DN521 - 80V Synchronous 4-Switch Buck-Boost Controller Delivers Hundreds of Watts with 99% Efficiency

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
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