Effectively powering smart wearable devices

From products such as Google Glass, advanced fitness activity trackers, heads-up imaging displays, and blood pressure monitors, it is obvious that wearable devices have entered the military, industrial and high-end consumer markets. These devices are rapidly improving and becoming even “smarter”. A “wearable” device can be defined as a product that is worn by the user for an extended period of time and in some way, enhances the user’s experience as a result of the product being worn.

A smart wearable adds connectivity and independent processing capability to the device. It is estimated that the wearables market will grow to 130 million units by 2018 [Source: PwC, October 2014]. Wearables are divided into five application sub-categories: fitness/wellness (activity monitors, fitness bands, foot pods and heart rate monitors), healthcare/medical (pulse oximeters, hearing aids and blood pressure monitors), infotainment (smart glasses/goggles, smart watches and imaging devices), military (heads-up displays, exo-skeletons and smart clothing), and industrial (body-worn terminals) [Source: IHS Electronics and Media, 2013].

These categories have different market forces driving their adoption rates. In the wellness and medical segments, these include: rising life expectancy, the desire to prolong a healthy life and to reduce hospital stays. For military, it’s the desire to improve situational awareness, better manage maps & routes, increasing combat efficiency, and saving lives. For industrial, the main drivers are improving production line efficiency and tracking capability. For infotainment, the exploding gaming market with cutting edge imaging and virtual reality, as well as the increasing number of devices able to connect wirelessly to smart phones to become part of the “internet of things” (IoT).

Smart wearables architecture & problems

So, what’s “under the hood” of your smart wearable device? Think of it as a miniature embedded system. The exact partitioning will obviously depend on the device itself. However, generally speaking, the core architecture for a smart wearable is a combination of the following: a microprocessor or microcontroller or similar IC, some sort of micro-electromechanical sensors (MEMS), small mechanical actuators, Global Positioning System (GPS) IC, Bluetooth/cellular connectivity, imaging electronics, LEDs, computing resources, battery or battery pack, and support electronics.

A wearable unit’s primary goals are to have a compact form factor, low weight for wearability/comfort and provide ultralow energy consumption in order to extend battery run time. Wearables are obviously “cool” products - however powering them efficiently and accurately while charging batteries with minimal current draw - is another matter entirely.

Some of the key issues associated with powering smart wearables with ICs include the following:

• Low current consumption from the IC in a battery-powered device is paramount for increased run time. A micropower – or even better a nanopower – conversion IC is ideal.

• Some wearable device architectures use a multiple-battery approach, for example, 2xLithium 8.4V battery rather than a single-cell Lithium (4.2V). This increases capacity and gives longer system run time. However, a higher voltage IC is then required.

• A MEMS sensor requires power from a quiet regulated power source. Busy actuators may also benefit. An LDO works great for such rails since they have low output ripple.

• Bluetooth/RF connectivity system rails also require low noise. A low dropout regulator or, since output currents can be high, an LDO post-regulated switching regulator is a good choice.

• Processor power (the “brains” of the wearable). From TI OMAP, ARM Cortex MCUs, DSPs, GPS chips or FPGAs for example, have a variety of low-voltage rails, spanning low level to high currents. These can be powered by LDOs or switching regulators.

• Batteries need care and feeding so as to avoid overcharging which thus reduces battery cycle life. Accurate battery chargers with onboard termination algorithms ensure longer life for the cell(s).

• Compact size and low weight make the wearable device more comfortable for the user. ICs in compact packages provide small solution footprints, thus enabling the device to be offered in a small form factors.

• A feature-rich wearable product means many system rails. Multiple output regulators or power management integrated circuits (PMICs) may well fit the bill. Finally, compact ICs with battery chargers integrated onboard provide a higher level of integration and flexibility.

Ultralow IQ IC solutions

It is clear that an IC solution that solves the application needs, as well as the associated issues already discussed should have many of the following attributes:

•Ultra-low quiescent current, both in operating mode and shutdown

• Wide input voltage range to accommodate a variety of power sources

• The ability to efficiently power multiple system rails

• Accurate battery charge voltage to prevent overcharging

• The ability to charge popular battery chemistries such as Lithium

• Simple and autonomous charging operation with onboard charge termination algorithms (no µC needed)

• Small and low profile solution footprints

• Advanced packaging for improved thermal performance and space efficiency

For example, Linear’s recent ultralow IQ LTC3388/-x buck regulator family, and its LTC3553 combination buck regulator & single-cell Lithium battery charger PMIC have most of these attributes already. The LTC3388 is an ultralow quiescent current synchronous buck converter than can deliver up to 50mA of continuous output current from a 2.7V to 20V input supply.

The LTC3388’s no-load operating current of only 720nA makes it ideal for a wide range of battery-powered and low quiescent power applications, including “keep-alive” supplies and wearables. The LTC3388 utilizes hysteretic synchronous rectification to optimize efficiency over a wide range of load currents. It can offer over 90% efficiency for loads ranging from 15µA to 50mA and only requires 720nA of no load quiescent current in regulation, thereby extending battery life. The combination of a 3mm x 3mm DFN package (or MSOP-10) and only five external components offers a very simple and compact solution footprint for a wide array of low power applications. Figure 1 shows a typical LTC3388 application circuit.

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Figure 1. LTC3388-1/-3 Typical Application CircuitFor Higher Scale of Integration – Use a PMIC

The LTC3553 is a micropower multi-function PMIC for portable Li-Ion/Polymer battery applications. The LTC3553 integrates a USB-compatible linear PowerPath control manager, a standalone battery charger, a 200mA high efficiency synchronous buck regulator, a 150mA LDO and pushbutton control, all in a compact 3mm x 3mm QFN package. (Refer to Figure 2 for its typical application circuit.)

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Figure 2. LTC3553 Simplified Application Schematic

For applications operating with low currents in standby mode, the IC’s pin-selectable Standby Mode reduces battery drain current to only ~12µA while maintaining regulation of all outputs, see Figure 3’s graph for details. The LTC3553 is highly suited for personal navigation devices (PNDs), media players, portable medical and industrial devices and other small-battery portable device applications with small capacity batteries that consume low amounts of power.

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Figure 3. LTC3553 Battery Drain Current Operation

The smart wearables market is exploding and includes a wide variety of products for the health & fitness, medical, infotainment, military and industrial application spaces. The core architecture for a smart wearable device depends on the product type, but essentially boils down to a micro-controller, MEMS sensor(s), wireless connectivity, battery and support electronics.

Powering a low-current wearable device can prove very challenging. However, Linear Technology offers a large portfolio of leading-edge products capable of very high performance at such low power levels. Devices such as the ultralow IQ LTC3388 energy harvesting buck regulator, and for a higher level of integration, the LTC3553 Lithium battery charger and buck regulator/LDO combination PMIC can enhance the operating life and overall performance of many future wearable devices.