Mark Patrick, Mouser Electronics
People are becoming increasingly involved in their own healthcare – largely as a result of the new generation of highly portable medical devices. Real-time remote monitoring will become the new ‘normal’, giving greater convenience to patients, and relieving some of the pressure on medical facilities. However, for this vision to become a reality, designers will have to solve the challenge of powering these devices.
In common with most ‘tech’ devices, as new wearable fitness devices and smart watches are released to market, they offer greater functionality than the generation they replace. The same is true of other types of medical device – yet, in almost all cases, the greater functionality means that the device consumes more power. By their very nature, these wearable devices are intended to be highly portable, and this constrains the size of the battery that can be used, especially as these devices are worn for extended periods of time. In many cases the wearer will be elderly or infirm, so a heavy battery can be a source of fatigue. In order to meet these challenges, designers are finding that they must use ever-more innovative power management techniques to ensure the lowest possible current draw.
Basic design criteria
Wearable devices may be small, but they are fully functioning systems in their own right and will typically comprise most, if not all, of the following:
- A microcontroller unit (MCU), to run the code as well as managing and processing data.
- A battery or other form of energy storage to power the device.
- Sensors to monitor key physical parameters – these may include temperature, heart rate, movement or others.
- A communication interface, to communicate with a gateway device such as a smartphone or tablet. Common protocols include Bluetooth Low Energy (BLE) or Near Field Communication (NFC).
- Cryptographic mechanisms, to ensure that the highly sensitive medical data, as well as the code required to run the device, cannot be intercepted or otherwise interfered with.
Most devices are intended to fulfil a particular application or use, and this defines the choice of the MCU, sensors and other aspects of the device. For example, accuracy and reliability are key requirements for clinical medical devices, as is the ability for them to be easily managed and securely protect the data they contain. Longevity is another requirement, as these devices can be required to record for over a year when monitoring patients with long-term conditions.
Any device that requires sophisticated power management is often segmented into multiple functional blocks, with each section having similar power needs. Each section (‘domain’) can then be designed to be as energy efficient as possible. Each domain will have separate power feeds, allowing power to be applied or removed as needed. For example, if the RF section only needs power when communicating or the sensors only need power when a reading is being taken, and by removing power at other times, the design becomes highly efficient. Part of the MCU’s role is to manage this power control, while ensuring that the peripheral devices remain highly integrated into the system.
Figure 1: Typical internal circuit configuration of a wearable device with various power domains.
Energy storage technology
Most wearable devices use a battery that is either lithium-ion (Li-ion) or lithium-ion polymer (LiPo). All batteries based on Li-ion chemistry are formed of organic electrolytes and deliver a primary and secondary nominal voltage in the range 3.2V to 4.0V. The difference between these two popular chemistries is in the amount of energy they can store; Li-ion can store more energy per unit volume than LiPo, giving it an advantage in compact devices, such as wearables.
Li-ion batteries are generally small and lightweight – both desirable characteristics for wearables – and they are also reasonably environmentally friendly. As they obey the laws of physics, the storage capacity of a Li-ion battery is proportional to its size, so future batteries are likely to include other materials to allow more energy to be stored per unit volume.
For example, there are multiple research projects looking into graphene – a material that is believed to offer far greater energy storage density than Li-ion. Supercapacitors are another possible energy source for wearables, and are looking more promising based on the good progress made in the area of nanotechnology.
Low-power building blocks play a role
The MCU has a pivotal role within the wearable device, yet designers are seeking devices that consume well under a milliamp when operating and just nanoamps while in standby mode. Apart from its processing tasks, dealing with code and data, the MCU also understands when peripherals will be needed by the system and manages the power domains accordingly, ensuring none of the valuable stored energy is wasted. One such device is Maxim Integrated’s MAX32660 MCU, which is intended specifically for use in wearable devices. This MCU has become popular as it marries high levels of processing performance with excellent energy efficiency. At the heart of the MAX32660 is a 32-bit ARM Cortex-M4 processor that contains a floating point unit (FPU) processor. It is capable of managing sensors and the external memory needed to run sophisticated processing algorithms. The tiny device is housed in a WLP package with a 1.6mm x 1.6mm footprint, and it offers one of the best power/performance ratios on the market today (50μW/MHz).
Figure 2: Block diagram for the MAX32660.
Another supplier of ultra-low power 32-bit MCUs for medical wearables is Microchip. Its range offers entry-level ARM Cortex-M0+ based SAM D MCUs, highly energy-efficient SAM L devices and the powerful PIC32MX XLP products, among others. In terms of energy performance, they can run as low as 35µA/MHz when active and require just 200nA in sleep mode. Despite the low power performance, the Microchip MCUs include operational amplifiers, real-time clocks, display ports, DMA and USB connectivity, and mTouch sensing.
Figure 3: Microchip PIC32MX block diagram.
Silicon Labs EFM32 Giant Gecko ARM Cortex-M3 based devices are more 32-bit MCUs that are finding applications in the wearable device space. These devices include low-energy peripherals, a communications UART, sensor control hardware and analogue operational amplifiers. They provide the security needed for medical wearables through the included AES encryption.
Figure 4: EFM32 block diagram.
Batteryless operation
Even if energy goals are met, it is still necessary to replace the battery at regular intervals, putting a burden on the patient (who may not be capable or may forget) or a caregiver. In fact, many believe that this inconvenience is a significant contributor to the reason why wearable devices have not seen the levels of adoption that many predicted. Despite the reduction in energy consumption and the improvements in battery chemistry, many people believe that the ultimate solution lies in energy harvesting.
Instead of relying solely on stored energy, energy harvesting obtains energy from natural sources such as the Sun, external heat energy, or even the movement of the person wearing the wearable device.
If the energy is to be harvested from the wearer as heat or movement (or a combination of the two) then the person must be active enough to generate the energy necessary to power the wearable device. On average, a person dissipates around 100 Megajoules per day – which equates to a significant battery bank and represents more than sufficient energy, provided it can be harvested without too much wastage.
Heat is a potential source of energy for harvesting by converting the difference in temperature into a voltage proportional to the temperature gradient. To achieve this, a Peltier cell is created using the Seebeck effect from a pair of semiconductor devices. In wearable applications, the wearer is the hot side and ambient air forms the cooler side so there is always a temperature gradient, especially if the wearer is exercising. Unlike solar power using a photovoltaic cell that is only effective during daylight hours, this type of thermo-electric power provides energy 24/7.
Movement of the patient is another energy source that can be exploited for energy harvesting. As the patient (or any device wearer) moves around, small piezo-electric devices would generate currents in response to the movement. This means that while performing everyday activities such as walking, exercising or even breathing, energy is being generated to replenish a reservoir on the device. For designers looking to develop solutions around this technology, the Energy Harvesting Solution to Go development kit from Wurth Electronics is a valuable resource for completing designs quickly.
Onboard power conversion
Especially when using energy harvesting techniques, the voltage available to the wearable device can fluctuate. Many wearable devices incorporate a DC/DC converter to ensure that a constant voltage is always available. The DC/DC converter will be controlled by the MCU and will control the power delivered to each domain.
Efficiency is incredibly important for DC/DC converters so that they use as little of the available energy as possible. One device that is aimed specifically at wearable solutions is Linear Technology’s LTC3107. This device includes the ability to manage both a battery and a thermal energy harvesting system, and can increase the life of the onboard battery substantially, thereby increasing user convenience and reducing costs.
Figure 5: Typical application circuit for the LTC3107 DC/DC converter.
As wearable devices become smaller and gain functionality, power management becomes more important to avoid frequent (and inconvenient) battery replacement. Energy harvesting is now a popular technique to extend battery life, and in some cases replace the battery entirely. Combining this with intelligent power management and low consumption components (MCU, peripherals) enables the development of highly featured wearable devices that can last for a long time.