If you're designing true-wireless earbuds (Figure 1), there are some key features they need to have:
- Small size
- Fast and efficient battery charging
Figure 1. True-wireless earbud
Let's look at each of these closely related features in more detail to understand why they are important and find out what can get in your way of making them a reality in your earbud designs.
Ears come in all shapes and sizes, but even the person with the smallest ears needs to find earbuds that are comfortable to wear. It's great that "true-wireless" earbuds don't have any wires, but the electronics to make this possible have to go somewhere. It might be stating the obvious, but you need to seize on every opportunity to save space by reducing the number of components you use, to make the enclosure as small as possible. It might sound clever to use a wireless connection to allow your earbuds to communicate with their charging case, but this requires an extra IC that takes up space (and also needs power)—exactly what we are trying to avoid. With this in mind, let's move on and look at the next important feature.
When not in use, wireless earbuds are usually placed back in their charging dock. Many earbuds use three (or more) pins to connect to the dock (Figure 2).
Figure 2. Typical 3-pin earbud interface.
However, more pins mean more points of failure, not only during the manufacturing process, but also in the field (or "real life," as the customer likes to call it). Remember, each earbud will be taken out of a person's ear and replaced in its dock, typically several times a day (maybe thousands of times in a lifetime). And it won't always be shown much love each time this happens, being forced to endure all types of harsh treatment and sometimes unpleasant conditions along the way! When designing wireless earbuds, you should always be looking for ways to minimize the number of pins at the earbud/charger interface (ideally only two) to make them less likely to break.
Fast and Efficient Battery Charging
The one downside of a true-wireless earbud (compared to its wired earphone cousin) is that it has a battery. Batteries, as we all know too well, go flat and when this happens, the music stops - literally! Getting up and going again depends on how quickly the battery can be recharged – clearly, the faster the better, but we need to be careful here. In the world of the engineer, efficiency is synonymous with "heat" and if earbuds are not charged efficiently, they can get hot very quickly. Apart from the fact that heat is not very good for battery life, nobody wants to put hot earbuds in their ears. What you are looking for is controlled charging that allows the battery to recharge quickly, while keeping the earbuds cool. But this has implications for our previous point – controlled charging requires communication between the earbud and its charger, which is why it has been common to use three (or more) pins at the earbud/charger interface.
So, we now find ourselves at the point where, if we want controlled battery charging, we need a charging interface with more than two pins, but this will reduce reliability. One way around this would be to use wireless communication between the earbud and charger, but, as we said before, this would increase the size of the earbud and cause the battery to drain quicker. There appears to be no optimal solution…or is there?
The circuit arrangement in Figure 3 uses a combination of two ICs to overcome these problems.
A more efficient way for an earbud to communicate with its charging case is to combine data and power transfer into a single wired channel, effectively superimposing the data signal onto the power. This is known as "powerline communication" (similar to the way in which electrical outlets can be used to extend wired network communications). MAX20340 implements a novel variation of this technology by providing a bi-directional DC powerline communication interface that is suitable for use in applications where space is limited. Using this IC, the number of pins at the interface can be reduced to two, the ideal solution, allowing bidirectional data transfer at rates up to 166.7kbps. A master IC is located in the charging case with addressable slave ICs located in each earbud.
Figure 3. Diagram showing communication IC and buck-boost converter for controlled earbud battery charging.
As we've already said, to prevent heating, earbud battery recharging needs to be done as efficiently as possible. By looking closely at the process, we can find a source of power loss that can go unnoticed. The lithium-ion battery in the charging case (typically 3.7V) is normally boosted to 5V using a DC-DC converter IC. This voltage is then used by a linear charger in the earbud for battery charging. However, even as the earbud battery voltage rises during charging, it always remains lower than 5V. The excess voltage causes power to be wasted in the form of heat. To prevent this, the voltage difference between the input to the linear charger (supplied by the boost converter) and the battery voltage should be continuously minimized as the battery voltage rises during charging. The MAX20343 shown in Figure 3 above is a buck-boost converter which can manage this, using a technique called dynamic voltage scaling (or DVS). At regular intervals, the MAX20340 queries the earbud battery voltage and gives this information to the case-side microcontroller. The micro then adjusts the output voltage of the MAX20343 so that it matches the earbud battery voltage plus the additional overhead required by the linear charger. This has the advantage of minimizing energy waste in the case-side battery, while also reducing heating in the earbud.
So, there you have it—as they say—problems are there to be solved and the MAX20340 and MAX20343 have allowed us to solve the challenges around designing small, reliable earbuds that can charge quickly with minimal heating.