Bench Talk

Micro speakers are now commonly found in a variety of consumer devices, such as gaming devices/accessories, smart home IoT, and wearables. Their basic components are similar to those found in traditional speakers: a diaphragm, voice coil, and magnet. However, they are much smaller and simpler in construction to offer a smaller, thinner form factor.

Since these speakers are so small, they are very limited with their loudness (sound pressure level) and overall bass response. The smaller the box, the higher the resonant frequency, which leads to reduced bass response and a very tinny sound. However, if an amplifier can continuously monitor and protect the speaker against the conditions by which it is most likely to fail, it can significantly increase the loudness and bass response of the same speaker. 

There are two key conditions that speakers are most likely to fail under, which define a very conservative power rating for the speakers since standard amplifiers cannot monitor and protect against these conditions:

• Condition 1: Exceeding Voice Coil Thermal Limit
  • Voice Coil Thermal Limit - how hot the voice coil can get before parts of the micro speaker melt.
• Condition 2: Exceeding Mechanical Excursion Limit
  • Mechanical Excursion Limit - how far the diaphragm can move before mechanical breakage occurs.

Therefore, driving speakers to their thermal and mechanical limits to increase loudness and bass response requires speaker protection.

For proper speaker protection, the amplifier that boosts the loudness of audio signals must know the speaker’s characteristics, such as the resonant frequency within its enclosure, excursion limit, and voice coil thermal limit. Typically, a time-consuming, complex speaker characterization effort is needed to gather this data and then significant time to customize the programming of the amplifier for said specific speaker. Both of these steps will need to be redone if there are any changes to the speaker or the enclosure.

These days, consumers don’t want to sacrifice audio performance for form factor—they want both. Fortunately, Maxim’s patented Dynamic Speaker Management (DSM™) technology provides a way to safely drive micro speakers beyond their specified maximum power rating. The DSM algorithm delivers up to 2.5x louder sound (Sound Pressure Level) and extends the bass response up to two whole octaves below the resonant frequency, all with industry-leading power consumption. Employing Maxim’s proven thermal protection, which models the speaker based on the temperature coefficient of the voice coil (Ohms/C) and DC resistance (DCR), allows design engineers to take advantage of Maxim’s highly-efficient integrated boost to safely push speakers well beyond their specified power rating to maximize the loudness. Typically bass response is limited by the resonant frequency. DSM’s excursion protection extends the bass response beyond the resonant frequency up to two octaves based on the characterization of the speaker leading to a much more balance sound.

Maxim’s DSM smart amplifiers consist of high-performance current and voltage (IV) sensing amplifiers integrated with the patented DSM algorithm into an easy-to-use, fixed-function DSP. By choosing a fixed function DSP Maxim only requires that you load a complete register map to customize the protection algorithm and amplifier configuration for your design. This register map can be completely customized and designed using Maxim’s easy-to-use DSM Sound Studio GUI. Furthermore, the incredibly powerful DSM Sound Studio GUI also makes the aforementioned speaker characterization easier than ever. Now you can characterize and customize the protection algorithm for your speaker in ~3 minutes! 

Maxim’s newest smart amplifier with integrated DSM is the MAX98390, a boosted, digital Class DG DSM smart amplifier. The MAX98390, available in a 6.3mm² package, unleashes a system’s full audio potential by safely driving higher power levels (up to 5.1W) into tiny speakers typically rated for much lower power (up to about 3W), but can also be used with 5W speakers to increase loudness and bass response as well.

To evaluate the MAX98390 for your next micro speaker-based design, check out the MAX98390 evaluation system and the DSM User’s Guide.