Build a True Wireless Fitness Hearable—Part 3: Wireless Power Management
Contributed By Digi-Key's North American Editors
Editor’s Note: Although they have great potential, fitness hearables present significant design challenges in three key areas: biomeasurement, audio processing, and wireless charging. This series of three articles explores each of those challenges one by one and shows developers how to take advantage of ultra-low-power devices able to more effectively create fitness hearables. Part 1 addressed biomeasurement of heart rate and SpO2. Part 2 looked at audio processing. Here, Part 3 discusses solutions for power management and wireless charging for fitness hearable designs.
Power optimization has become a fundamental requirement across most application segments, but fitness hearables raise unique concerns even beyond those found in conventional "wireless" earbuds. The latter use a Bluetooth connection for audio streaming but retain a wired connection to a battery that is typically encapsulated with volume controls and a power connector in an inline package. In contrast, true wireless designs eliminate all wired connection, requiring product designers to build rechargeable batteries into each earbud.
As a result, system engineers must find design solutions able to comply with tight packaging requirements while ensuring both extended battery life and a simple battery recharging process for users.
This article discusses efficient approaches for extending battery life and simplifying battery recharging while supplying multiple supply rails for the biosensing, audio, and processor devices underlying these designs. It then briefly describes how wireless charging works before introducing standards-based wireless power solutions developers can use to rapidly implement sophisticated true wireless products, able to take full advantage of a rapidly expanding base of compatible third-party charging platforms. Solutions will be introduced from vendors such as Maxim Integrated, Analog Devices, STMicroelectronics, and Texas Instruments.
How to manage power in fitness hearables
As discussed in the first two articles in this series, engineers can draw on ultra-low-power system-on-chip (SoC) biosensor, audio, and Bluetooth devices to minimize power consumption and extend battery life (Figure 1).
Figure 1: For true wireless fitness hearable designs, highly integrated power management integrated circuits (PMICs) and fuel gauge ICs provide the foundation for battery and power management, requiring only a wireless power source for battery charging. (Image source: Digi-Key Electronics, based on source material from Maxim Integrated)
Along with their extensive functional capabilities, these SoCs integrate their own power management features that variously use low-power operating modes, clock or voltage gating capabilities, or internal voltage regulators for supplying different power domains from a single power source. Although these features help simplify the implementation of power optimized designs, they often bring requirements for several power rails suited to each device. For example, the SoCs discussed earlier in this series (Maxim Integrated MAXM86161 biosensor; MAX98090 audio codec; and ON Semiconductor’s RSL10 Bluetooth microcontroller) present diverse supply requirements (Table 1).
Table 1: Voltage supply ranges for the primary SoCs in a fitness hearable design. (Table source: Digi-Key Electronics, based on source material from Maxim Integrated and ON Semiconductor)
Rather than a set of individual voltage regulator devices, a multi-rail PMIC such as the Maxim Integrated MAX77654 offers a simpler single-chip solution. Designed specifically for space-constrained low-power applications such as hearables, the MAX77654 provides three buck-boost switching regulator outputs and two low-dropout (LDO) regulators in a 2.79 millimeter (mm) x 2.34 mm package with a low operating current of 6 microamps (mA) and shutdown current of 0.3 μA. Developers can program the MAX77654's three buck-boost regulators individually in 50 millivolt (mV) steps to deliver regulated outputs from 0.8 to 5.5 volts. Similarly, the two LDO regulator outputs can be programmed in 25 mV steps to deliver outputs ranging from 0.8 to 3.975 volts.
Based on a single-inductor multiple-output (SIMO) buck-boost regulator, the device helps reduce BOM and design footprint with its ability to provide a complete power management solution with only a few additional components (Figure 2).
Figure 2: The Maxim Integrated MAX77654 PMIC simplifies development with its ability to supply multiple programmable voltage rails with two LDOs and three buck-boost regulators that require only a single inductor thanks to the device's SIMO technology. (Image source: Maxim Integrated)
Within a complete system, the MAX77654's off-on controller and power sequencer manage the internal power state transitions and timing needed to bring up (or shut down) power rails according the specific sequence required for the application. In a fitness hearable design, for example, developers could program the device to apply power sequentially to individual SoCs and subsystems to reduce peak current demands or to avoid audio artifacts.
Besides its system power management capabilities, the MAX77654 integrates a complete lithium-ion battery charger that provides a programmable constant-current charge rate of 95 milliamps (mA) to 475 mA from a wide variety of sources including USB. Maxim's Smart Power Selector technology automatically switches power from the input power source (CHGIN) to the battery (BATT) and system (SYS) as needed. When charging is finished, the Smart Power Selector automatically disconnects the battery from the input source.
The MAX77654 offers an extensive set of status registers that allow developers to monitor and control every aspect of device operation. By setting interrupt control registers, developers can program the device to alert the host processor to a wide range of operating conditions and faults including system over and under voltage, temperature, charging errors, and battery faults.
For a consumer product, however, developers would typically combine the PMIC with a battery fuel gauge IC such as the Maxim Integrated MAX17260. Drawing only 5.1 mA, the MAX17260 uses Maxim's ModelGauge m5 battery age forecasting algorithm to provide dynamic estimates of remaining battery life during operation and charge completion time during charging. Developers can program the device to generate an interrupt to the host processor when the remaining state of charge falls below a specified threshold during operation. In a fitness hearable, developers could use this feature to gracefully degrade application features using strategies such as reducing heart rate update times from the biosensor or reducing audio bandwidth, eventually alerting the user before power falls below sustainable limits.
The combination of the MAX77654 PMIC and MAX17260 fuel gauge IC provides an effective solution for battery management. Providing a suitable charging source is the remaining significant challenge in creating a true wireless fitness hearable. By definition, that source cannot take advantage of conventional wired approaches using line power adaptors or USB. For this, the availability of wireless power technologies and associated silicon solutions offers a ready solution.
Practical wireless power methods take advantage of tightly coupled induction between a primary and secondary wire coil or loosely coupled resonant induction between a pair of coils operating at the same resonant frequency (see, "Inductive Versus Resonant Wireless Charging.")
Widely used for many years to recharge consumer products like electronic toothbrushes or medical products like hearing aids, inductive wireless power has achieved a level of maturity and device support that makes it a safe choice for even the most advanced electronic products. As a result, developers can, in principle, implement wireless power chargers with little more than an Analog Devices LTC4124 wireless Li-Ion charger receiving power from a coil coupled by induction to a transmitter coil driven by an Analog Devices LTC6990 voltage controlled oscillator (VCO). Along with the LTC4124 receiver and LTC6990 VCO, the complete wireless power supply design requires only a MOSFET, a few passive components, and a pair of coils such as the Würth Elektronik 760308101216 7.2 microhenry (µH) receiver (RX) coil and Würth Elektronik 760308103206 7.5 µH transmitter (TX) coil (Figure 3).
Figure 3: Using the Analog Devices LTC4124 wireless power receiver and Analog Devices LTC6990 voltage controlled oscillator, developers can implement a complete proprietary wireless power supply with only a few additional components. (Image source: Analog Devices)
Although resulting in simple designs, earlier wireless power solutions have become less suitable for consumer products as users have rapidly embraced standard wireless power offerings based on the Wireless Power Consortium (WPC) Qi specifications (see, "Qi Compliant Wireless Charging.") Such simple designs, like the one shown above, designed for proprietary wireless products and their associated charging bases, lack key capabilities such as communication between receiver and transmitter, foreign objection detection (FOD), and other requirements laid out in the WPC Qi specifications.
Besides enabling a more sophisticated wireless charging process, the rapid acceptance of Qi-compatible wireless power has fueled the rise of low-cost wireless power transmitter platforms. As a result, developers of consumer products like fitness hearables that require a wireless power source can largely focus on the design of a compatible wireless power receiver with the expectation that potential users have (and prefer to use) existing off-the-shelf wireless charging pads.
Taking advantage of commonly available wireless charging products does, however, require a fundamental shift in design perspective. To a rough approximation, efficient coupling and power transfer requires well-matched transmitter and receiver coils of similar size and with a ratio of secondary to primary coil inductance typically in the single digits. Consequently, using the very small diameter coil needed to fit in a fitness hearable would complicate design of a wireless power system able to meet user expectations for rapid charge times. In addition, the very tight tolerances for inter-coil alignment and positioning would necessitate a product design that would use a custom case or other mounting fixture to consistently place the earbud coil closely to a charging coil.
Because of these multiple challenges, true wireless earbud products typically take a more practical approach that builds a Qi-compatible wireless receiver into the earbud case. When the earbuds are placed into the case, pins built into each earbud meet power contacts built into the bed of the case. In turn, when the case is placed on a compatible third-party wireless charging pad, power flows wirelessly from the pad to the case receiver and from there through the contact points to the earbuds. Using this approach, implementing wireless charging for fitness hearables becomes a much more straightforward problem that is supported by a broad set of Qi-compatible wireless power receivers.
Wireless receiver solutions
Fortunately, developers can find a wide range of wireless power receivers designed specifically to support WPC Qi standards. In fact, available devices go well beyond the minimum requirements for supporting standard wireless power transfer and offer features designed to simplify overall system design. For example, as with many devices in this class, the STMicroelectronics STWLC03 wireless power receiver supports a simple approach for disabling wireless charging for designs that allow users to provide power to the charging case through an external power adaptor or USB connection (Figure 4).
Figure 4: As with other devices in this class, the STMicroelectronics STWLC03 wireless power receiver provides a simple option for disabling wireless power transfer when an external power source is detected. (Image source: STMicroelectronics)
Many Qi-compatible wireless power receivers also integrate battery charging capabilities, allowing developers to add batteries to the case for backup power when wireless charging is not available or convenient. For example, the Texas Instruments BQ51050B supports a three-step charge sequence including pre-charge, fast-charge constant current, and constant voltage, with only a simple connection to a battery pack (Figure 5).
Figure 5: The Texas Instruments BQ51050B wireless power receiver can support battery pack charging with minimal additional development effort. (Image source: Texas Instruments)
Along with support for external supplies and battery charging, Qi-compatible wireless power receivers can also support emerging peer-to-peer wireless charging scenarios that rely on one mobile product, such as a smartphone, to wirelessly charge another product. For example, the Maxim Integrated MAX77950 combines support for existing wireless power usage with support for peer-to-peer charging that requires minimal additional development effort (Figure 6).
Figure 6: Along with support for more conventional wireless charging configurations, the Maxim Integrated MAX77950 wireless power receiver supports peer-to-peer wireless power transfer. (Image source: Maxim Integrated)
Wireless power development support
Despite the continuing evolution of wireless power features and associated devices, developers can find a ready supply of development support resources including development boards, design guides, and application notes. For example, each of the wireless power devices mentioned in this article is available with an associated development kit.
For its LTC4124 wireless power receiver, Analog Devices offers a series of kits that provide both transmitter and receiver boards to demonstrate wireless power transfer at increasing levels of received charging current. The Analog Devices DC2769A-A-KIT and DC2769A-B-KIT kits demonstrate charging currents of 10 mA and 25 mA, respectively. Based largely on the LTC4124 design described earlier (see Figure 3), the transmitter board uses an Analog Devices LTC6990 VCO while the receiver board uses the Analog Devices LTC4124 wireless receiver. To demonstrate higher charger currents, the Analog Devices DC2770A-A-KIT and DC2770A-B-KIT feature 50 mA and 100 mA charging currents, respectively, with an LTC4124-based receiver board, but each kit's transmitter board is instead based on the Analog Devices LTC4125 wireless power transmitter.
For its devices, STMicroelectronics offers the STEVAL-ISB036V1 evaluation board for the STWLC03 wireless power receiver; Texas Instruments has the BQ51050BEVM evaluation board to support development for the BQ51050B wireless power receiver; and Maxim Integrated provides the MAX77950EVKIT evaluation kit for its MAX77950 wireless power receiver. Along with evaluation kit hardware, each manufacturer provides a full set of design resources typically including BOM, schematic, and physical design layout guide for developers building custom designs.
For software development, drivers and evaluation software are also typically available for immediate download or by request. For example, the Maxim Integrated MAX77950 Evaluation Kit Software package allows developers to monitor and modify MAX77950 registers and configuration via USB connection from their Windows® 10 computer to the MAX77950EVKIT, where an integrated microcontroller updates the MAX77950 through a shared I2C bus (Figure 7).
Figure 7: The Maxim Integrated MAX77950 Evaluation Kit Software package and associated documentation help walk developers through different MAX77950 device settings to explore the effect of different device configurations on wireless power performance. (Image source: Maxim Integrated)
Designs for true wireless fitness hearables present designers with challenges to implement ever more efficient systems, while also encouraging the use of advanced wireless charging technology. As shown, highly integrated PMICs and fuel gauge ICs provide an effective solution for power and battery management. For wireless power, the availability of standards-based wireless power devices offers developers multiple options for implementing wireless charging features in fitness hearable products. Using these standard solutions, developers can rapidly implement sophisticated true wireless products able to take full advantage of a rapidly expanding base of compatible third-party wireless charging platforms.
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