Cirrus Logic AN257 - Manual

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Copyright © Cirrus Logic, Inc. 2005
(All Rights Reserved)
www.cirrus.com
AN257
Application Note
MEASURING THE PERFORMANCE OF POWER SUPPLY
REJECTION FEEDBACK AS USED IN CS44800/44600
DIGITAL CLASS-D PWM AMPLIFIERS
by Randy Boudreaux and Sean Davis
1. Introduction
Most currently available digital Class-D amplifiers are termed “open-loop” systems since no
mechanisms have been developed to provide real-time feedback to reject unwanted tones (rip-
ple) generated by the power supply, or the noise coupled onto the power rail due to ripple cur-
rents flowing through large decoupling capacitors. Since the high sides of the power MOSFETs
are connected directly to the power supply rail, all tones and associated harmonics generated on
the power supply rail are coupled onto the output of the audio amplifier channel.
Costly detection and feedback circuitry must be added to the power supply to account for output
voltage variations due to load current fluctuations. In addition, the output voltage rail must be de-
signed to remain stable as the input AC voltage deviates from the nominal 120 V. Any drift from
the nominal design point will be directly reflected in the output DC voltage rail causing audible
volume changes during playback. To overcome these inherent limitations of conventional “open-
loop” Class-D amplifiers, tightly regulated power supplies are commonly employed.
Utilizing the integrated PSR Feedback, it is shown that a digital Class-D PWM amplifier imple-
mented with the CS44800/44600 PWM Controller is now an effective power supply “closed-loop”
system by employing real-time feedback of the voltage rail. Real-time voltage feedback now en-
ables this digital Class-D PWM amplifier to operate with any type of power supply, with any mea-
sure of output regulation. To demonstrate the rejection/compensation for the above-mentioned
deficiencies, a procedure is described below that uses the CDB44800 development board to em-
ulate a system which contains a large amount of tones and harmonics on the power supply rail.
Power supply costs can now be reduced and other power supply technologies used. “Open-loop”
digital Class-D PWM amplifiers prohibit the use of traditional low-cost, dependable, low-EMI-ra-
diated-noise, unregulated, linear power supplies. Published application notes for current Class-
D amplifier systems on the market recommend the use of switch mode power supplies with very
low output voltage ripple and tight regulation for both load and line variations, in conjunction with
large, expensive, low-ESR electrolytic capacitors. The CS44800/44600 PSR circuitry eliminates
these dependencies and allows for inexpensive power supply alternatives to be used.
FEB ‘05
AN257REV1