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Integration of power:communication interfaces in smart true wireless headset designs

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March 12, 2019

Horst GetherMartinDenda,-March 12, 2019

True wireless headsets (TWS headsets) continue to become more attractive due to an increase in battery life, enhanced features, attractive design and better price points.  With headset manufacturers focusing on miniaturization and design improvements and quickly adopting features to enhance user experience they are able to attract the most demanding consumers in a strong and competitive market.

Looking at these new in-ear systems, at first glance they look like rather simple devices. On the contrary, TWS systems require a lot of electronics to be smart and user friendly, as illustrated in the high-level system overview in Figure 1.

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Figure 1: System Overview (Source: ams AG)

When thinking about TWS headsets in daily use, there are several ways for enhancing user experience and hassle-free user interface integration. A key issue that TWS system designers face is playtime duration, as battery space can be particularly limited in TWS headphones. Typical battery sizes of 25-80mA/h can be reached, which results in a playtime of 2-4 hours. Once the battery is empty, the earbud needs a fresh charge before it is ready for its next use.

Currently, the most advanced TWS headphones are shipped with a charger cradle for charging the batteries, rather than having a wire connected to each of the earpieces. The cradle includes a bigger battery and acts as handy compartment – as it is easy to lose the tiny earphones. This allows the user to charge the earbuds on-the-go without being dependent on a power outlet. The goal of this cradle/earbud configuration is to guarantee fully-loaded batteries at all times. This avoids the frustration of realizing at the beginning of a workout that your earbuds are not ready to use because the batteries are empty! Another aspect to enhance user experience is automatic start-up and pairing of the earbuds. The user doesn’t want to wait for the devices to pair or start when the earbuds have been inserted in the ear. It should be a seamless process without pressing any buttons to begin the pairing.

To make a standard TWS headset smart and user friendly, a key requirement is data exchange between the charger cradle and earbuds.

If the cradle senses the battery status of the earbuds, it can automatically start re-charging the earbuds. This continuous re-charging process is necessary due to the quiescent current consumption caused by the always-on Microcontroller Unit (MCU), as shown in Figure 2. Conversely, if the earbud senses an empty charging cradle, it can automatically inform the user via a Bluetooth notification to charge the cradle’s battery.

In terms of automatic start up and pairing, a smart connection would also be beneficial. If the cradle informs the earpiece that the compartment lid has been opened, the earbuds wake from their sleep mode and prepare the BT pairing process without the need to press a button on the earbuds to enable them.

As well as the enhanced user experience, a link between cradle and earbud can enable better industrial design, software updates, personalization of earbuds (name, EQ data) and transfer of music data to the earbuds, to name just a few application examples of a feature-rich and differentiating product in the market.

To get a clearer picture of the technical implementation, let’s dig deeper and have a look at the system in more detail, as shown in Figure 2.

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Figure 2: Detailed block diagram (Source: ams AG)

On the cradle side, the most important thing is, of course, the Li-Ion Battery and accompanying charger to enable the charging of its battery with the help of a standard 5V supply connected to a USB outlet. Power management blocks – like LDOs and DCDC converters – distribute the required supply voltages to the MCU and other devices placed in the cradle. A dedicated 5V supply is mandatory to provide power to the earbuds for charging their batteries. The always-on MCU acts as the central control unit of the cradle and is usually connected to several other sensors (lid detection, earbud detection), as well as to the charger to receive cradle-battery-status updates.

After a trigger event – such as opening the lid, inserting an earphone, or a request sent by the earphone – it exchanges the required information or sends commands/ data to the earbuds.

On the earbud side, the topology is basically very similar, but of course the Bluetooth SOC is additionally required. The MCU in the earbud directly communicates with the MCU on the cradle side, exchanging information back and forth.

Sensor-wise, there may be additional devices such as proximity sensors for ear-insertion detection, accelerator sensors, hear rate sensors for fitness devices, temperature sensors, and touch sensors.

As shown in Figure 2, multiple pins are needed to implement the smart functionality of the charger cradle and the earpieces. This fact comes with several disadvantages: to reach a high customer-acceptance level, TWS solutions can’t be dramatically larger than their wired competitors. So, the placement of additional poles on the earbud always leads to a compromise between space and features. In addition, the design and appearance are also affected negatively if several poles need to be placed on the earpiece. Certainly, one option is implementing a BLE (Bluetooth Low Energy Connection) link, however this would significantly affect the bill of material cost and increase software implementation effort.

A more elegant compromise is to enhance the features of the standard and mandatory two poles which are used for charging (GND and 5V) the earbuds. If the functionality of the two-wire connection is extended to allow for charging and simultaneous communication to the earpiece in parallel, all the smart and user-friendly features could be realized without drawbacks in physical space or design expectations. The resulting user experience can be further improved with a dedicated app, which benefits from the load of information the earbud is now able to provide to any smart device. A few examples are shown in the list below.

  • Battery status left

  • Battery status right

  • Cradle battery status

  • Pairing status

  • Name of left and right earbud or if it matches

  • Temperature

  • Check for SW updates for charger cradle

  • Notification info about empty cradle battery (especially with altered battery)

To incorporate the functions listed above, a few modifications to Figure 2 are necessary that however make the system slightly more complicated. We need to find a way to utilize the 5 Volt power supply signal line to be able to transfer power as well as data over a single wire.

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Figure 3: Proposal for data communication over 5V power line (Source: ams AG)

A possible and simplified timing diagram is shown in Figure 3, which shows the 5V power signal line with data modulated directly to the signal line. The host side provides the 5V supply to be transferred to the client to charge a battery and the client could potentially modulate data which can be transferred to the host side. In ideal case semi-duplex communication can also be implemented where client and master share the single power line to alternatively modulate data, so as to be able to exchange data between the cradle and earbud.

For a proper implementation of such a single-wire-communication principle, various new system blocks are necessary to replace the two-serial-communication interface signal lines shown in Figure 2. On the host side, the easiest way to implement such a modulation principle is with a coil to reject high-frequency modulation content and a modulation resistor to modulate a voltage drop to the 5V power supply signal line. In addition to the coil, there is also a data modulator which could be implemented by a simple current sink.

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