The market for smart wearable devices has experienced explosive growth in recent years, with a variety of products emerging for applications in healthcare and fitness, medical, entertainment, military, and industrial sectors. This includes a new wave of products, including healthcare wearable devices that use sensors, creating opportunities for adopting a more active and healthier lifestyle. This article will introduce the design requirements for smart wearable devices and the power conversion solutions offered by ADI that can be applied to low-power wireless devices.
Wearable devices require ultra-low energy consumption to extend battery life
The number and types of wireless sensors that support the Internet of Things (IoT) are rapidly increasing. This trend has created a demand for small, compact, and highly efficient power converters customized for low-power wireless devices. One of the emerging sub-markets in the IoT industry is the wearable electronic products market. A prominent and evident application within this market is health monitoring. Whether it's for patients in a hospital setting or individuals who are highly conscious of their health, medical wearable devices are being used to record biometric data. This data includes essential physiological indicators such as body temperature, pulse/heart rate, respiratory rate, and blood pressure. These biometric measurements are used to assess the basic functions of the human body and monitor health status.
These vital signs are crucial because any adverse changes in this data could indicate a decline in health, and vice versa. Traditionally, various devices found in hospitals and doctor's offices were required to measure these biometric data. However, having the ability to measure such biometric data conveniently, in real-time, and efficiently enables individuals to adjust their lifestyle and behaviors based on real-time vital signs data. This, in turn, can help improve health and potentially extend lifespans or even save lives.
The core architecture of smart wearable devices depends on the type of product, and the specific components vary based on the device. However, in general, the core architecture of a smart wearable device typically includes a microprocessor or microcontroller, or a similar IC. It may also include Micro-Electro-Mechanical Systems (MEMS) sensors, small mechanical actuators, Global Positioning System (GPS) ICs, Bluetooth/cellular/Wi-Fi connectivity for data collection/processing and synchronization, imaging electronic components, LEDs, computational resources, rechargeable or primary (non-rechargeable) batteries or battery packs, and supporting electronic components. Therefore, the primary design goals for wearable devices are compact form factor, lightweight for wearability and comfort, and ultra-low energy consumption to extend battery runtime/lifespan.
Energy harvesting systems extend the usage time of wearable devices
To extend the usage time of wearable devices, in addition to using the main battery as a power source, the incorporation of an energy harvesting system can significantly prolong the device's usage time. There are various off-the-shelf energy harvesting technologies available in the market, such as vibration energy harvesting products and indoor or wearable photovoltaic cells, which can generate power in the mW range under typical working conditions. While this power level may seem limited, when considering the cost per unit of energy provided, energy harvesting products are roughly comparable to long-life primary batteries.
While some primary batteries claim to provide up to 10 years of life, this largely depends on the level of power drawn from them and the frequency of power extraction. Systems with energy harvesting capabilities can recharge once the power is depleted, which is not possible for systems powered solely by primary batteries. However, most implementation schemes use an environmental energy source as the primary power source, with the primary battery serving as a backup for the environmental energy source. This can be considered a 'battery life extender' capability, providing the system with an extended operational life, approaching the life of the battery, typically around 12 years for a lithium thionyl chloride chemical composition.
Of course, the energy provided by an energy harvesting power source depends on how long that source can operate. Therefore, the primary comparative metric for energy harvesting power sources is power density rather than energy density. Energy harvesting power sources typically provide low, variable, and unpredictable available power, so a hybrid structure is often used, connecting the harvester to an auxiliary power source. The auxiliary power source may be a rechargeable battery or a storage capacitor. The collector becomes the system's energy source due to its infinite energy supply. The auxiliary power storage repository (be it a battery or a capacitor) generates higher output power but stores less energy. It provides power as needed, and at other times, it periodically receives charges from the energy collector.
In these common wearable devices, the energy harvesting system must use power conversion ICs capable of handling extremely low power and very small currents, often in the range of tens of microwatts and tens of nanoamperes. ADI offers a range of power conversion ICs designed with the necessary features and performance to manage the low harvested power typically found in wearable devices.
Highly integrated DC/DC converters extend battery life
The LTC3107 is a highly integrated DC/DC converter designed to extend the life of a primary battery in low-power wireless systems by harvesting and managing surplus energy from very low input voltage sources, such as thermoelectric generators (TEGs) and thermopiles. It operates in a step-up topology and can work with input voltages as low as 20mV.
The LTC3107 uses a small step-up transformer to provide a complete power management solution suitable for typical wireless sensor applications that rely on a primary battery. The 2.2V LDO can be used to power an external microcontroller, and the main output voltage can be automatically adjusted to match the primary battery voltage. In cases where harvested energy is available, the LTC3107 can seamlessly transitions from battery power to harvested power, thus extending the battery life. The BAT_OFF indicator can be used to track battery usage. An optional storage capacitor accumulates surplus harvested energy, further extending the battery life.
When using the LTC3107, the space required for an energy-harvesting module is minimal, only enough to accommodate the LTC3107's 3mm x 3mm DFN package and a few external components. By generating an output voltage that tracks the existing primary battery voltage, the LTC3107 seamlessly brings the cost savings of harvesting free thermal energy to new and existing battery-powered designs. Additionally, the LTC3107 can extend battery life along with a small thermal energy source, sometimes exceeding the shelf life of the battery, reducing the recurring maintenance costs associated with battery replacement. The LTC3107 is designed to supplement or even fully power the load from the battery, depending on the load and available harvested energy.
A DC/DC converter for high-voltage energy harvesting power sources
Another product example from ADI is the LTC3331, which integrates a high-voltage energy harvesting power source with a rechargeable battery-powered buck-boost DC/DC converter to create a single-output power supply for alternative energy applications. A 10mA shunt allows for simple battery charging using harvested energy, and a low battery disconnect function is included to prevent the battery from deep discharge.
The LTC3331 consists of an integrated full-wave bridge rectifier and a high-voltage buck-boost DC/DC converter to harvest energy from piezoelectric sources, solar, or magnetic sources. Either DC/DC converter can supply power to a single output. The buck converter operates when harvested energy is available, reducing the static current drawn from the battery to as low as 200nA, extending the battery life. When there is no harvested energy available, the buck-boost converter will supply power to VOUT only.
The LTC3331 provides a complete energy harvesting regulation solution that can deliver a continuous output current of up to 50mA to extend the battery life when harvested energy is available. It doesn't require the battery to supply power current to the load when harvested energy is providing a steady power source, and it consumes only 950nA of operating current when powered by the battery under no-load conditions.
The LTC3331 integrates a high-voltage energy harvesting power source with a synchronous buck-boost DC/DC converter, powered by a rechargeable primary battery, to offer uninterrupted output for energy harvesting applications like wireless sensor nodes, IoT devices, and wearable devices.
Additionally, the device includes a supercapacitor balancer to enhance output storage. Input and output voltage and current set points can be configured via pin-strapped logic inputs. The LTC3331 is available in a 5mm x 5mm QFN-32 package.
A high-efficiency, low quiescent current buck-boost DC/DC converter
Additionally, ADI has introduced LTC3335, a high-efficiency, low quiescent current (680nA) micropower buck-boost DC/DC converter. It features an integrated high-precision Coulomb counter that can monitor the accumulated battery discharge in long-life battery-powered applications, making it suitable for wireless sensor networks and general energy harvesting applications. The buck-boost circuit can operate with an input voltage as low as 1.8V and offers 8 pin-selectable output voltages, along with an output current of up to 50mA.
The integrated Coulomb counter in LTC3335 monitors the accumulated battery discharge in long-life battery-powered applications. This counter stores the accumulated battery discharge in an internal register accessible via the I2C interface. LTC3335 includes a programmable discharge alarm threshold, generating an interrupt on the IRQ pin when the threshold is reached.
To accommodate various battery types and sizes, LTC3335 allows the peak input current to be selected within a range of 5mA to 250mA. The full-scale Coulomb counter offers a programmable range of 32,768:1. LTC3335 is available in a 3mm x 4mm QFN-20 package.
Conclusion
Most smart wearable devices are battery-powered and aim for the longest possible battery life and longevity. When combined with energy harvesting technology, it can significantly extend the operational time of wearable devices. Therefore, low-power conversion solutions will be a significant driver for wearable devices. However, powering low-current wearable devices can be quite challenging. ADI offers a range of leading products capable of delivering high performance at low power levels, making them an ideal choice for developing wearable devices.