3A Output, 96% Efficient Buck-Boost DC/DC Converter Sets the Standard for Power Density and Noise Performance

design features
3A Output, 96% Efficient Buck-Boost DC/DC Converter
Sets the Standard for Power Density and Noise Performance
Richard Cook
High power density has become a primary requirement
for DC/DC converters, as they must keep up with ever
increasing functional density of electronics. Likewise, power
dissipation is a major concern for today’s functionally rich,
tightly packed devices pushing the need for highly efficient
solutions to minimize temperature rise. For applications
where the input voltage source can be above or below
the regulated output voltage, finding an efficient compact
solution can be a challenge, especially at elevated power
levels. Conventional design approaches, such as using
a dual inductor SEPIC converter, produce relatively
low efficiencies and relatively large solution sizes.
The LTC3113 single inductor buck-boost
converter offers a compact, highly efficient alternative. Internal low resistance
switches allow the converter to support
an impressive 3A of load current in a tiny
4mm × 5mm package. The LTC3113 offers
an extended input and output operating voltage range from 1.8V to 5.5V, with
peak efficiencies reaching 96%. The
internal PWM controller is designed for
low noise performance and offers a seamless transition between buck and boost
modes. The combination of these features
allows the LTC3113 to easily meet challenging high density power requirements.
Figure 1 shows an 11mm × 14mm ×
2.5mm LTC3113-based solution that can
supply up to 12W of output power from a
Li-ion battery. This translates to a power
density of 31mW/mm3 (511W/in3). A complete SEPIC design would require twice as
much PCB area, resulting in half the power
density and significantly lower efficiency,
which complicates thermal design.
15mm
Figure 1. A typical application occupies 154mm2
The LTC3113 offers a number of options
to optimize performance for specific
applications, including the ability to
adjust the operating frequency from
300kHz to 2MHz, internal soft-start, user
selectable Burst Mode® operation for
improved efficiency at light load currents,
and a host of fault protection features
including short-circuit protection and
thermal shutdown. The LTC3113 is available in both a 4mm × 5mm DFN and
a 20-pin thermally enhanced TSSOP.
Figure 2. Pulsed load or portable RF power amplifier power supply and typical output response
2.2µH
VIN
3.3V
±10%
SW1
4.7µF
47µF
OFF ON
PWM BURST
SW2
VIN
VOUT
845k
LTC3113
RUN
FB
BURST
VC
RT
SGND
90.9k
10k
200µF
4.7µF
VOUT
3.8V
0A TO 3A
VOUT
200mV/DIV
33pF
68k
220pF
PGND
10pF
158k
ILOAD
2A/DIV
100µs/DIV
April 2011 : LT Journal of Analog Innovation | 19
The LTC3113 single inductor buck-boost converter offers a unique combination of features
to meet challenging high density power requirements. Internal low resistance switches
allow the converter to support an impressive 3A of load current in a tiny 4mm × 5mm
package, with peak efficiencies reaching 96%. The internal PWM controller is designed for
low noise performance and offers a seamless transition between buck and boost modes.
GSM APPLICATION
NOISE PERFORMANCE
30
Many GSM applications require expensive
supercapacitors on the DC/DC output supply rail to support the temporary heavy
loads placed on the output by the power
amplifier during transmission bursts.
In many cases, the high output current
capability of the LTC3113 is sufficient to
support the transmit current without the
need for supercapacitors. Figure 2 shows
such a circuit and associated typical load
transient for an RF power amplifier using
a standard, inexpensive 100µF ceramic
capacitor on the 3.8V output.
Many applications, including RF transmission, are sensitive to noise generated by
switching converters. The LTC3113 uses a
low noise switching architecture to reduce
unwanted subharmonic frequencies, which
occur below the operating frequency and
can interfere with other sensitive circuitry.
These subharmonics usually occur when
VIN and VOUT are approximately equal.
Buck-boost converters operating in this
region typically produce pulse width
and frequency jitter—a result of all four
switches changing state during a single
switching cycle. The LTC3113 minimizes
the magnitude of the jitter or subharmonic frequencies to satisfy the requirements of noise-sensitive RF applications.
20
The oscilloscope photo shows the response
of the 3.8V output when a 3A load pulse
lasting 580µs is applied. For this extreme
case the output voltage undershoots
only 150mV (4.5%) and quickly recovers. The output voltage overshoot when
the load is removed shows a similar
response. For this external load pulse,
the transient response has been optimized by tailoring the compensation to
minimize the effects of the load step.
Figure 3 shows worst-case spectral
comparisons of the LTC3113 and a competitive buck-boost converter without
the low noise architecture of the LTC3113.
The worst-case condition was achieved
by placing a fixed 1A load on the output
and slowly increasing or decreasing the
input voltage until the greatest harmonic content in the converter spectrum
LTC3113 fSW = 2MHz
MAGNITUDE (dB)
10
SUBHARMONICS
0
COMPETITOR
fSW = 2.4MHz
–10
–20
–30
–40
–50
–60
–70
0
0.5
2
1
1.5
FREQUENCY (MHz)
2.5
3
Figure 3. Spectral comparison of the LTC3113 and
typical competitor’s part
was observed. The LTC3113 exhibits an
expected single large magnitude tone
at its switching frequency of 2MHz. In
contrast, the competing buck-boost
exhibits several high magnitude subharmonic and harmonic tones across
the entire frequency range, indicative of
significant pulse width jitter and potential
noise interference issues. Note also that
the overall noise floor of the LTC3113 is
10d B to 20d B lower than the competition across the entire frequency range.
Figure 4. Li-ion to 3.3V supply and efficiency
100
2.2µH
SW1
47µF
OFF ON
PWM BURST
SW2
VIN
VOUT
825k
LTC3113
RUN
FB
BURST
VC
RT
SGND
47pF
49.9k
680pF
182k
80
70
PGND
90.9k
12pF
20 | April 2011 : LT Journal of Analog Innovation
6.49k
VOUT
3.3V
100µF 3A
EFFICIENCY (%)
90
VIN
2.5V TO 4.2V
Li-Ion
60
0.001
VIN = 3V
VIN = 3.7V
VIN = 4.2V
VIN = 3V BURST
VIN = 3.7V BURST
VIN = 4.2V BURST
0.01
0.1
1
LOAD CURRENT (A)
10
design features
The LTC3113 monolithic buck-boost converter breaks
new ground in power density, low noise operation and
high efficiency across a wide range of load currents.
SINGLE LI-ION TO
3.3V, 10W CONVERTER
BACKUP POWER SYSTEMS
Figure 5 shows a supercapacitor-powered
backup power supply system, where the
LTC3113 is used to provide a regulated
3.3V output at a constant 1.5A load. In this
application, two series 30F supercapacitors
have been charged to 4.5V during normal
operation to provide the needed backup
energy when the primary power is lost.
Besides generating bias voltages for
RF power amplifiers, creating a 3.3V rail
from an input source such as a Li-ion
battery is another common application
for a buck-boost converter. The LTC3113
can provide up to 10W (3.3V/3A) of output
power over the Li-ion battery’s usable
range. Figure 4 shows a typical application schematic used to generate 3.3V. Also
shown are the associated efficiency curves
for different battery voltages over a range
of load currents for this application. The
efficiency peaks at 92% and efficiencies
greater than 80% are achieved from loads
ranging from 60m A to 3A. Burst Mode
operation employs a variable frequencyswitching algorithm to maintain highly
efficient conversion at lighter loads.
The scope photo shows that the LTC3113
can regulate the output for 22.5s when
powered only by the two series 30F capacitors. Over this time, the capacitors
discharge from an initial 4.5V to just
below 1.8V—output regulation over this
input range is only possible because of
the LTC3113’s low input voltage capability. In this example, the amount
of energy supplied by the input is:
Setting the BURST pin to a voltage greater
than 1.2V allows the LTC3113 to enter
Burst Mode operation at light loads to
maximize efficiency. For noise sensitive
applications the converter can be forced
into fixed frequency operation by keeping
the voltage on the BURST pin below 0.3V.
EIN =
1
2
2

• C • ( VINITIAL ) − ( VFINAL ) 


2
EIN =
1
• 15F • ( 4.5V 2 − 1.8 V 2 ) = 127.6J
2
The output is regulated to 3.3V with a
constant load of 1.5A for 22.5s, which
yields output energy of:
EOUT = IOUT • VOUT • t
= 1.5A • 3.3V • 22.5s
= 111.4J
This shows that about 87% of the available input energy is converted to output
power. The solution size for this application is about 11mm × 14mm, excluding the area of the supercapacitors.
CONCLUSION
The LTC3113 monolithic buck-boost
converter breaks new ground in power
density, low noise operation and high
efficiency across a wide range of load
currents. The LTC3113 is an ideal solution for battery-powered products,
backup power supply systems and RF or
other noise-sensitive applications. n
Figure 5. Supercapacitor-powered supply and typical output response with 1.5A load
2.2µH
VIN
1.8V TO 4.5V
SW1
30F
30F
VIN
0.1µF
VIN
2V/DIV
SW2
VOUT
825k
LTC3113
OFF ON
PWM BURST
RUN
FB
BURST
VC
RT
SGND
6.49k
47pF
49.9k
680pF
PGND
90.9k
12pF
182k
100µF
VOUT
3.3V
VOUT
2V/DIV
RUN
2V/DIV
5s/DIV
April 2011 : LT Journal of Analog Innovation | 21
Similar pages