Why current and magnetic sensors are crucial to the design of TWS (true wireless earphones)

In recent years, TWS (True Wireless Stereo) has been rapidly rising in the headphone market. Now, users no longer have to worry about the tangled headphone wires when using streaming devices. True wireless headphones are wireless headphones based on Bluetooth®, with the left and right channels separated into two independent and paired individuals. Although this innovative design eliminates the need for users to connect their phones or other devices with wires, it brings a series of new design challenges to headphone manufacturers.

To maximize battery life and battery runtime, the earphones must be properly positioned in the charging case and be efficiently charged while charging. A cost-effective approach is to use a current sensing amplifier to monitor the charging of the earbuds, and to use Hall effect switches for the opening and closing of the wireless charging case and the placement of the earbuds to maximize the battery charging efficiency and battery life of this application scenario and improve the user experience.

Designing with Current Sense Amplifiers

The battery capacity of TWS is usually less than 100-mAh. Therefore, in order to protect and accurately charge these small capacity batteries, we need more accurate current measurement. Traditional battery chargers and fuel gauges are good at monitoring the current of larger batteries (such as the batteries in charging boxes), but often do not perform well at monitoring lower currents.

Dedicated current sense amplifiers are more accurate when measuring small currents. If you already have a microcontroller (MCU) or power management integrated circuit (PMIC) in your design, you can use the output of these amplifiers to monitor and measure battery usage and battery life based on algorithms written to the MCU or PMIC. Figure 1 shows a fuel gauge with an external current sense amplifier and controller.

Figure 1: Fuel gauge with external current sense amplifier and controller measurement

Placing two small current sense amplifiers (such as INA216) in the wireless earbud charging case enables high-precision charging current measurement. Alternatively, if solution size is a priority, a single dual-channel current sense amplifier like INA2180 is recommended.

If accuracy is not an important consideration, and assuming equal current sharing, a single current sensor can monitor the charging of both earbuds. Placing a bidirectional current-sense amplifier, such as the INA191 or INA210, in the earbuds allows for both charging and measurement functions. Regardless of which topology you use, these devices can better protect the battery, since even small current changes can affect battery life.

Designing with Hall Effect Sensors

The new features of TWS are innovations around charging and listening. The opening and closing of the charging box cover can be used to turn the Bluetooth connection on or off, while the in/out detection can determine whether to stop charging and pair the left and right ears. Other sensor technologies may not be able to cost-effectively and correctly implement these functions with the right sensitivity, so choosing the right sensor is crucial. Figure 2 shows the arrangement of TWS sensors.

Figure 2: Wireless earbud sensor placement and usage

Hall effect sensors are a good candidate for detecting the lid of the charging case and the charging of the earbuds. Since magnets are already used to close the lid of the charging case, using a magnetic sensing solution in the form of a Hall effect switch to detect the lid is an obvious solution to connect/disconnect Bluetooth without the need for additional parts. Additionally, placing magnets in the earbuds is an effective way to detect the presence of the earbuds in the charging case and thus charge them.

Choosing the right digital Hall effect sensor is important, and the low frequency/low power consumption makes the DRV5032 the perfect choice. For the Hall effect sensor in the earbuds, it is perfectly possible to provide magnet detection information 5 times per second. This frequency allows you to use the low power option of the DRV5032, which consumes only about 0.5µA of quiescent current, without significantly affecting battery life.

Determining the state of charge and charging case lid detection are both critical for earbuds that use small capacity batteries and wireless connectivity. Current sense amplifiers and Hall effect sensors provide solutions for those struggling to design around these new features and challenges.