Class D Audio Power Amplifier - iPod Dock

Class D Audio Power Amplifier - iPod Dock
Version 1.0
Publication Date: 2010/02/22
Copyright © 2010 XMOS Ltd. All Rights Reserved.

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Class D Audio Power Amplifier - iPod Dock (1.0)
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Overview
Class D power amplifiers (PA) offer excellent power efficiency when compared to
other classes of PA. The output stage switches at high frequency between fully on
and fully off and can be controlled by a pulse width modulation (PWM) signal. The
audio signal is kept in the digital domain right up until the final PA stage. This avoids
degradation of the audio through analog stages, and also eliminates the additional
bill of materials cost of a high quality DAC.
The XS1-L1 provides an excellent platform for an all digital iPod dock solution. Digital
audio samples are acquired from the iPod across its USB interface and then used to
drive a class D audio PA. A single XS1-L1 device provides all of the processing and
interfacing requirements of the system: implementing the USB host to obtain the
audio samples; performing digital signal processing (DSP) on the samples; driving
the PWM to control the class D PA.
A general description of class D PAs is contained within a separate application
note Class D Audio Power Amplifier Overview. This document describes a specific
implementation of a class D PA integrated with the XMOS iPod Dock Reference
Design.
2
iPod Class D Demonstration Hardware
The iPod class D PA demonstration was implemented on an iPod Dock Reference
Design board (V1.0) connected to a class D prototype PCB. Four 1-bit ports are
required to drive the stereo class D PA board, so wire modifications were made to
the iPod Dock board to route these signals to the class D board (see Figure.1). The
PA also needs a 12V power supply.
The class D PA is a stereo single ended implementation. The major components used
on this PCB are listed in Table 1.
Manufacturer Description
Part No
Qty
Cost($)* Total Cost($)
Fairchild
Dual N-type and Ptype FDS4897C
2
0.21
0.42
MOSFET
Fairchild
MOSFET driver
FAN3227T
2
0.44
0.88
Panasonic
470uF DC Blocking Cap EEU-FM1A471
2
0.08
0.16
Panasonic
220uF Reservoir Cap
EEU-FM1E2211 1
0.08
0.08
1.54
* 1000 off prices obtained from Digikey.com on 2009/12/18
Table 1: Class D PA Bill Of Material
www.xmos.com

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Class D Audio Power Amplifier - iPod Dock (1.0)
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Figure 1: Photo of the iPod Dock Class D PA Hardware
www.xmos.com

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The XS1-L1 operating at an XCore frequency of 400MHz has eight 50MHz threads.
T6
XS1-L1
Interpolate
Left
T5
Channel
T8
USB
T1-4
USB Host
Timing
PMW Port
PWM
Control
Driver
T7
Interpolate
Right
Channel
Figure 2: iPod Dock Class D PA Thread Diagram
Table 2 shows how the threads are allocated to the software tasks.
Thread
Thread Name
Description
1­4
USB Host
iPod Dock reference design
5
Timing Control
Generates 1ms timing packets to the iPod. Splits
the left and right samples and buffers them.
6
Interpolate Left Channel
Upsamples and filters the audio input samples to
the PWM frequency (for example 48kHz to 384kHz).
7
Interpolate Right Channel
Upsamples and filters the audio input samples to
the PWM frequency (for example 48kHz to 384kHz).
8
PWM Port Driver
Drives the PWM ports for both channels using the
PWM frequency samples.
Table 2: Class D iPod Implementaton: 50MHz Thread Requirements
Table 3 lists the performance figures for the Class D iPod Implementaton:
Power per Channel into 4R load
3WRMS
Signal to Noise Ratio
-90.3 dB
Dynamic Range
85.4 dB
Total Harmonic Distortion + Noise (at maximum power -3dB)
0.345%
Table 3: iPod Dock Class D Audio Performance
www.xmos.com

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Class D Audio Power Amplifier - iPod Dock (1.0)
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Related Documents
Information about XMOS technology is primarily available from the XMOS web site;
please see http://xmos.com/documentation for the latest documents or click on one
of the links below to find out more information.
Document title
Class D Audio Power Amplifier Overview_amp
4
Document History
Date
Release
Comment
2010-02-22
1.0
First release
Disclaimer
XMOS Ltd. is the owner or licensee of this design, code, or Information (collectively,
the "Information") and is providing it to you "AS IS" with no warranty of any kind,
express or implied and shall have no liability in relation to its use. XMOS Ltd. makes
no representation that the Information, or any particular implementation thereof, is
or will be free from any claims of infringement and again, shall have no liability in
relation to any such claims.
Copyright ©2009-10 XMOS Ltd. All Rights Reserved. XMOS and the XMOS logo
are registered trademarks of XMOS Ltd in the United Kingdom and other countries,
and may not be used without written permission. Company and product names
mentioned in this document are the trademarks or registered trademarks of their
respective owners. Where those designations appear in this document, and XMOS
was aware of a trademark claim, the designations have been printed with initial
capital letters or in all capitals.
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