generated in the nonsynchronous catch diode. The advantages offered by a synchronous buck or boost topology can also be applied to a buck-boost topology, where the converter’s output voltage falls within its input range. In this case, a synchronous 4-switch buck-boost converter using a single inductor offers the same advantages as a 2-switch synchronous buck or boost. L1 22µH M1 + CIN2 4.7µF ×4 CIN1 220µF ×2 M2 M4 TO DIODE DB2 TG1 BOOST1 SW1 BG1 CSP 1nF 10Ω 1nF 10Ω CSN 10mΩ 392k GND CSNIN CSPOUT CSNOUT SHDN 10k INTVCC GATEVCC LT8705 4.7µF SRVO_FBIN FBIN 4.7µF SRVO_FBOUT RT SRVO_IIN SS SRVO_IOUT IMON_IN 1µF 4.7µF 98 FBOUT SWEN VC 301k 99 EXTVCC MODE 20k 100 BG2 SW2 BOOST2 TG2 CSPIN LDO33 576k CLKOUT SYNC IMON_OUT 4Ω DB1 DB2 TO TO BOOST1 BOOST2 56k 1µF 220pF 97 96 95 94 93 92 91 3.3nF 90 DN521 F01 CIN1, COUT2: 220µF, 100V CIN2, COUT1: 4.7µF, 100V, TDK C453X7S2A475M DB1, DB2: CENTRAL SEMI CMMR1U-02-LTE L1: COILCRAFT SER2915H-223 M1: INFINEON BSC057N08NS3 M2: INFINEON BSC123N08NS3 M3, M4: INFINEON BSC039N06NS fSW = 150kHz Figure 1. LT8705 240W, 48V, 5A Buck-Boost Converter for Telecom Voltage Stabilization 12/13/521 VOUT 48V COUT2 5A 220µF ×2 0.22µF VIN 100k + COUT1 4.7µF ×6 M3 TO DIODE DB1 0.22µF L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. EFFICIENCY (%) VIN 36V TO 80V The LT®8705 is a synchronous 4-switch buck-boost controller IC that can deliver hundreds of watts with high efficiency for constant voltage or constant current applications from wide-ranging inputs (up to 80V). It uses a robust synchronous switch topology and adds the versatility of four servo loops (voltage and current at input and output), making it possible to design high power battery chargers and solar panel converters with minimum component count. These are only two examples of the many high power, high current telecom, automotive and industrial solutions the LT8705 can produce. 30 35 40 45 50 55 VIN (V) 60 65 70 75 DN521 F02 Figure 2. The Efficiency of the LT8705, 48V Converter Is as High as 99% 240W 48V 5A Telecom Power Supply In telecom applications the input voltage has a wide (36V to 72V) range. Power converters that deliver a stabilized 48V DC voltage to the loads are commonly used. The LT8705 can easily handle hundreds of watts at 48V output voltage, at efficiency as high as 99%. Figure 1 shows one example with 5A (240W) output. 500W Charger for 12S LiFePO4 battery Figure 3 shows a circuit for charging a lithium iron phosphate battery from a 48V (±10%) input voltage. The battery has 12 cells in series, so the maximum charge voltage is 44V. This means that the circuit will operate in step-down (buck) mode most of the time, but at lowest input voltage it must operate in buck-boost mode. At a 48V input voltage the circuit has an efficiency of 99% at full load. The efficiency is high because only the input stage (M1, M2) is switching at high duty cycle and M4 is on continuously. The efficiency drops slightly if the input voltage is reduced to the minimum input voltage (43.2V), as the LT8705 circuit then must operate in buck-boost mode when all MOSFETs are switching. VIN 48V ±10% 4.7µF 100V ×2 CIN 220µF 100V ×2 Four Servo Loops and Wide Voltage Range The LT8705’s 2.8V to 80V input and its 1.3V to 80V output ranges, combined with its four servo loops, allow it to easily solve a number of traditionally complex problems. The four servo loops can be used to control input and output voltages and currents. For instance, the input voltage and current can be regulated along with the output voltage and current for maximum power point solar panel applications. The IC outputs a flag for each servo loop, indicating which is in control at any given time. This is particularly useful information for microcontrollers in battery chargers and solar panel converters. Conclusion The LT8705 is an 80V synchronous 4-switch buckboost controller, that can provide hundreds of watts at up to 99% efficiency with a single inductor. Its four servo-loops allow it to regulate current and/or voltage for both the input and output. L1 15µH M1 + An external microcontroller can be used for the charge algorithm, and for controlling current and voltage from the LT8705 power converter. M2 GVCC M4 2.2µF ROUT 100V 0.004Ω ×2 M3 GVCC 0.004Ω 10Ω D1 10Ω 1nF SHDN 20k 4Ω GVCC 10µF 1nF CSP CSN BG2 SW2 BOOST2 TG2 CSPOUT CSNOUT EXTVCC LT8705 GATEVCC INTVCC VC IMONIN MODE GND SS RT 375k 1µF 100V FBOUT 10k IMONOUT SWEN LDO33 FBIN 23.7k 10nF 100k 301k 150kHz 5.1k 1000pF VOUT 44V COUT 11.5A 220µF 100V ×2 0.22µF SRVO_FBIN SRVO_IIN SRVO_IOUT SRVO_FBOUT CLKOUT CSNIN TG1 BOOST1 SW1 BG1 CSPIN VIN SHDN SYNC 71.5k + D2 0.22µF 4.7µF 100V 4.7µF 100V ×2 22nF 1µF 4.7µF DN521 F03 CIN, COUT: 2 x 220µF 100V ALUMINUM ELECTROLYTIC D1, D2: CMMR1U-02 L1: COILCRAFT SER2915L-153KL M1-M4: INFINEON BSC039N06NS 60VDS fSW = 150kHz Figure 3. LT8705 500W, 44V, 11.5A Buck-Boost Converter for High Powered Battery Supply Data Sheet Download www.linear.com/LT8705 Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 3550 dn521f LT/AP 1213 111K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2013