What kind of "charger” will improve the battery life of TWS headsets?

According to the latest data released by IDC, global wearable device shipments are expected to reach 396 million units in 2020, an increase of 14.5% compared to 2019. In the wearable market segment, hearables account for nearly 60% of total shipments – an impressive performance indeed.

Figure 1: Analysis of global wearable market shipments (Source: IDC)

One of the key factors driving the demand for hearables is the rise of TWS true wireless headsets. With the launch of AirPods in 2016, Apple has achieved huge market success in an extremely short period of time. The familiar daily sight of commuters wearing white "tadpoles” in their ears as they travel to and from work attests the extent of Apple’s market penetration.

The enormous popularity of AirPods inspired other manufacturers to follow suit, ultimately contributing to the overall prosperity of the TWS market. According to IDC’s earlier market analysis, TWS headsets have maintained a 100% growth rate in the past few years. Shipments reached 120 million units in 2019, and are expected to exceed 200 million units by the end of 2020.

Figure 2: Global TWS headset market trends and market share of each manufacturer (Source: IDC)

However, the market analysis clearly shows that despite being the presence of "wolves” hungry to make a killing of their own, Apple still dominates the TWS market with a share of more than 40%. Though this can be partly attributed to its marketing strategy, the fact is that there are very few commercially available products that can rival the Apple AirPods. Many manufacturers, including some leaders in the field of Bluetooth headsets, have faltered in the design and production of their first-generation TWS products, confirming that TWS is still a relatively advanced technology with a high application threshold. With industry benchmarks like AirPods blazing the trail, latecomers to the game face considerable hurdles.

Meanwhile, end-users have come to expect more than ever before – sound quality cannot be too “bland”, and audio lag must be as short as possible so that gaming is uninterrupted. Noise reduction, moreover, whether passive or active, must meet high expectations. And let’s not forget the most critical indicator – the battery life of TWS, which ideally should last indefinitely.

In all honesty, it is already challenging to squeeze the master Bluetooth chip, audio decoder, speaker, microphone, memory, sensor and other components into a headset weighing only a few grams. Larger batteries with greater capacities are even more difficult to accommodate, making TWS headsets generally inferior to traditional Bluetooth headsets in terms of battery life. However, with the unveiling of AirPods, Apple has presented us with a reasonable solution for this, which is to use a charging case to store the headset so that the headset can be charged when not in use. This ensures that it is fully charged every time you take it out of the case, significantly prolonging the cumulative battery life of the headset. While the total battery life of first-generation TWS is generally 10-20 hours, through the use of charging bins the latest TWS can last for up to 25-30 hou

Compared with earphones that are small and extremely limited in space, the design of TWS chargers may seem much easier. The challenges, however, are greater than imagined, beginning with these two major ones:

• Smaller size: As a portable device, the size of the TWS charger is also extremely limited. Going by current trends, new functions such as wireless charging will continue to be added to the charger, making space utilization equally important.
• Higher efficiency: Low efficiency may cause the headset to heat up during the charging process. This will affect the charging speed while causing more energy loss and negatively impacting total battery life.

The solution to these problems requires the assistance of specially optimized products. The good news is that there are products and solutions that have been specifically designed to address these issues, and they are already available on the market.

For example, Maxim Integrated just recently proposed a novel solution that enables simplifying the earphone charging circuit and compressing system space. 

In the traditional TWS charging process, three or more pins are required, two of which are used for charging while the rest are used to establish a communication (or data) channel between the charging box and the headset to track the charging status of the headset battery and control the charging process, or to upgrade the device firmware and/or for factory debugging. In some solutions, a dedicated (pogo) pin is also used to detect whether the headset is placed in the charging box. But more pins mean increased circuit wiring complexity, which poses more risks in terms of reliability.

Figure 3: The typical charging case features an earphone charging circuit that requires at least three pins (Image source: Maxim)

To address this issue, Maxim Integrated has proposed a "power line communication" solution with the introduction of the MAX20340 DC power line communication management IC. This solution combines data and power transmission into a single channel, superimposing the data signal on the power supply so that only two pins are necessary. These pins can realize power transmission (maximum charging current is 1.2A) and bidirectional communication (rate up to 166.7kbp) at the same time, thus simplifying the entire charging system.

Figure 4: MAX20340 only needs two pins to realize data and power transmission between the headset and the charging case (Image source: Maxim)

Maxim Integrated proposed yet another unique solution to improve the headset charging efficiency of charging cases.

In a typical charging solution, the lithium-ion battery in the charging compartment will charge the TWS headset with a voltage of 5V. Even if the headset battery voltage gradually increases with the charging process, the output voltage of the charging compartment – which is the input voltage of the headset’s linear charger – will remain constant at 5V. This high voltage will cause more power to be dissipated as heat, undermining the overall efficiency.

Maxim Integrated uses dynamic voltage regulation (DVS) technology to solve this problem with the newly launched MAX20343 boost/buck converter. During the charging process, the output voltage output of the charging case (input voltage of the headset’s linear charger) changes with the difference in voltage of the battery detected by the boost converter, thereby reducing energy loss and improving efficiency.

Figure 5: MAX20343 boost/buck converter uses DVS technology to improve charging efficiency (Image source: Maxim)

The combination of the above-mentioned MAX20340 and MAX20343 constitutes a TWS charging case solution characterized by miniaturization and high efficiency. MAX20340 intermittently queries the headset battery voltage and provides this information to the microcontroller in the charging compartment, after which the microcontroller adjusts the output voltage of the MAX20343 to match the headset battery voltage plus the additional margin required by the linear charger. This minimizes the energy waste of the battery in the charging compartment and avoids overheating when the headset is charging, thereby accelerating the charging rate.

Figure 6: TWS charging case solution realized by combining MAX20340 and MAX20343 (Image source: Maxim)

Like other battery-powered and wirelessly-connected products in the IoT era, TWS also faces severe power management challenges. Just as an electric car needs charging stations for replenishing its power en route to its destination, TWS charging cases are integral to extending the battery life of hearables. Fortunately, innovative technology is continuously extending the battery life of TWS, which will drive the ongoing development of the entire TWS market and give rise to new product concepts for next-generation hearables.