要約

This design solution discusses the barriers to high-quality sound from micro speakers due to their size limitations. Maxim's Dynamic Speaker Management Sound Studio GUI overcomes these barriers, simplifying design and improving time to market. The DSM algorithm is integrated into a fixed-function DSP to alleviate the complexities of programming. DSM, based on MAX98390 Class DG audio amplifier, significantly increases loudness (SPL) and bass response from micro speakers while maximizing efficiency for longer battery operation.

Introduction

You're watching your favorite movie on your tablet, and the sound from the micro speakers comes through with such richness, loudness, and clarity that you almost feel like you're listening to full-size speakers (Figure 1). Now you want to deliver this level of audio performance in even smaller devices, such as wearable, medical, AR/VR, gaming, and smart home IoT solutions.

The designers outside of the smartphone and tablet markets are looking to leverage the same micro speaker and IV-sense (current and voltage sensing) amplifier technologies for compact designs in a multitude of end markets. However, due to their small size, micro speakers have limited ability to accurately reproduce sound. Hence, the burden shifts to the audio amplifier, which, through a combination of hardware and software system integration, picks up the slack and manages to deliver sound quality that turns music lovers' heads while still minimizing power consumption for longer battery life.

Improving sound quality with IV-sense amplifiers relies on an accurate speaker model to tune the amplification algorithm. Proper speaker characterization is difficult and expensive work that requires extensive expertise and special, expensive equipment.

The limited availability of equipment and expertise for IV-sense amplifiers presents a huge barrier to their proliferation in low-volume applications. This design solution presents a disruptive approach that removes the characterization barrier with a quick, cost-effective, high-performance solution.

The Micro in Micro Speaker

Micro speakers are diminutive devices that must fit into small portable gadgets like medical wearables, smart home IoT, and gaming devices. Figure 2 shows the size comparison of two micro speakers (one with wires) next to a SD memory card and a more traditional speaker.

The small size of a micro speaker limits its ability to reproduce sound with high fidelity. Figure 3 illustrates the relationship between a speaker's size and range of operation.

In particular, the small size of a micro speaker also limits its excursion and confines its response to the treble frequency range, producing a tinny audio response. The challenge for the micro speaker designer is to overcome these limitations. One way is by including integrated boost and digital signal processing (DSP) with an IV-sense audio amplifier. This vastly improves sound quality by increasing loudness (sound pressure level) by 2.5X and significantly extending bass frequency response well past the resonant frequency of a micro speaker.

Micro Speaker Operation

Figure 4 shows the basic elements of a micro speaker's operation. The voice coil is immersed in the magnetic field established by the permanent magnet. When a variable current (the audio signal) flows into the voice coil, a corresponding variable force is applied to the coil (Lenz's Law). The diaphragm, attached to the voice coil, vibrates accordingly and produces a sound of amplitude proportional to the diaphragm deviation from the state of rest (this is known as excursion).

The micro speaker is encased in a box, which functions to block the bottom half of the sound waves and avoid sound cancellation.

Class DG Amplifier

The key to better portability and longer battery life is to drive a speaker with an audio amplifier that has a switching Class DG architecture (Figure 5). This option is better as opposed to using a classic linear one, which is notoriously lossy. In Figure 5, the audio signal (V_{AUDIO_I}) is digitized by the modulator (Comparator + V_TRIANGLE), which embeds signal information into the duty cycle of the power output (V_{AUDIO_CLASS_DG}). Finally, the original sound amplified through the power transistors T1 and T2 (I_{AUDIO_OUT}) is recovered by the lowpass filter and is applied to the speaker. The Class DG amplifier uses a multilevel output stage to sense the magnitude of the output signal. It then switches the supply rails (V_BATT, V_BOOST), as needed, to supply the required signal power more efficiently.

Although amplification is its obvious function, at the heart of a modern audio amplifier is a digital signal processor (DSP) that incorporates a micro speaker protection algorithm. The DSP helps overcome the intrinsic limitations of a micro speaker by extending its response and capabilities.

Pushing the Envelope

In simple terms, micro speakers present several challenges to delivering big sound in a small box. The speaker power rating limits are determined by the voice coil thermal limit and the diaphragm excursion mechanical limits. With excessive current flow, the voice coil can get so hot that the mechanical components comprising the voice coil and diaphragm system can melt.

The micro speaker's small size also limits its capability to produce lower bass frequencies. An increase in bass response can cause an over-excursion event if the response is extended below the resonant frequency, which is quite high (～1kHz) for these micro speakers. The sound pressure can become so extreme that the diaphragm breaks apart resulting in what is known as a blown speaker.

Speaker manufacturers rate their products very conservatively, with a built-in engineering margin to protect against all kinds of abuse. However, the quest for better sound within smaller form factors in products such as smartphones has fueled the race to squeeze more out of a micro speaker. With a combination of speaker parameter characterization and active speaker management during operation, a 1W speaker can now put out up to 5W safely.

Currently, this type of performance doesn't come cheap. For speaker protection to function, the speaker must be characterized, which requires significant expertise and expensive special equipment. On the performance side, it requires the development and porting of a complex management algorithm. The extracted parameters and the management algorithm are loaded into a DSP, requiring specialized software development that is unique for different processor cores. This cumbersome process needs to be repeated many times since several micro speakers must be evaluated in order to arrive at a final choice.

Wouldn't it be nice if someone made it easier to implement such novel and complex technology in one step? Maxim Integrated comes to rescue with the Dynamic Speaker Management™ (DSM) smart amplifier solution.

Dynamic Speaker Management (DSM)

DSM is a system solution that significantly increases loudness (SPL) and bass response from micro speakers. The MAX98390 DSM smart amplifier integrates a high-performance IV-sense amplifier and an easy-to-use, fixed-function DSP loaded with a proven DSM algorithm. The MAX98390 (Figure 6) is a high-efficiency mono smart amplifier that also features a sophisticated, high-efficiency integrated boost converter with boost-bypass and high-resolution envelope tracking to provide maximum battery life, especially at lower output levels. The integrated fixed-function DSP has been hard-coded with the DSM algorithm to eliminate the complex time-consuming programming steps that are necessary with other general-purpose DSP implementations.

With DSM, the speaker performance envelope (Figure 7) is safely extended upward, increasing the audio volume (+6-8dB SPL). It also goes downward, dramatically extending the bass response to a quarter of the resonance frequency (Fc).

Power Management with DSM

The audio amplifier's efficiency is maximized in many ways. FET scaling at low power reduces switching losses. High-resolution envelope tracking on the boost converter assures that the audio signal headroom is minimized. The boost is also bypassed as soon as possible, always operating the amplifier with the lowest possible power rail. In addition to its highly efficient amplifier architecture, Maxim's Perceptual Power Reduction (PPR) algorithm (Figure 8) further increases efficiency up to 25% with no loss in audio fidelity. PPR is an adaptive and dynamic filter that uses the human hearing threshold to model what the user would perceive. Based on the speaker's characterized response, it removes content that would not be audible based on the content and volume level. However, if there is a case where power savings is more important than audio fidelity (i.e., in a 10% "low-power mode" smartphone option), the PPR threshold can be set more aggressively to trade-off some of the bass response in favor of desired audio fidelity and battery life.

DSM Sound Studio

The complexities of parameter extraction, tuning, and evaluation are eliminated by using the MAX98390 Evaluation Kit and the DSM Sound Studio GUI. The use of both empowers the audio designer from initial design to production (Figure 9).

One of the most compelling and unique features of the DSM Sound Studio is its ability to easily and quickly evaluate key parameters of multiple speakers (Table 1).

Speaker parameter extraction is broken down into two main steps. The first step automatically extracts and reports F_c, Q, R_e, and T_amb parameters. The second step includes setting thermal and excursion limits. The excursion (X_max) calibration is performed with an acoustic approximation represented by the amplifier voltage drive (V_drX), necessary for the speaker tone to reach an approximate excursion limit (Figure 10).

The voice coil thermal limit is normally provided by one of Maxim's preferred micro speaker vendors. However, if this value is not readily available, we suggest using 80°C as a starting point until the specification is supplied by the micro speaker vendor.

Once the final speaker is selected, the designer has the option to send it to Maxim for a final verification of its characterization. This final service is included with the purchase of an evaluation kit and is limited to one speaker characterization per evaluation kit purchase. This service includes finetuning the V_drX vs. excursion X_max limits, directly measured with a highly accurate laser micrometer.

Conclusion

In this design solution, we discussed the obstacles in obtaining high-quality sound from micro speakers. Existing solutions rely on complex systems and procedures to model the micro speaker and compensate for deficiencies that are barriers for many designers. To overcome this, we presented Maxim's DSM Sound Studio GUI which simplifies design and improves time-to-market. Furthermore, we examined Maxim's DSM Smart Amplifier which integrates the DSM algorithm into a fixed-function DSP to alleviate the complexities of programming and to help minimize quiescent power consumption. DSM is a fast turnaround system solution, based on the MAX98390 Class DG audio amplifier, that significantly increases loudness (SPL) and bass response from micro speakers while maximizing efficiency for longer battery operation.

Glossary

AEC: Acoustic/Echo Cancellation
AR: Augmented Reality
Buzzing: The interaction of mechanical rattling with port resonance
DSM: Dynamic Speaker Management
EQ: Equalization
HPF Highpass Filter
MP: Moving Picture. An audio file format.
PPR: Perceptual Power Reduction
RMA: Real Media Audio File
SPL: Sound Pressure Level
VR: Virtual Reality

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