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MEMS chips help extend battery life of wearables

Posted: 17 Jun 2015 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS? wearables? battery life?

Microelectromechanical system (MEMS) sensors that are used in wearables have two critical features: ultra-low power and ultra-small size.

MEMS manufacturer mCube Inc. discovered this in its research and has set out to meet that not-so-tiny goal. Since mCube already had the industry's smallest MEMS+CMOS die at 1mm x 1mmallowing room for larger batteries in wearablesthey decided to concentrate on technologies that would help extend the battery life of fitness, health monitoring and activity tracking wearables from hours or days, to weeks or months, according to mCube CEO Ben Lee.

"In a nutshell we already make the world's smallest accelerometera one-by-one die in a two-by-two package," Lee told EE Times in advance of their wearables announcement. "And we wanted to be the first to design an accelerometer specifically for the wearables market, so we redesigned our ASIC to fulfil their needs."

3D MEMS+CMOS process

The 3D MEMS+CMOS process used by mCube (right) grows the MEMS mechanical structure above the complementary metal oxide semiconductor (CMOS) unlike Invensense (middle) which caps its MEMS die with the ASIC die and the others (left) which stack two die then connect them with wire bonding. (Source: mCube)

As an example, Lee described how their wearable MEMS chips save power for a Bluetooth headset. When you set it down it goes into "sniff" activity-mode, which consumes only 0.6?A by only sampling the accelerometer often enough to sense it being picked back up. When the user does pick it back up, mCube's redesigned ASIC is fast enough to turn the headset back on and pair before the user even has it in their ear, according to Lee.

mCube die

The mCube die (left) uses only six wire bonds from the chip to the pad of the package, but competitors use from 20 (right) to 21 (middle) thus increasing RF-interference, which degrades signal-to-noise ratios (SNRs) as well as make the chip less reliable when wire bonds come loose after dropping a device. (Source: mCube)

In fact, mCube has designed into its new ASIC three low-power modes for different functionssniff at 0.6?A, single-sample mode at 0.9?A and a 25Hz sample rate, and a 4.7?A mode with a 50Hz sample rate and full operation. It also allows the designer to adjust the sample rate from between 0.4Hz and 400Hz as well as adjust its resolution for 8-, 10- or 12bits (with a 32-sample first-in first-out, FIFO, buffer) or at 14bits for single samples, giving wearable designers a full spectrum of options for saving the maximum amount of power while meeting the specifications of their application, thus extending the battery life of the wearable, according to mCube.


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