Bench Talk

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Numerous wearable technology products have been introduced in recent years. Hearables are becoming one of the most popular subsets of wearable technology. Of this subset, wireless earbuds are extremely popular because of the mobility and freedom from wires they provided. Earbuds are big in three areas—cellphone interface for music and podcasts, hearing aids, and personal sound amplification products (PSAPs), which are similar to hearing aids but without medical certification. All three often provide adjustable frequency response and active background noise cancellation. Some earbuds also have 4GB of Flash for offline playback. All use a battery in each earbud that requires recharging.

Although plenty of wired earbuds are in use, the true wireless stereo (TWS) variety is where earbuds are headed. In the third quarter of 2018, vendors shipped 15 million TWS earbuds globally, according to Statista. By the third quarter of 2019, this figure reached 42.5 million. The continued success of the rechargeable hearable market will depend on integrating these small portable devices with a reliable and efficient power management system. Here, we will discuss some of the most promising applications for earbud technology and explore a power management solution for portable devices that simplifies the connection between charger and hearable by managing power and data transfer.

Your Ears are Taking on a Whole New Theater of Options

Hearables can measure the user's pulse rate—and those with microphones—can be used to give voice commands to computers and smart devices that incorporate digital assistants like Google Assistant and Amazon Alexa. Hearables are also capable of integrating white and pink noise generators, used to drown out annoying background noise to enhance concentration, relaxation, or sleep. These applications are likely to expand the market for earbuds significantly.

Rechargeable Design

TWS wireless hearable devices must recharge their batteries in their supplied cases, usually every four to eight hours. Their charging regimen is critical to their market success. The cases also have a battery so the user can recharge the hearables on the go. The charging case takes about 1.5 hours to charge from a USB port. 

Most hearable devices use a three-pin connector in their charging case to supply 5V_DC and a communication channel. This communications channel is used to monitor batteries in both the charging case and hearables. This link is also used for firmware updates. Some models also have a fourth pin to detect when each earbud has been placed in its case.

A Better Way: The MAX20340 Power Management IC

Given the size of hearable devices, having three or four contacts for the interface is a problem. An improved method uses only two pins with the communications link combined on the power pins. The MAX20340 bidirectional DC powerline communication management IC combines power and data transfer into a single channel, meaning only two pins are required for interface with its charging case (Figure 1).

Figure 1: The MAX20340 comes in a very small 9-bump, 1.358mm square WLP package. (Source: Mouser Electronics)

An ideal solution to this problem, the MAX20340, provides a bidirectional 166.7kbps bit rate I²C link and power management for up to 1.2A of charge current. The device is used in both the charging case (master) and the hearing device (slave).

MAX20340 also has automatic slave detection, which means that neither a Hall-effect sensor nor a dedicated pin is required for the charging case to recognize that a hearable device has been docked. The chip includes high ESD protection, removing the need for additional TVS diodes. A master IC is in the charging case with addressable slave ICs located within each earbud, as shown in the system diagram (Figure 2).

Optimized Charging

The hearable’s charging cases will typically contain two sources of power: a battery and a USB connector. An external buck-boost converter regulates the selected source to ensure it is stable. Figure 2 shows a diagram of a hearable charging system using the Maxim MAX20340 Power Management IC.

Figure 2: Diagram of an earbud charging system using the MAX20340. The diagram shows the master IC located in the charging case and detectable slave ICs in each earbud. The charging station also includes a USB, power-path charger, and buck-boost converter. (Source: Maxim Integrated)

The MAX20343 ultra-low quiescent current, 3.5W, buck-boost regulator can be used with the MAX20340 to reduce power dissipation in hearable devices. The MAX20340 intermittently queries the two earbud battery voltages and provides this information to the case-side microcontroller. The MCU then adjusts the output voltage of the buck-boost converter so it matches the earbud battery voltage plus overhead required by the linear charger. This has the advantage of minimizing energy losses of the case-side battery and significantly reducing the heat dissipated in the earbud. This optimization also means the earbuds can be charged at a faster rate. This MAX20343 is available in a choice of 16-bump, 1.77mm x 2.01mm, 0.4mm pitch WLP or a 12-pin, 2.50mm x 2.50mm, 0.5mm pitch FC2QFN packages. A MAX20343 evaluation kit is available, the MAX20340EVKIT#.

Hearables are a popular subset of the wearables market. True wireless earbuds are rapidly gaining popularity. Who needs that tangle of wires around? A certain elegance is necessary for a good wireless earbud design. The tiny devices are, of course, size limited, but power management for their batteries is crucial. A synergy between the earbud case and the two devices is essential. The MAX20340 can provide that synergy and, combined with the MAX20343 buck-boost regulator, offer the best possible power management.