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issue: August 2009 APPLIANCE Magazine

Appliance Engineer - Electronics Report
Running Low and Long


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A dc-to-dc convertor increases usage time and reduces required PCB area for portable electronic devices.

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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