Transcript Getting Beyond the “Tiny Tripath”

Class D Amp Notes
Putzey’s Open Loop PWM Model
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AES Convention Paper 6690, 2006
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“All amplifiers are analogue, but some amplifiers are more
analogue than others”
Open Loop PWM Challenges
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Output is highly dependent on power supply quality
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Output filter is load-dependent
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No feedback from filter results in some interaction with the
loudspeaker (peaking or roll-off)
Fixed clock frequency results in higher EMI
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High currents modulate the signal and result in distortion
products
Power supply should be a switcher with sense feedback or a
linear regulator for best results
Power concentrated in fixed frequencies
Modulator linearity
Output Stage Feedback
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Feedback taken after output devices improves Power
Supply rejection (PSRR)
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Output feedback can improve modulator linearity
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Most analog input Class D chips use output feedback (including Tripath)
TI and Apogee do not use output feedback, but new TI output stage uses
local feedback for improved PSRR
Carrier-based modulators benefit most from output feedback
Output feedback can improve output stage fidelity
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This was an issue for early Class D designs with “dead-time”
issues, but not a big driver for new designs
Modulators
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Two major classes in use today
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Lowest distortion achieved with Sigma-Delta + feedback
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Analog Devices achieves .005% distortion levels
“Open Loop” Sigma-Delta achieves good results
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Sigma-Delta
Carrier-based (triangle wave/comparator)
Basis for Apogee and TI designs—distortion approx .02%
Carrier-based with feedback is also good
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Feedback reduces distortion to .01% levels
Feedback difficult to optimize (power-level dependent)
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See “Carrier Distortion in Hysteretic Self-Oscillating Class-D Audio Power Amplifiers”, IEEE
TRANSACTIONS ON POWER ELECTRONICS,VOL. 24, NO. 3, MARCH 2009
Other Design Issues
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Self-oscillating vs clocked
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Filter feedback
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Results in lower output impedance and less interaction with the
loudspeaker
Low noise
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Self-oscillating reduces EMI and results in simplified design for
analog-input Class D
Digital input is always clocked, but some digital modulators (e.g.
Apogee) use spread spectrum to reduce EMI
Early Class D amps suffer from Fab process that have high noise
levels in analog stages (e.g. Tripath)
Digital input
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Simplifies the design for multi-channel (6-8 DAC’s with I-V
converters and filters consumes a lot of board space)
Output Power
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Three distinct tiers at higher outputs:
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125W: 50V H-bridge chips (STA517B or TAS5631)
300W: TDA8954 or Sanyo STK-428 (plus and minus 40V)
500W+: Discrete outputs (plus and minus 60 to 80)
Note: power ratings are for 8ohms—double for 4ohm
Tripath version at 500W:
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TA0105A/TA2500 (or RA2500), based on TC2000 with drivers
plus discrete outputs
More complex than newer IRF or ICE designs and with higher
distortion
Poor reputation for reliability
Other high power options
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UCD self-oscillating amps (carrier-based)
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IR IRF2092
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Marketed by Hypex; chief evangelist is Bruno Putzeys
Elegantly simple design
Fairly low distortion
Uses output feedback (after the filters) to work with many loads
Simple, low-distortion design
Uses Sigma-Delta modulator with output stage feedback (but before
the filters)
Zetex (Diodes, Inc)
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Used in NAD M2
High power and very low distortion
Direct Digital Feedback Amplifier uses DSP to control modulator
with feedback after the filters.
Chip Comparison
output feedback filter feedback Delta-sigma self-oscillating low-noise Digital input Power @ 8ohm
Apogee + STA517b
Tripath + STA517b
Tripath RA-0105
TI + old output chip
TI + TAS5631
Analog: ADAU1592
ADAU1592+booster
TDA8954 (NXP)
IRF2092
Hypex UCD
no
yes
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yes
yes
sort-of
sort-of
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yes
yes
yes
no
yes
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no
no
yes
no
no
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no
yes
yes
yes
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no
yes
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yes
sort-of
yes
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no
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125
125
400
80
125
15
400?
300
500
500
Modulator + IRS20957
Zetex ZXCZM800
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N/A
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yes
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500
Yes – “digital”
Yes
Yes?
no
yes
Yes
500