Research On Critical Circuit Techniques For Low EMI Class D Audio Ampliifer

Class D audio amplifier is widely used in many portable apparatuses such asPDA,mobile phones and MP3s due to its high efficiency and low power dissipation.However, the switching operation of class D power amplifiers generates signals withhigh di/dt and dv/dt as well as wide disturbance bandwidths. These voltage and currentspikes induce large ac displacement currents in both physical and parasitic circuitelements. It creates conducted noise and radiated noise. The application of Class Damplifier is thus encountering more challenges with the increasingly rigorous standardsfor EMI. Therefore, much attention has been paid to resolve the issue of EMI in Class Damplifier nowadays.Compared to traditional PCB layout optimization, controlling the sources of EMIin circuit level is more valuable to improve system performance and reduce board space.Different spread spectrum techniques have been widely studied to overcome theproblems faced in filter solutions. However, the design complexity, power dissipation,efficiency issues, and chip cost limit the applications. Moreover, the total harmonicdistortion (THD) effects and power efficiency reduction issues are the major problemswhen applying a spread spectrum technique to a switching power amplifier design.Besides, when the device is working without output filtering and the cable connected tothe speaker is long, high-frequency emissions will be serious if output power is large. Atthis time, optimizing the gate driver circuits to alter PWM waveforms of the amplifier isneeded rather than changing the clock frequency simply. It will increase circuitcomplexity and chip area contributed by more power transistors unluckily.Based on analyzing advanced technological achievements reported before, thecircuits and system design for low EMI filterless Class D audio amplifier are researchedintensively, including temperature compensation for bandgap reference, digital spreadspectrum to reduce conductive EMI and low EMI gate driver. Some critical techniques,containing non-resistor curvature compensation, pseudorandom modulation, currentsource gate driver and so on, are presented in this paper. The innovations of the workcan be listed as follows: 1. According to the theory for high-order temperature compensation, a novelnon-resistor compensation technique (NRCT, IEEE Trans. Circuits Syst.II,2010, pp.767-771) is proposed. The nonlinearity of a base-emitter voltage is directlycompensated using a bias current with high-order temperature coefficient (TC), which isrealized by a current subtracter and a translinear circuit. Implemented in0.5μm CMOSprocess, the active area of the reference is500×200μm~2. The power consumption of theproposed reference is0.648mW at3.6V. A temperature coefficient of11.8ppm/℃andPSRR of more than31dB at low frequencies are easily achieved. At the same time, twopiecewise compensation structures based on saturation current of a PN junction(2.8ppm/℃) and a variable gain current mirror as well as second correction (1.3ppm/℃)are presented to achieve low TC voltage references.2. The basic theory of spread spectrum and comparison of EMI reduction resultsbetween periodic and random modulation are discussed. A spread spectrum clockgenenrator (SSCG) by ultilizing linear feedback shift register to realize pseudorandommodulation (PRM, IEEE Trans. Power Electron.,2011, pp.638-646) has therefore beenproposed to reduce EMI greatly; the important sub-circuits and system feedback loopwith multiple low-pass filters are optimized as well to overcome nonideal effects fromSSCG that may deteriorate audio performance. Implemented in0.5μm CMOS process,the active area of SSCG is191×251μm~2, which is only about2%of the die size and thedesigned maximal power is10.08μW at3.6V. Peak power reduction of more than12dB is measured with a frequency deviation of20%. The amplifier meets the FCC ClassB standard for radiated emissions without any LC filtering.3. Based on analyzing origins of EMI from power stage and some typical low EMIgate drivers, a low power current source gate driver (CSGD) for H bridge has beenproposed. It relies on optimizing the charging or discharging slope at the gate of powerMOS and dead time to restrain high-frequency EMI emissions. At the same time, toovercome deterioration from CSGD, a system with dual-loop feedback and feedforwardcontrol is presented to keep good THD and PSRR performance. Implemented in0.6μmBCD process, high-frequency emissons with more than20dB energy reductioncompared to the traditional ones are achieved and the THD for input frequency below10kHz is less than0.5%. The simulation results prove superiority of the audio amplifier.