PWM Audio AmplifiersZhiming Deng Chinwuba Ezekwe Dimitrios Katsis

OutlinePWM Basics

Digital Modulator Signal Flow

Pulse Edge Delay Error Correction

Volume Control

PWM BasicsEfficient modulation method when only two states (ON & OFF) are availableLinear modulation: Average pulse ON time proportional to signalExtract modulating signal with low pass filterModulation depth (M

Natural PWM (NPWM)What about PCM input?www.powerdesigners.comStraightforward analog implementation

Uniformly Sampled PWM (UPWM)Jorge Varona, http://www.eecg.toronto.edu/~kphang/ece1371/pwrdac.pdfSample modulating signal at the edges of the sawtoothUse this to estimate the pulse width Problem: Introduces an error!

Weighted PWM (WPWM)Estimates pulse width using the information from two consecutive samples:NPWM approximation in the digital domainYields improved THD compared to UPWMMorten Kjaer Johansen et al, A review and comparison of digital PWM methods for digital PMA systems, Proceedings of the 107th AES Convention, 1999.

Digital Modulator Signal FlowOversampling gives better dynamic rangeWPWM is used to simulate NPWM in order to decrease THDNoise shaping also helps lower the bit-rate while keeping a high resolutionSteen Munk et al, State of the Art Digital Pulse Modulated Amplifier System, AES 23rd International Conference, 2003.

Sigma-Delta Noise shaperUse noise shaping to preserve in-band SNR

Amplification Errors in Analog Power StageNon-stable supply causes amplitude errors.Dead time in switches causes delay of rising edge of output PWM signal.Variation of load changes the frequency characteristics of the demodulation filter.Finite On resistance of semiconductor switch and output filter resistance lead to output resistance.Non-linearity of the demodulation filter leads to errors in the demodulated output signal.

Error CorrectionAnalog signal referenced control systemNOT POSSIBLE in a digital PMA because of the absence of analog reference signal.

Pulse Edge Delay Error Correction (PEDEC) PEDEC is a pulse referenced control system that eliminates all types of error by re-timing the edges of the PWM inputs.

Pulse Edge Delay Error CorrectionKarstem Nielsen, Digital Pulse Modulation Amplifier (PMA) topologies based on PEDEC Control, Proceedings of the 106th AES Convention, 1999.Pulse referenced control systemEliminates error by retiming pulse edges

Edge Delay (ED) UnitSteen Munk et al, State of the Art Digital Pulse Modulated Amplifier System, AES 23rd International Conference, 2003.forMakes absolute correctionNo quantization error

Limited correction range saturates if correction range is exceededIncreasing phase lag of feedback signal w.r.t. reference signal leads to saturation for very short or long pulses

Voltage Feedback before Demodulation FilterFeedback from switching power stage output Vp.Feedback path compensator A(s) is a simple attenuation.No reference signal shapingKarstem Nielsen, Digital Pulse Modulation Amplifier (PMA) topologies based on PEDEC Control, Proceedings of the 106th AES Convention, 1999.

Voltage Feedback before Demodulation Filter1st order reference shaping with matched 1st order output feedback shaping

Karstem Nielsen, Digital Pulse Modulation Amplifier (PMA) topologies based on PEDEC Control, Proceedings of the 106th AES Convention, 1999.

Voltage Feedback after Demodulation FilterUsing global feedback from demodulator output Vo.2nd order reference shapingThis is the ONLY topology that eliminates the errors in the demodulation filterKarstem Nielsen, Digital Pulse Modulation Amplifier (PMA) topologies based on PEDEC Control, Proceedings of the 106th AES Convention, 1999.

Volume ControlA digital volume control is simple to implement but will decrease the dynamic range as the signal is attenuated.

An analog volume control can retain the dynamic range, but it is not applicable to a digital PMA.

GOAL: a volume control system that will not decrease dynamic range. Intelligent Volume Control (IVC)

Intelligent Volume Control (IVC)Use multiple supply voltages in power stage for coarse controlUse digital attenuation through modulation depth modification for fine adjustmentSteen Munk et al, State of the Art Digital Pulse Modulated Amplifier System, AES 23rd International Conference, 2003.

IVC (two-level example)Steen Munk et al, State of the Art Digital Pulse Modulated Amplifier System, AES 23rd International Conference, 2003.

IVC AdvantagesImproved dynamic range

Improved edge related noise

Improved efficiency

Improved electromagnetic interference (EMI) characteristics

SummaryImprove THD with WPWM

Reduce noise by noise shaping

Correct various errors through PEDEC

Improve SNR of volume control by using multiple supply voltages