DN369 - Industry's First 4-Switch Buck-Boost Controller Achieves Highest Efficiency Using a Single Inductor

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Industry’s First 4-Switch Buck-Boost Controller Achieves Highest
Efficiency Using a Single Inducutor – Design Note 369
Wilson Zhou and Theo Phillips
Introduction
One of the most common DC/DC converter problems
is generating a regulated voltage that falls somewhere
in the middle of a wide range of input voltages. When
the input voltage can be above, below or equal to the
output voltage, the converter must perform step-down
and step-up functions. Unlike solutions requiring bulky
transformers, the LTC ®3780 meets these requirements
in the most compact and efficient manner, using just
one off-the-shelf inductor and a single current sense
resistor.
High Efficiency 4-Switch Buck-Boost Converter
Figure 1 shows a simplified LTC3780 4-switch buckboost converter. When VIN exceeds VOUT, the LTC3780
operates in buck mode. With switch D on and switch C
off, switches A and B turn on and turn off alternately,
as they would in a typical synchronous buck regulator.
Conversely, when VIN is lower than VOUT, the LTC3780
operates in boost mode. With switch A on and synchronous switch B off, switch C and synchronous switch D
turn on and turn off alternately, behaving as a typical
synchronous boost regulator.
The LTC3780 uses a constant frequency current mode
architecture which allows seamless transitions between
buck, boost and buck/boost modes with a wide 4V to 30V
(36V maximum) input and output range. Burst Mode®
operation and skip cycle mode provide high efficiency
operation at light loads, while forced continuous mode
and discontinuous mode reduce output voltage ripple by
operating at a constant frequency. A soft-start feature
reduces output overshoot and inrush currents during
start-up. Overvoltage protection, current foldback and
on-time limitation provide protection for fault conditions,
including short circuit, overvoltage and inductor current
runaway. The LTC3780 is available in low profile 24-pin
TSSOP and 32-lead 5mm × 5mm QFN packages.
When VIN is close to VOUT, the controller is in buckboost mode. Switches A and D are on for most of each
period. Brief connections between VIN and ground, and
VOUT and ground, are made through the inductor and
switches B-D and A-C to regulate the output voltage.
In buck-boost mode, inductor peak-to-peak current is
much lower than that of SEPIC converters and traditional
buck/boost converters. Figure 2 shows the inductor
current and switch node waveforms.
VIN
A
CIN
SW2
L
SW1
B
D
VOUT
Low inductor ripple current and the use of synchronous rectifiers allow the LTC3780 to achieve very high
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
SW2
10V/DIV
COUT
C
SW1
10V/DIV
SNS+
RSENSE
VOUT
100mV/DIV
AC COUPLED
SNS–
R1
LTC3780
SNS+
SNS–
IL
2A/DIV
R2
5μs/DIV
DN369 F02
DN369 F01
Figure 1. 4-Switch Buck-Boost Converter
08/05/369_conv
Figure 2. Switch Nodes and Inductor Current Waveforms
(VIN = VOUT = 12V)
100
9
90
SEPIC
CONVERTER
85
80
75
70
5
10
15
20
VIN (V)
DN369 F04
8
7
90
6
5
85
4
80
3
POWER LOSS (W)
EFFICIENCY (%)
LTC3780
95
Figure 4. Efficiency Comparison Between the LTC3780
and a SEPIC Converter (VOUT = 12V, ILOAD = 5A)
10
95
100
EFFICIENCY (%)
efficiency over a wide VIN range. When the input and
output voltages are both 12V, the 4-switch buckboost has 99% efficiency at 2A load and 98% at its
maximum 5A load (Figure 3). With its current mode
control architecture, the converter has excellent load
and line transition response, minimizing the required
filter capacitance and simplifying loop compensation.
As a result, very little filter capacitance is required. The
single sense resistor structure dissipates little power
(compared with multiple resistor sensing schemes)
and provides consistent current information for short
circuit and overcurrent protection.
2
75
1
0
70
0
5
10
20
15
VIN (V)
25
30
SEPIC
35
DN369 F03
Figure 3. Efficiency and Power Loss
(VOUT = 12V, ILOAD = 5A)
Replacing a SEPIC Converter
This single inductor buck-boost approach has high
power density and high efficiency. Compared with a
coupled inductor SEPIC converter, its efficiency can be
8% higher. Figure 4 shows the efficiency comparison
between the LTC3780 4-switch buck-boost and a typical SEPIC converter. Note that a SEPIC converter has
a maximum switch voltage equal to the input voltage
plus the output voltage. So for a given maximum input
voltage, a SEPIC would dictate the use of a higher voltage external switch than is required with the LTC3780.
Moreover, the typical inductor occupies about 1/5th of
a SEPIC transformer’s footprint, less than 1/15th the
volume and less than one-half the profile, as shown
in Figure 5.
Protection for Boost Operation
The basic boost regulator topology provides no short
circuit protection. When the output is pulled low, a
large current can flow from the input to the output.
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Linear Technology Corporation
LTC3780 Buck Boost
Figure 5. Inductor Size Comparison Between the LTC3780
5A/12V Converter (Right, 12.7mm × 12.7mm × 4mm) and
a Typical SEPIC (Left, 21mm × 21mm × 10.8mm)
Without shutting down the whole circuit, the LTC3780
circumvents this problem by forcing the converter
into buck mode and using current foldback to limit the
inductor current.
Simplify
For certain applications such as those requiring low
current or not requiring current sinking, Switch D can be
replaced with a Schottky diode. This simplified topology
has approximately 2% lower efficiency.
Conclusion
The LTC3780 is a constant frequency current mode
buck-boost switching regulator controller that allows
the input voltage to be above, below or equal to the
output voltage. Its high efficiency, high power density
and single inductor topology make this product ideal
for automotive, telecom, medical and battery-powered
systems.
For applications help,
call (408) 432-1900
dn369f_conv LT/TP 0805 305K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2005
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