Introduction to TWS Bluetooth headsets and TI low power solutions

TWS (True Wireless Stereo) Bluetooth headsets are extremely popular audio products in recent years. With the help of Bluetooth chips, it first establishes a wireless connection between the mobile phone and the main headset, and then establishes wireless communication between the main headset and the auxiliary headset, thus completely abandoning the wire connection between traditional headsets, which greatly facilitates the use of users. In addition, the main headset can be used alone, which is fully capable of meeting the application requirements of single Bluetooth headsets on the current market, and has very powerful functions. Therefore, since Apple released the first TWS headset, Airpods, in September 2016, the market response has been very enthusiastic. Subsequent audio manufacturers quickly followed up and crowded into the layout of TWS Bluetooth headsets, making the TWS headset market colorful. Next, Bluetooth 5 will bring a more exciting user experience, and the new charging box design will make consumers more convenient.

Lightweight and easy to carry are the most important design goals of TWS headphones. Limited by the small space of the charging box and the headphones, the battery capacity of these two parts cannot be increased. The capacity of the charging box is generally within 1000mAh (200-700mAh is the most common), and the capacity of the headphone end is even smaller, most of which are less than 100mAh. Therefore, whether it is a charging box or a headset, the low power consumption design of the system should be emphasized to ensure that the product has a long service life.

1. Introduction to the charging box system
The detailed charging box system block diagram is as follows:

Figure 1 TWS charging box system block diagram

In the signal chain, the sensors mainly include Hall sensors to detect the opening and closing of the box. The LED lights provide a cool display effect, and the Bluetooth chip can transmit the box information to the mobile phone, so that the mobile phone can check the box power information. Key detection may require some logic devices, such as the D flip-flop SN74LVC1G74, which can convert the pulse edge of the key into a level flip, so that the MCU can record the key information.

For the power rail, the input port is generally made into a 5V micro USB interface (Apple's Airpods are lightning interfaces, also 5V). Considering that there are many adapters that support high-voltage fast charging, the charging box needs an overvoltage protection chip for mis-insertion protection, and a charger to charge the lithium battery. At present, many chargers have integrated overvoltage protection functions, but the overvoltage response time is mostly at the us level. It is recommended to add an additional overvoltage protection chip for fast protection. For the charger, it is recommended to use a charger with power-path (i.e. path management). On the one hand, when the battery power of the box is low, plugging in the adapter can immediately get a higher system voltage to ensure that the box can immediately power the low-power headphones; on the other hand, when the fast charging current is set to a small value, if the load needs to be constantly loaded (such as driving an LED light), this part of the load is likely to be near the cut-off current of the charger. Without the power-path function, the charger may not be able to determine that the battery is fully charged, and the user experience will be worse.

The battery is usually a single-cell lithium battery, which is usually provided by the battery pack manufacturer, and the fuel gauge and secondary protection IC are already packaged in it to ensure more reliable operation of the battery. The power supply of a single-cell lithium battery is mainly supplied to two parts: one part is boosted to 5V to power the headset, and the other part is stepped down to 3V or below to power the MCU/Sensor in the box.

TI has a wide range of products that can cover most of the needs of charging boxes. Here are a few ICs that are perfect for use in charging boxes. The recommended indicators are mainly packaging and power consumption. The power consumption is mainly the static current of the IC when it is enabled. It should be mentioned that since the overvoltage protection and charger only work when charging, the adapter is external and the power supply is sufficient, so these two parts do not require a small Iq, but the leakage current of the charger should be taken seriously.

Table 1 Summary of TI charging box solutions

2. Headphone system introduction

TWS generally has two earphones, and their systems are the same. The detailed block diagram is as follows:

Figure 2 Headphone system block diagram

In the signal chain, the Bluetooth chip is responsible for receiving data sent by the mobile phone, and then driving the headphones through the headphone amplifier. The sensor is mainly a gravity sensor, which detects signals such as the shaking of the headphones. Currently, the integration of Bluetooth chips is very high, and they can also perform some tuning functions.

As for the power rail, due to the very small earphone space, the earphone end can no longer be made into a power supply port for the micro USB interface, and is usually changed to a specific metal contact piece. The earphone input power comes from the 5V after the charging box is boosted and stabilized, so there is no risk of overvoltage at the earphone input, and there is no need to add overvoltage protection. A single lithium battery can be charged directly through the charger. Similarly, the battery is also provided by the battery pack manufacturer, with an integrated fuel meter and secondary protection. The battery provides 2.5V or 1.8V power to the system through the LDO.

Compared with the design of the charging box, the earphone end has a smaller space, which puts forward more stringent requirements on the chip packaging and power consumption. In view of this situation, TI's earphone end solution is summarized as follows:

Table 2 Summary of TI headphone solutions

3. TI reference design

TI has a reference design for TWS earphones, TIDA-050007. The following figure is the system block diagram of the reference design. The power consumption of this reference design is only 18uA, and a simple algorithm is used to achieve dynamic regulation of boost. When the battery voltage of the charging box is higher than the battery voltage of the earphone, the boost chip TPS61099 can also enter the pass-through mode to further improve the charging efficiency.

Figure 3 TIDA-050007 reference design system block diagram

For headphone applications, the TI website also has a wealth of headphone design solutions for reference, including product block diagrams, reference designs, popular products, and more.

Figure 4 TI online resource wireless headset system block diagram

In summary, the design of TWS headphones needs to comprehensively consider power consumption, packaging, and performance to give the product a better experience. With the help of TI's rich resources, you can easily realize the design of your product.