DN499 - High Efficiency 4-Switch Buck-Boost Controller Provides Accurate Output Current Limit

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High Efficiency 4-Switch Buck-Boost Controller Provides
Accurate Output Current Limit – Design Note 499
Tage Bjorklund
Introduction
A 4-switch buck-boost converter (Figure 1) is often a
better alternative to a transformer-based topology when
a converter’s input voltage can be above or below the
regulated output voltage and isolation is not required.
A buck-boost converter provides a wider input voltage
range, better efficiency and has no need of a bulky
transformer. A buck-boost converter is much more
efficient than a comparable SEPIC converter.
The LTC ®3789 is a buck-boost switching regulator
controller that operates in current mode at a constant
switching frequency. Current mode control simplifies
loop compensation and yields excellent load and line
transient response with only small output and input
capacitance. Furthermore, an accurate inductor current
limit allows use of a small size inductor.
The LTC3789 includes two internal series regulators that
provide 5.5V for gate drivers and control circuitry. During
start-up the current is drawn from VIN (40V maximum
rating), but switches to EXTVCC (14V maximum rating)
as soon as the voltage on EXTVCC exceeds 4.8V, to
decrease power loss.
This 4-switch topology allows the output to be disconnected from the input during shutdown. LTC3789
features adjustable soft-start, no reverse current during
start-up and 1% output voltage accuracy.
A
SW2
L
SW1
D
VOUT
VIN
CIN
B
COUT
C
SENSE+
RSENSE
SENSE–
LTC3789 Features
The internal oscillator can be programmed by a resistor, applied voltage, or phase-locked to an external
clock within a 200kHz to 600kHz frequency range. The
LTC3789’s wide 4V to 38V (40V maximum) input and
output range and seamless, low noise transitions between operating regions is ideal for automotive, telecom
and battery-powered systems. Higher input voltage is
easily enabled by adding a high voltage gate driver for
the input side MOSFETs, for example the LTC4444-5.
Figure 1 shows a simplified schematic of a buck-boost
converter. The LTC3789 controls four external N-channel
MOSFETs (A, B, C, D). The power stage uses one inductor
(L), and a current sense resistor (RSENSE) for current
mode control and inductor current limit. Figures 2
to 4 show the switch waveforms for buck, buck-boost,
and boost modes of operation.
The LTC3789 is available in low profile 28-pin 4mm ×
5mm QFN and narrow SSOP packages.
12V, 5A Output from a 4V to 38V Input
Figure 5 shows a buck-boost regulator that takes a 4V
to 38V input and converts it to a fixed 12V.
01/12/499
R1
LTC3789
SENSE+
SENSE–
R2
DN499 F01
Figure 1. 4-Switch Buck-Boost Converter Using the
LTC3789
5V/DIV
1A/DIV
5V/DIV
1μs/DIV
DN499 F02
Figure 2. Buck Mode [VIN = 20V, VOUT = 12V]
L, LT, LTC, LTM, Linear Technology, the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
A power good output pin (PGOOD) indicates when the
output is within 10% of its set point.
1A/DIV
Accurate Output (or Input) Current Limit
The optional output current feedback loop shown in
Figure 6 provides support for battery charging and other
constant current applications, where an accurate output
current limit is essential. The output current limit function
provides a constant current characteristic—the output
current is limited to a constant level even as the output
voltage is pulled down by an overcurrent condition.
Alternatively the sense resistor can simply be moved
to the input side to accurately control the maximum
input current.
5V/DIV
5V/DIV
1μs/DIV
DN499 F03
Figure 3. Buck-Boost Mode [VIN = 11V, VOUT = 12V]
1A/DIV
RSENSE2
FROM
CONTROLLER
VOUT
RF
100Ω
TO
SYSTEM
VOUT
+
RF
100Ω
CF
5V/DIV
2
IOSENSE+
5V/DIV
1
IOSENSE–
LTC3789
DN499 F06
1μs/DIV
Figure 6. Optional Output Current Limit
DN499 F04
Figure 4. Boost Mode [VIN = 8V, VOUT = 12V]
The MODE/PLLIN pin can be used to select between
pulse-skipping mode and forced continuous mode
operation, or allows the IC to be synchronized to an
external clock. Pulse-skipping mode offers the lowest
ripple at light loads, while forced continuous mode
operates at a constant frequency for noise-sensitive
applications.
+
Conclusion
The LTC3789 is a constant frequency current mode
buck-boost switching regulator controller that accepts a
wide 4V to 38V input voltage range. The single inductor
topology yields high power density and high efficiency
in a compact footprint. Its optional current-limit function is useful for applications such as battery charging,
where accurate current control is necessary.
1μF
CER
4.7μF
ILIM INTVCC IOSENSE–
PGOOD
VIN
22μF
50V
CER A
VINSNS
IOSENSE+
0.010Ω
EXTVCC
VOUTSNS
D
TG2
LTC3789
BOOST1
BOOST2
TG1
0.1μF
SW1
B
BG1
SS
RUN
8k
1000pF
0.01μF
SGND
C
BG2
MODE/PLLIN
ON/OFF
VFB
FREQ
SENSE+ SENSE– PGND
VOUT
12V
5A
330μF
16V
0.1μF
SW2
ITH
+
121k
105k
1%
7.5k
1%
Efficiency and Power Loss
100
12
95
10
90
8
85
6
80
4
75
0.010Ω
2
VOUT = 12V
ILOAD = 5A
0
70
4.7μH
0
DN499 F05a
POWER LOSS (W)
2200pF
10μF
16V
CER
EFFICIENCY (%)
VIN
4V TO
38V
5
10
15
20
25
VIN (V)
30
35
40
DN499 F05b
Figure 5. Regulated 12V, 5A Output from a 4V to 38V Input and Associated Efficiency and Power Loss Curves
Data Sheet Download
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For applications help,
call (408) 432-1900, Ext. 3550
Linear Technology Corporation
dn499 LT/TP 0112 196K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2012
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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