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