DN540 - 36V Input, 8.5A Buck-Boost μModule Regulator Easily Parallels for Increased Power

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36V Input, 8.5A Buck-Boost µModule Regulator Easily
Parallels for Increased Power
Design Note 540
Andy Radosevich
Introduction
The LTM®8055 is a buck-boost µModule® regulator with
a 5V to 36V input range that can be easily paralleled to
extend load current capability. Its 4-switch buck-boost
topology features high efficiency while allowing the
input voltage to be below, at or above the output—with
smooth transitions between regulation modes. The
LTM8055 can regulate a constant-voltage (CV) and a
constant-current (CC) output, with analog programming of output current. Its out-of-phase clock output,
combined with switching frequency synchronization,
0.22µF
C5
68µF
C7
22µF
VOUT
12V
12A AT 18V ≤ VIN ≤ 36V
DERATE IOUT
BELOW 18VIN
6A MAXIMUM IOUT AT 7VIN
100k
GND
IINMON
+
11k
IIN
IOUT
VIN
SVIN
RUN
CLKOUT
SS
U2
LTM8055
CTL
MODE
LL
IOUTMON
SYNC
RT
0.01µF
C3
22µF
36.5k
600kHz
47pF
9.31k
R2
0.008Ω
VOUT
C4
22µF
+
C6
68µF
C8
22µF
100k
GND
C2
4.7µF
2×
R1
0.008Ω
VOUT
47pF
36.5k
600kHz
IINMON
+ –
U3
FB
RT
0.01µF
COMP
0.22µF
L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are
registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
FB
4.7µF
2×
High efficiency is an inherent benefit of a 4-switch
buck-boost converter design, due mainly to low power
losses in NMOS switches. 2-switch converters are
IIN
IOUT
VIN
SVIN
RUN
CLKOUT
SS
U1
LTM8055
CTL
MODE
LL
IOUTMON
SYNC
COMP
VIN
7V TO 36V
enables easy phase interleaving in parallel operation.
A complete solution fits a space not much larger than
the 15mm × 15mm footprint of the LTM8055, including
input capacitors, output capacitors and current sense
resistors as needed.
C1, C2 MURATA GRM32ER71H475K
C3, C4, C7, C8 SAMSUNG CL21A226MAQNNNE
C5, C6 AVX TPSE686M025R0200
R1, R2 VISHAY WSL20108L000FEA
U3 LT1636CMS8
DN4AR F01
Figure 1. 144W Buck-Boost Regulator Circuit: Two LTM8055s Can Be Easily Paralleled to Increase Output Power.
This Parallel Circuit Is Capable of 12A of Output Current When the Input Is Between 18V and 36V
07/15/540
similarly efficient but limited to buck or boost topologies. Furthermore, DC/DC supplies that use a single
switch and a rectifier diode, instead of two switches,
are less efficient because of power losses in the diode.
Parallel Operation
It is easy to parallel multiple LTM8055s to increase
output power capability. Figure 1 shows a 12V output
regulator composed of two parallel LTM8055s, one CV
master regulator U1 and one CC slave regulator U2, capable of 12A of output current when the input is from 18V
to 36V. Figure 2 shows that the efficiency of the Figure 1
circuit is as high as 93% at 24V input. The output current
is derated at input voltages lower than 18V.
CV operation of the master regulator and CC operation
of the slave regulator assures balanced current sharing. IOUTMON monitors the output current, while the
CTL pin programs the output current of the current
source regulator. The master is programmed to the
desired output voltage while the slave is programmed
to a higher output voltage, since it normally regulates
current and not voltage. The master regulates the output
voltage and its IOUTMON pin connects to the CTL pin of
the slave which forces the LTM8055s to share the load
current equally. A unity gain buffer allows the master
IOUTMON pin to drive the CTL pin of the slave. Figure 3
shows load sharing of the two parallel regulators in
Figure 1, where the IOUTMON outputs remain equal
during a 6A to 10A load transient test.
The two switching cycles are synchronized 180° apart
by simply connecting CLKOUT of U1 to the SYNC pin of
U2. Out-of-phase synchronization reduces the required
capacitance on the input and output. Synchronization
prevents any beat frequency noise that could be caused
by a random switching cycle relationship between the
LTM8055s.
More than two LTM8055s can be paralleled by connecting the IOUTMON pin of the master to the CTL pins of the
slaves using a single unity gain buffer. An external clock
generator with two or more phases may be necessary
for proper channel interleaving.
Current Regulation and Monitoring
Parallel operation is not the only use for the LTM8055's
current regulation feature. It can be used for battery
and supercapacitor charging or to protect the output
from overcurrent and short faults. The LTM8055 also
provides input current regulation and monitoring. Input
current limit can prevent the LTM8055 from overloading
its input supply.
Conclusion
Minimal design effort is required to produce a compact
and high performance converter with the LTM8055.
This 36V buck-boost µModule regulator can be easily
paralleled for increased output power, with 2-phase
designs requiring no external clock.
100
95
VOUT
200mV/DIV
EFFICIENCY (%)
90
85
80
IOUTMON, MASTER
500mV/DIV
75
IOUTMON, SLAVE
500mV/DIV
70
VIN = 12V
VIN = 18V
VIN = 24V
65
60
0 1 2 3 4 5 6 7 8 9 10 11 12 13
IOUT (A)
VIN = 24V
100µs/DIV
IOUT
5A/DIV
DN540 F03
DN4AR F02
Figure 2. The Efficiency for the Figure 1 Circuit Is
as High as 93% at 24V Input. The Output Current
Is Derated at Input Voltages Lower Than 18V
Data Sheet Download
www.linear.com/LTM8055
Linear Technology Corporation
Figure 3. The IOUTMON Outputs of the Master and Slave
Regulators of Figure 1 Show Balanced Current Sharing
During a 6A to 10A Load Transient
For applications help,
call (408) 432-1900, Ext. 3568
dn540f LT/AP 0715 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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 LINEAR TECHNOLOGY CORPORATION 2015
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