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DC/DCs extend battery life in CDMA, WCDMA handsets

Posted: 27 Sep 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Semtech? SC250? SC251? CDMA? WCDMA?

Semtech's SC250 and SC251 DC/DC <a href=power converters" ALIGN="RIGHT">

Semtech's SC250 and SC251 are the first members of a DC/DC step-down (buck) power converter family designed to improve battery life by providing adaptive power management for CDMA and WCDMA handsets.

"These devices mark the launching of our new family of application specific buck regulators targeted at improving power amplifier efficiency," said Ven Shan, VP for the power management group at Semtech. Power amplifier current consumption is the biggest contributor to battery drain in a CDMA or WCDMA handset. "The true innovation of [our] devices lies in the adaptive nature of the power transfer scheme as well as the integrated PA biasing and their small size and low profile, which allow designers to add this functionality without impacting the size of their phones."

Power efficiency
CDMA and WCDMA handsets experience wide swings in power consumption based on their distance from a base station. The SC250 and SC251 are designed to work with the phone's baseband processor to enhance the phone's efficiency by adaptively controlling the DC power supply to the power amplifier. The SC250, delivering a maximum average current of 600mA at 0.3-3.6V from a 2.7-5.5V input, performs a buck conversion to generate an output voltage that is linearly proportional to an analog input control voltage from the baseband processor. The buck conversion process reduces battery current when the PA outputs low power, greatly increasing talk time compared to directly connecting the power amplifier to the battery.

The SC251, rated for a maximum average current of 800mA at 0.5-3.3V, performs a similar function, but uses a nonlinear transfer function that can eliminate the need for a software look-up table. The nonlinear transfer function is matched to the power requirements of the given CDMA and WCDMA power amplifiers, reducing the need for software development and optimizing talk time. The SC251 also provides the option to switch between two different transfer functions when used with a dual-mode power amplifier. Each transfer function is optimized to match the DC power requirements of the different power amplifier gain modes.

Pass-through mode
The adaptive buck converter starts up and responds to changes in requested output voltage in less than 40?s. For increased efficiency at maximum transmit power, the devices feature a pass-through mode where an integrated switch connects directly between the battery and the output, bypassing the inductor. The SC250 has a 75-milliohm switch, and the SC251 a 200-milliohm switch, with a pin for driving an optional external FET when ultralow series resistance is required.

Both devices integrate an LDO for biasing a power amplifier. The SC250 and SC251 provide a regulated 2.85V, with the SC250 providing up to 20mA and the SC251 providing up to 10mA. Both devices include a full range of protection features, including over current limit, thermal protection, and under-voltage lockout

The SC250, in a 2.3-by-2.3mm MLPD-W8 package, is priced at $1.36 each in 1,000-piece lots. The SC251, in a 3-by-3mm MLPD-10 package, is $1.38 each.

- Vince Biancomano

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