How to Design True Wireless Earbuds Using Hall Effect Sensors and Current Sense Amplifiers

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How to Design True Wireless Earbuds Using Hall Effect Sensors and Current Sense Amplifiers [Copy link]

TWS (True Wireless Stereo) is rapidly gaining ground in the headphone market. Now, users no longer have to worry about the tangled wires when using streaming devices. True wireless headphones are Bluetooth-based wireless headphones with 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.
　　In order to maximize battery life and battery run time, the headphones must ensure the correct position in the charging case and can charge efficiently when charging. A cost-effective approach is to use current sensing amplifiers to monitor earbud charging, and 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.
　

　　Battery capacities for TWS are typically less than 100-mAh. Therefore, to protect and accurately charge these small batteries, more accurate current measurements are needed. Traditional battery chargers and fuel gauges excel at monitoring currents for larger batteries, such as those in a charging case, but often fare poorly at monitoring lower currents.
　　Dedicated current-sense amplifiers are more accurate at measuring small currents. If you already have a microcontroller (MCU) or power management integrated circuit (PMIC) in your design, you can use the outputs 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.
　　How to Use Hall Effect Sensors and Current-Sense Amplifiers for True Wireless Earbud Designs Figure 1: Fuel Gauge with External Current-Sense Amplifier and Controller Measurements Placing two small current-sense amplifiers, such as the INA216, in the wireless earbud charging case enables high-precision charging current measurements. Alternatively, if solution size is a priority, a single dual-channel current-sense amplifier, such as the 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.
　

　　The new features of TWS are innovations around charging and charging. The opening and closing of the charging box lid 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.
　　How to use Hall effect sensors and current detection amplifiers for true wireless earphones design Figure 2: Arrangement and use of wireless earbud sensors
　　Hall effect sensors are well suited for detecting the charging box lid and earbud charging. Since magnets are used to close the charging box lid, 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 additional parts. In addition, placing a magnet in the earbud is an effective way to detect whether the earbud is in the charging box and thus charge it effectively.
　　Choosing the right digital Hall effect sensor is very important, and the low frequency/low power consumption characteristics make the DRV5032 the only choice. For the application of Hall effect sensors in earbuds, it is fully 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 0.5A of quiescent current without significantly affecting battery life.
　　Determining the state of charge and charging case cover detection are 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.