Wearable Electronics
How to design a safe wearable device with 4× faster battery charging
By Brandon Hurst, field applications engineer, Analog Devices T
his article demonstrates a reference design emulating the power architecture for a true wireless stereo (TWS) earbud application. It will
show designers how they can improve the fast-charging speed of their application by nearly 4× while optimising solution size and system BOM cost. Test results showing lower temperatures compared with a traditional solution are presented using thermistor and thermal imaging measurements. The design demonstrates numerous benefits offered by a solution with a single-inductor, multiple output (SIMO) architecture with autonomous headroom tracking.
Introduction
As the revolution in wearable devices continues, the demand for solid power architectures continues to increase. We’ve seen a large amount of growth of wearable health monitoring devices over the past decade, and the next generation of these devices is likely to include more features packed into the same tiny solution size. Typical requirements for wearable devices may include Wi-Fi, Bluetooth, and vital sign monitoring (VSM). The need for more features requires system-level and IC-level designers to get increasingly clever about their choices for powering wearable devices.
Power challenges for wireless earbuds
A true wireless stereo earbud application currently requires several separate regulators fitting into a small solution size - after all, the entire system needs to fit in our pocket!
The typical power system for a TWS 44 July/August 2024
Figure 1a. A diagram of a typical power architecture for a TWS earbud application.
Figure 1b. A diagram of a typical power architecture for a TWS earbud application with PLC.
earbuds application is shown in Figure 1a. A DC-to-DC converter between the cradle and earbud is used to elevate the voltage to 5V USB levels from VSYS. This provides enough headroom to keep the earbud’s linear charger out of dropout. However, a drawback of this solution is large efficiency loss due to the dropout voltage and losses across the linear charging FET on the earbuds. This is especially true when the earbud battery is at low state-of-charge. Charging at low efficiency increases thermal
Components in Electronics
dissipation, which lowers the system battery life and product reliability. In some cases (Figure 1b), additional power line communications (PLCs) are added to increase the efficiency of the system by tracking the linear charger’s headroom with the assistance of a buck- boost regulator on the cradle side. However, the solution size for wearable products comes at a premium. The PLC chips and the inductors required by the buck outputs supplying the wearable
device’s peripherals directly impact the size and the cost of the product for both of these traditional solutions.
A better solution: SIMO architectures and autonomous headroom tracking
SIMO power management ICs (PMICs) provide the architecture and efficiency required to satisfy compact design requirements. Battery-powered wearable Continues on page 46
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