LINEAR TECHNOLOGY LTC3101 Technical data
L DESIGN FEATURES
LTC3101
BUCK-BOOST
LDO
ON/OFF
AC
ADAPTER
USB
or
HOT SWAP OUT
3.xV AT 300 to 800mA
1.xV AT 350mA
1.xV AT 350mA
1.8V AT 50mA
3.xV AT 100mA
x.xV AT 200mA
TRACKING OUT
LI-ION
USB
BAT
Power Management IC with
Pushbutton Control Generates Six
Voltage Rails from USB or 2 AA Cells
Via Low Loss PowerPath Topology
Introduction
As the complexity of portable electronic
devices continues to increase, the demands placed on power supplies, and
their designers, expand dramatically.
Not only must typical power systems
accommodate multiple input sources,
with voltages as low as 1.8V for two
AA cells, but they must also provide
an increasing number of independent
output rails to support a wide range
of requirements—for memory, microprocessors, backlights, audio and RF
components. To further complicate
matters, expanding feature sets add up
to increased power dissipation, making
it important to optimize overall power
system efficiency. This is particularly
challenging given that the constant
drive to minimize the required board
area and profile height of the power
system is at direct odds with improving efficiency.
The LTC3101 addresses all of these
challenges with a single-IC power
management solution that allows a
designer to easily maximize overall
power system efficiency while minimizing space requirements. The LTC3101
can generate six power rails by integrating three synchronous switching
converters, two protected switched
Figure 2. Complete portable power
solution with a 16mm × 19mm footprint
12
by John Canfield
Figure 1. Six output rails, a low loss PowerPath and integrated pushbutton control
power outputs, and an LDO. Its integrated low loss PowerPath™ topology
allows each switching converter to
run directly from either of two input
power sources.
Two 350mA, high efficiency low
voltage rails, typically used to power
processors and memory, are generated
by synchronous buck converters. Each
converter is able to operate down to an
input voltage of 1.8V thereby enabling
single stage conversion from any input
power source.
A single inductor buck-boost
converter generates a high efficiency
intermediate output rail, typically at
3V or 3.3V, and is able to operate from
either input power source and with
input voltages that are above, below,
or even equal to the output regulation
voltage. The buck-boost converter
can supply a 300mA load at 3.3V for
battery voltages down to 1.8V and an
800mA load for input voltages of 3.0V
and greater.
Two always-alive outputs—MAX,
which tracks the higher voltage input
power source and LDO, a fixed 1.8V
output—provide power to critical
functions that must remain powered
under all conditions. An integrated
pushbutton controller with programmable µP reset generator provides
complete ON/OFF control using only
a minimal number of external components while independent enables allow
total power-up sequencing flexibility.
This complete portable power solution
is packaged in a single low profile 24lead 4mm × 4mm QFN package and
the entire power supply, including
all external components, occupies a
PCB area of less than 3cm2 as shown
in Figure 2.
Zero Loss PowerPath
Topology Maximizes
Efficiency
Although rechargeable Li-Ion and
Li-Polymer batteries are the leading
chemistries for powering portable
devices due to their high energy density and long cycle life, many portable
devices continue to be powered by
alkaline and NiMH cells. This allows
indefinite periods of use away from a
Linear Technology Magazine • June 2009
BAT1
USB1
C
RS
ENA1
ENA2
ENA3
PWRKEY
PWRON
PWM
PBSTAT
RESET
USB2
FB3
LDO
SW2
FB2
HSO
MAX
10µF
10µF
2 AA
CELLS
USB/WALL
ADAPTER
1.8V TO 5.5V
10µF
1M
V
OUT3
= 3.3V
300mA FOR VIN ≥ 1.8V
800mA FOR VIN ≥ 3V
221k
Hot Swap OUTPUT: 3.3V AT 100mA
TRACKING OUTPUT: 200mA
4.7µH
4.7µF
1.8V AT 50mA
V
OUT2
1.8V
350mA
V
OUT1
1.5V
350mA
10µF
221k
110k
4.7µH
SW1
FB1
10µF
221k
147k
4.7µH
0.1µF
ON/OFF
BAT2 SW3A
LTC3101
GND
µP
SW3B OUT3
DIS ENA
+
USB
BAT
+
BUCK
V
OUT
USB
BAT
+
BUCK-BOOST
V
OUT
charging socket—which is particularly
important for devices intended for use
in remote locales such as handheld
personal navigation devices or portable
medical devices. Voice recorders, digital still cameras and ultra-small video
recorders are additional examples of
devices that benefit from the ability to
operate from a pair of commonly available batteries, rather than requiring
the lengthy recharging cycle needed
for an internal Li-Ion battery.
Even in portable devices where the
primary power source is restricted
to AA or AAA form factor cells, there
still exist a wide variety of compatible chemistries including alkaline,
rechargeable alkaline, NiMH and
single-use lithium. As a result, the
AA/AAA powered device must accommodate a wide range of input voltages,
from 1.8V for two series alkaline cells
near end of life, to approximately 3.7V
for a pair of fresh non-rechargeable
lithium cells. With its wide 1.8V to
5.5V input voltage range, the LTC3101
can easily support all of these battery chemistries. In addition, the
LTC3101 is able to operate from a
single standard Li-Ion/Polymer cell in
cases where recharging is performed
independently.
Although rechargeable cells are
usually charged outside these types
of devices, the power supply must
accommodate a secondary tethered
power source such as USB or a regulated wall adapter. Consequently, the
power supply must include a means to
generate every power rail from either of
two input sources, and the ubiquitous
3.3V rail must be generated from input
power sources that can be higher or
lower voltage.
In many devices, the capability to
handle dual power sources is provided
by using discrete power MOSFETs to
switch regulator inputs between the
two input power sources or by utilizing
two regulators for generation of each
rail (for example, a buck converter
that generates a 3.3V rail from the
USB input in conjunction with a boost
converter that generates the 3.3V rail
from the battery input).
Both of these approaches suffer
from significant drawbacks. The par-
Linear Technology Magazine • June 2009
allel converter approach increases
system cost and size given that only
one converter is ever active at any given
time and often suffers from glitches
and disruptions to the output rails
during the transition between the
two input power sources. Similarly,
the discrete power switch technique
reduces efficiency due to the addition
of extra series elements in the power
path, increases component count,
and can also lead to disruptions in
the output rails unless the supply
crossover is carefully controlled.
The LTC3101 avoids these problems by using a low loss PowerPath
topology as shown in Figure 4, where
each converter is able to operate directly from either input power source.
In this architecture, each switching
converter utilizes an additional power
switch, which is connected to the
alternate power input. As a result,
each converter is able to run with
maximum efficiency from either input
power source so no efficiency penalty
DESIGN FEATURES L
Figure 3. Typical application
is incurred in supporting dual input
power sources.
The total solution area is minimized
by the fact that the same inductor is
used in either case. In addition, the
automatic transition between the
two input power sources is seamless—there is no interruption to any
of the output rails. Figure 5 shows the
transient response of the buck-boost
converter as the input power source
transitions from battery power to USB
power in response to a live cable plug
into a USB port.
Integrated Buck-Boost
Provides High Efficiency
3V/3.3V Rail from
Any Power Source
In many portable devices an intermediate supply rail, typically regulated to
3.3V, is required to power an RF stage
or audio amplifiers. Often this rail is
generated from the two series AA cells
using a boost converter. However, the
higher cell voltage of single-use lithium
Figure 4. The low loss PowerPath architecture
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