Maxim's Active-Emissions-Limiting Circuitry Demystified
IntroductionMaxim's 2nd-generation Class D audio power amplifiers feature a unique spread-spectrum-modulation mode that flattens wideband spectral components, improving EMI emissions that may be radiated by the speaker and cables. Maxim's 3rd-generation Class D audio power amplifiers employ the same spread-spectrum technology with a new active-emissions-limiting circuit that further reduces narrow-band spectral components without degrading audio performance. The combination of these two unique technologies allows Maxim's Class D audio power amplifiers, such as the MAX9705, to run 'filterless' with up to 24 inches of unshielded cable and still meet EN55022 electromagnetic-interference (EMI) regulation standards. This article explains the technology Maxim employs in active-emissions-limiting Class D amplifiers to minimize radiated emissions levels, while maintaining the high efficiency expected of Class D amplifiers.
Maxim's 3rd-Generation Class D TechnologyMaxim's 3rd-generation Class D technology features Maxim's spread-spectrum-modulation technology with a new active-emissions-limiting circuit that further reduces narrow-band spectral components.¹ Active-emissions-limiting circuitry greatly reduces EMI emissions by actively controlling the output-FET gate transitions under all possible transient output-voltage conditions. The edge rate of each gate transition is intelligently controlled such that the near rail-to-rail swing and the fast switching frequency contribution to emissions is reduced with as little impact on amplifier efficiency as possible.
Since the high efficiency of a Class D amplifier is due to its near rail-to-rail swing and the fast on/off transitions of its output FET (Figure 1), edge-rate control has the potential to negatively impact the Class D amplifier's efficiency.
Figure 1. Class D output waveform illustrating regions of loss due to its switching edge rate and transistor RDS(ON).
Fast transitions on the output contribute to high efficiency by minimizing loss in rise or fall time. If the on/off transition time is increased, then efficiency decreases. With active-emissions limiting, the output-FET gate transitions are intelligently controlled so as to maximize Class D efficiency while still achieving excellent emissions performance (Figure 2).
Figure 2. Active-emissions-limiting circuitry intelligently controls the edge rate of the Class D switching waveform and reduces radiated emissions with very little degradation in efficiency.
The MAX9705 Class D, Active-Emissions-Limiting AmplifierThe MAX9705 is an ultra-low EMI, mono, Class D amplifier featuring Maxim's 3rd-generation Class D technology. Offering efficiencies up to 90%, the MAX9705 boasts of 15.8dB quasi-peaked margin to the FCC limit with 12 inches of unshielded, twisted-pair speaker cable (Figure 3). More impressively, the MAX9705 can pass the FCC limit with up to 24 inches of unshielded, twisted-pair speaker cable (Figure 4).
Figure 3. MAX9705 radiated emissions data (MAX9705EVKit, spread-spectrum-modulation mode, 12 inch, unshielded, twisted-pair speaker cable).
Figure 4. MAX9705 radiated emissions data (MAX9705EVKit, spread-spectrum-modulation mode, 24 inch, unshielded twisted-pair speaker cable).
In addition to excellent EMI performance, the MAX9705 IC features a low 0.02% THD+N, a high 75dB PSRR, and a low-power 0.3µA shutdown mode. The MAX9705 is available in 10-pin TDFN (3mm x 3mm x 0.8mm), 10-pin µMAX®, and 12-bump UCSP™ (1.5mm x 2mm x 0.6mm) packages. For more a more detailed description and additional specifications, refer to the MAX9705 data sheet.
ConclusionThe emissions performance of Maxim's 3rd-generation Class D amplifiers enables the truly 'filterless' operation of Class D amplifiers in consumer electronic equipment. In this way, spread-spectrum modulation and active-emissions-limiting circuitry reduce component count, extend battery life, and enable Maxim's 3rd-generation Class D amplifiers to provide Class AB performance without sacrificing efficiency. Consequently, this 'filterless' technology allows for smaller, more cost-effective solutions, making these amplifiers ideal for cellular-phone, MP3, PDA, and notebook applications.
¹For a detailed discussion of Maxim's spread-spectrum-modulation technology, refer to application note 3881, "Spread-Spectrum-Modulation Mode Minimizes Electromagnetic Interference in Class D Amplifiers."