15V, 2.5A Monolithic Buck-Boost DC/DC Converter with 95% Efficiency and Low Noise Operation

design features
15V, 2.5A Monolithic Buck-Boost DC/DC Converter with
95% Efficiency and Low Noise Operation
Eddy Wells
Power-hungry handheld devices and industrial instruments often require multicell or high
capacity batteries to support their ever-increasing processing needs. A wide voltage range,
high efficiency buck-boost DC/DC converter is the ideal solution for longer battery run
times and handling multiple input sources. The LTC3112 is a 2.2V to 15V input capable
2.5A buck-boost converter. The extended voltage range allows conversion from a variety of
power sources such as one, two or three Li-ion cells, lead acid batteries, supercapacitors,
USB cables and wall adapters to output voltages programmed between 2.5V and 14V.
The LTC3112 features the latest generation
buck-boost PWM control scheme, effectively eliminating jitter when crossing the
barrier between buck and boost operation. Safeguards such as current limit,
overvoltage protection, thermal shutdown,
and short-circuit protection provide
robust operation in harsh environments.
For demanding applications where component size or conversion efficiency is critical, the LTC3112’s 750kHz default switching
frequency can be synchronized between
300kHz and 1.5MHz. For designs where
output current needs to be controlled
or measured, an output current monitor
pin is available. Selectable Burst Mode®
operation extends the operating life when
the battery-powered device is idle.
The LTC3112-based converter shown in
Figure 1 can generate 30W of power with
a 12V output. The solution footprint
is less than 200mm2, which cannot be
matched by a controller-based buck-boost
or complex dual-inductor SEPIC design
at similar power levels. The main external components are limited to the input
and output filter caps and the power
inductor. The LTC3112 is offered in a
thermally enhanced 16-lead 4mm × 5mm
DFN or 20-lead TSSOP package.
extends high efficiency operation for
more than two decades of load current.
Figure 1. LTC3112 based 30W solution
OPERATION FROM MULTIPLE INPUT
SOURCES
The LTC3112’s wide operating range allows
devices to be powered from multiple input
sources. Figure 2 shows an application
where the LTC4412 PowerPath controller (TSOT-23 package) provides a low loss
selection between two input sources. The
LTC4412 maintains a 20mV forward voltage
across the selected P-channel MOSFET, keeping losses to a minimum. In this circuit,
the LTC4412 automatically switches the
greater of a single Li-ion cell or 12V wall
adapter to the input of the LTC3112.
Efficiency curves based on the two
input sources are given in Figure 3. Peak
efficiencies of greater than 90% are
achieved with either input. Selectable
Burst Mode operation (dashed lines)
with 50µ A of typical sleep current
A feedforward network (CFF, RFF of
Figure 2) connected between the VIN and
FB pins provides improved transient
response when the wall adapter voltage is
applied. Feedforward values were selected
by first measuring the voltage change in
voltage at COMP as VIN transitions from
3.6V to 12V. A 380mV change at COMP was
observed, optimal values for VIN and
RFF can now be calculated as follows:
CFF =
∆VCOMP
• (CFB + CP ) = 33pF
∆VIN
RFF =
RFB • CFB
= 681k
CFF
VOUT regulation is maintained within
300mV or 6% during the 15µs transition with a 47µF output cap (Figure 4)
and 500m A load. A falling VIN edge is
about 10-times slower, resulting in
an even smaller transient.
A 3.6V input, 5V output load step response
using the compensation components of
Figure 2 is shown in Figure 5. In this
case, a 250m A to 1A load step results in
only a 250mV transient on VOUT with a
47µF output capacitor. Figures 4 and
5 illustrate how the LTC3112’s loop
July 2012 : LT Journal of Analog Innovation | 17
The LTC3112 features the latest generation buck-boost
PWM control scheme, effectively eliminating jitter when
crossing the barrier between buck and boost operation.
AUXILIARY
P-CHANNEL
MOSFET
12V
WALL
ADAPTER
CFF
33pF
RFF
681k
0.1µF
VIN
Figure 2. LTC4412
PowerPath controller
selects highest voltage
input to power the
LTC3112 converter
BURST PWM
GATE
CTL
STAT
response can be configured to provide
excellent response to both input voltage and output current load steps.
SW2
BST1
VIN
BST2
VOUT
LTC3112
47µF
OFF ON
1µF
470k
PWM/SYNC
COMP
IOUT
GND
OVP
To protect data, some data systems require
a short period of time to backup data
when the primary power source fails. A
bank of supercapacitors is often used to
provide the required burst of energy. The
LTC3112’s wide input voltage range and
In this circuit, a stack of supercapacitors
totaling 22mF is charged to 15V while
the primary power source is active. A
lower ESR electrolytic or ceramic cap is
placed in parallel to minimize VIN ripple.
The VCC supply pin is back-driven from
the 5V output in this example, allowing the LTC3112’s gate drive circuits to
Figure 3. 5V output efficiency from a single Li-ion
cell (3.6V) or wall plug (12V)
Figure 4. 3.6V to 12V input step and resulting VOUT
transient
CFB
680pF
FB
RUN
ability to buck or boost make it ideal for
such an application, as shown in Figure 6.
5V BACKUP SUPPLY
0.1µF
22pF
LTC4412
VIN
SENSE
GND
SW1
VCC
PRIMARY
P-CHANNEL
MOSFET
Li-ION
BATTERY
CELL
4.7µH
845k
RFB
33k
47pF
10k
TO ADC
1V PER AMP
100pF
42.2k
VIN
5V/DIV
EFFICIENCY (%)
90
operate efficiently with an input voltage from 15V down to 2.2V. Available
energy at the input is given by:
1
2
2

EIN = • CIN • ( VINITIAL ) − ( VFINAL ) 


2
22mF  2
2
=
• 15 − 2.2 


2
= 2.4J
The results of the backup event are
shown in Figure 7. A resistive network
from VIN, VOUT and GND is used to drive
Figure 5. 250mA to 1A load step and resulting VOUT
transient
12V
3.6V
Burst Mode Operation
85
75
70
VOUT
500mV/DIV
VOUT
1V/DIV
80
VIN = 3.6V
VIN = 12V
0.1
1
10
100
ILOAD (mA)
1A
18 | July 2012 : LT Journal of Analog Innovation
10A
IL
1A/DIV
IL
1A/DIV
20µs/DIV
47µF
158k
95
PWM
VOUT
5V
1.5A
200µs/DIV
design features
The LTC3112’s ability to support large load currents make it ideal
for handheld devices with increased processing power. Solution
size and conversion efficiency benefit from 50mΩ N-channel
MOSFET switches and thermally enhanced packages.
VOUT
4.7µH
499k
BAT54
0.1µF
VIN
15V TO 2.2V
+
22mF
SUPERCAP
STACK
+
+
= 250mA • 5V • 1.7s
= 2.1J
The prior example can be easily scaled
depending on the voltage rating of the
Figure 7. Supercap discharge performance during
power supply backup event
BST2
VOUT
LTC3112
PWM/SYNC
499k
RUN
5V/DIV
ILOAD
500mA/DIV
500ms/DIV
680pF
VOUT
5V/500mA
33k
845k
IOUT
GND
OVP
1µF
Figure 8. Maximum output current versus VIN with
VOUT = 5V and VCC back-fed
4
3.5
TO ADC
1V PER AMP
100pF
47µF
47pF
FB
RUN
supercapacitors and the energy required
for backup. The IOUT pin (Figure 6) can
be monitored by an ADC to measure
load current during the backup event.
An important consideration in design is
the maximum output current capability
of the buck-boost converter. As shown
in Figure 8, the LTC3112 is able to support up to 4A of load current when
VIN >> VOUT. As the converter transitions
from buck to boost mode, the maximum load current drops accordingly.
MAXIMUM OUTPUT CURRENT (A)
VOUT
5V/DIV
0.1µF
22pF
4.5
VIN
5V/DIV
COMP
220µF
Figure 6. Backup Supply for 5V
rail runs down to VIN = 2.2V
EOUT = IOUT • VOUT • t
SW2
BST1
VIN
VCC
1M
+
the RUN pin to provide a clean shutdown
of VOUT. In this example, a constant
250m A load is drawn from the LTC3112
resulting in the VIN capacitors maintaining regulation for 1.7 seconds, and an
average conversion efficiency of 88%
including the supercapacitor losses.
SW1
42.2k
10k
158k
SUMMARY
The LTC3112 produces low noise buckboost conversion in a range of applications requiring an extended input or
output voltage range. The LTC3112’s
ability to support large load currents
make it ideal for handheld devices with
increased processing power. Solution size
and conversion efficiency benefit from
50mΩ N-Channel MOSFET switches and
thermally enhanced packages. To provide longer run times, a low Burst Mode
quiescent current extends high efficiency
over several decades of load current.
Features such as synchronized switching
frequency, programmable output voltage, a load current monitor and external
loop compensation allow the LTC3112 to
be tailored for a specific application. n
3
2.5
2
1.5
1
0.5
0
2 3 4 5 6 7 8 9 10 11 12 13 14 15
VIN (V)
July 2012 : LT Journal of Analog Innovation | 19
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