Despite the ultra-low current consumption of wearable electronics, batteries for wearable electronics present an array of design challenges. They must retain a usable charge sitting in box in a consumer electrics warehouse, and come alive for a time before a battery activation. From that starting point, the battery must offer 15 hours of wearable life. Powering a mobile handset with a rechargeable lithium-ion battery presents one kind of design challenge; the challenge of powering a fitness band with a coin cell is equally as formidable.
Semiconductor makers have their hands full when it comes to power management devices for wearables. Low-voltage operation is only one of the many challenges. Many wearable sensors function on voltages as low as 0.8 volts (stepped-down as efficiently as possible from a 3-volt source). If the load is bursty (as with a fitness band sampling once every several seconds) the microcontroller’s current consumption is low (35- or 40µA per MHz). It’s what the microcontroller does between samples — quiescent currents (IQ) — that influences overall power consumption. The rule is: keep it low.
The power management architecture for fitness bands shows only small similarity to the architecture of a smartphone. The wearable smart watch is quite a bit smaller than a smartphone. Its battery life may be a function of its formfactor: the smartphone battery, offering a 2200 mAh capacity (several days of talk time); the smart watch does what it needs to do with a chip-sized container for as little as 40 mAh.This means: the smaller the battery, the lower its capacity, and the greater the challenge in extracting functionality.
Texas Instruments’ Smart Watch Reference Design invites comparison with Smart Phones. The power chain includes an inductor, power transistors, and an USN port. Source: Texas Instruments.
The smart watch or fitness band may not need to surf the Internet looking for data. But it does have to coordinate readings from various sensor attachments: heart rate monitoring (via a pulse oximeter or resistive chest strap), electrocardiogram (ECG) monitoring, skin sweat and moisture. Thus, a fitness band must power a variety of sensors (with varying input impedances), and a microcontroller. In operation, the wearable’s controller will look for out-of-true measurements, and, where necessary, issue an alert to the fitness band wearer via a screen change, a haptic vibration, or a wireless Bluetooth signal (BTLE).
Multiple power rails are required. But rather than increase the number of batteries, the number of voltage regulators, or the number of inductors, the preferred power management architecture is “single inductor, multiple outputs” (SIMO).
Wearable sensor manufacturers often focus on the convergence (or lack of it) between the sensors worn by fitness advocates, and the sensors used for medical diagnostics and/or hospital patient monitoring. In terms of their sensitivity, the sensors work the same; they respond to the signals from the user (or the patient). But the clinical sensors — intended to inform physicians and medical personnel must reflect a higher degree of reliability (if not accuracy), and typical require — the gating factor — Food and Drug Administration (FDA) approval.
Design Challenges for Power in Wearables
The design challenges are mapped by wearable power management suppliers like Maxim Integrated. These include
- Ultra-Low Iq Regulators, which reduces standby power for “always on” sensors or peripherals. It also extends battery life and support the use of very small batteries
- Efficient Regulators reduce the active “in-use” power (e.g., for measurements or transmitting)
- Integration enables sophisticated power architectures in space-constrained designs
- System Management include buttons/resets controls, fuel gauges, haptics, switching noise suression, and I2C Control
The typical wearable will require multiple voltage rails: one for the microcontroller, one for the display, several for sensors and haptic feedback devices. The issue becomes how many and what type of devices attach to these rails. You want to avoid multiple inductors, but it is possible to have multiple outputs from one power management device. Thus, the proliferation of wearables with SIMO architectures — and an array of sensors — encourages a smorgasbord approach, offering a variety of custom power management interfaces.
The proliferation of wearables with SIMO architectures requires support for multiple voltage rails. The power management objective is to supply adequate operating current, while minimizing standby and leakage currents, and maintaining battery life. Source: Maxim Integrated.
Preserving battery life
A key issue is to preserve battery life under a wide variety of usage scenarios. The battery must yield enough current to power an ARM-based processor, for example. Its vendors tout a low 35- or 40µA per MHz of clock cycles current draw for wearables. But reporting a measurement produces one kind of current surge; If the sensor comes awake and quickly returns to a sleep state, the wearable user can legitimately expect the battery (say a 3-volt CR3022 coin cell) to last a couple of weeks (even months) before replacement.
Rechargeable batteries present different current discharge profiles than non-rechargeable types. A rechargeable battery (say, lithium-ion) has a totally different usage profile than a use-and-replace battery (zinc-air or alkaline) Zinc-air technology, used almost exclusively with hearing aids, uses oxygen from the atmosphere. The batteries are shipped in air-tight packaging, andthe production of voltage begins as soon as the package is opened. Though rated for 1.45 volts, the usable voltage out of the box is 1.1 to 1.3 volts. In its first contact with the atmosphere, a chemical reaction occurs, and the output voltage will increase. In hearing aids, the zinc batteries support extremely small form factors, but the battery life is only a few weeks.
Silver Oxide batteries, another non-rechargeable type, have a very “flat” voltage discharge curve, producing 1.55 volts until they are completely exhausted. Alkaline batteries, in contrast have a decreasing discharge profile in which their output drops gradually. Most smart watches and fitness monitors will start working when the voltage drops to 1.3 volts.
A requirement for consumer wearables is to keep built-in batteries from losing their charge, even as the wearable appliance sits idle on a store shelf. The buyer would expect a certain functionality out-of-the-box; that is, the battery must carry a small charge for the consumer to gain a day’s worth of functionality before the wearable gets its first full charge. This requires the battery and power transfer to maintain close to zero leakage current. And, often a user-removeable plastic tab ensures the battery will have no load on it while it sits on a shelf.
Rechargeable batteries have an advantage in terms of voltages available, but they are too bulky for wearables. Texas Instruments, for example, will enable charging for single-cell lithium batteries up to 4.35- or 4.4 volts for use in a 4.2-volt system. Its battery charger offers an intelligent algorithm with maximum power point tracking technology. The algorithm, which TI calls input current optimization (ICO), automatically detects the full capacity of input power.
For rechargeable power management battery settings, TI has introduced a power management device (a DC-DC converter) that can be used in a buck or boost configuration; that is, it can be used to step down or step up the converter’s voltage level. The buck-boost capability supports battery charging in which current flows into the battery, and current consumption in which current flows from the battery. TI recently introduced single-chip buck-boost battery charge controller includes USB Type- C interface for charging and/or power delivery support
Wearable power management development platforms
Meeting ultra-low current requirements requires a specialized impedance interface. Semiconductor suppliers, looking at the vast array of requirements, can develop separate power management devices for each application. A 100% impedance match enables low noise and a lengthy battery life. In other cases, suppliers while utilize a platform approach in which the power management device uses programmability to serve multiple needs.
Maxim, for one, offers a health sensor development platform (h-sensor) that helps is an integrated sensor platform that helps engineers evaluate medical and high-end fitness applications. The platform includes a specialized analog front-end, a pulse oximeter and heart-rate sensor, two human body temperature sensors, one 3-axis accelerometer, one 3D accelerometer and 3D gyroscope, and one barometric pressure sensor reading absolute pressure.
The Architecture of evolving fitness devices supports multiple outputs (SIMO) for larger functionality in a clinical setting. Source: Maxim Integrated.