A small footprint and
a long battery life are key to the success of portable devices. A new
3-MHz step-down dc-to-dc convertor from Analog Devices Inc. (ADI;
Norwood, MA, U.S.; www.analog.com) has been optimized to help designers
achieve better performance on both counts. The 600-mA ADP2108 convertor
has an operating voltage as low as 2.3 V, allowing designers to fully
use the available battery energy for 2.5-V and 3-V power loads before
recharging. This can eliminate the use of a more-expensive buck-boost
regulator in some applications.
The
convertor runs from very low supply voltages, thanks to carefully
designed internal circuits, says Ken Marasco, ADI’s system applications
manager, power management group. “And just as important, an additional
circuit is added to make sure that the convertor can move seamlessly in
and out of 100% duty cycle mode if the Vin drops down to the level of
the regulated Vout.” This enables designers to safely use the buck
convertor from a power supply of 2.3 V or higher.
For
additional battery life savings, the ADP2108 has a light-load power
saving mode (PSM) that allows it to maintain efficiency at light loads
by skipping switching cycles. “Proprietary circuitry allows the ADP2108
to seamlessly switch into PSM below 100 mA and always go back to
fixed-frequency pulse-width modulation (PWM) mode above 100 mA. Under
some conditions, our competitors’ parts can have hundreds of milliamps
of load current and still be in PSM,” says Marasco. “The ADP2108 has a
transition scheme where it periodically checks the load current with a
high-accuracy measurement circuit and selects the correct operating
mode. The device has a superior range over which it can stay in
fixed-frequency mode without compromising the low current efficiency.”
The
convertor saves not only power but also space. By using a design that
integrates the internal power switch, synchronous rectifier, and
control circuitry, the convertor requires only three 0603-size external
components—input and output decoupling capacitors and the switching
inductor. The small footprint and high conversion efficiency enable
designers to support a higher load in a smaller area.
Marasco
says for appliance engineers seeking to improve power system
efficiency, the ADP2108 can be used as a replacement for low-dropout
regulators (LDOs). “Using the ADP2108 as a replacement for an LDO helps
with heat management because power not delivered to the load is
dissipated as heat within the LDO,” which causes the temperature of the
device to rise and thus decrease the system efficiency.
ADI
offers the ADIsimPower designer tool on its Web site to facilitate the
design of high-efficiency dc-to-dc conversion blocks. “The user need
not be a power engineer since the only requirements are Vinmin, Vinmax,
Vout, Iout, and temperature,” says Marasco. After the designer chooses
a design goal from size, cost, efficiency, or part count, the tool does
the rest. “The software mimics the design process used by an
experienced power designer. Thousands of calculations are run to select
the best parts from a database of qualified parts,” he says. A blank
PCB is available for prototyping each custom design, although
“predictions for efficiency and stability are typically so accurate
that prototypes are not needed,” says Marasco.
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