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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

1 2 3 4 5 6 7

Podsumowanie treści

Strona 1 - Application Note

Strona 2 - channel A

AN2572 AN257REV12. PSR Feedback Measurement Procedure2.1 Power Supply Rejection Performance TestThe test platform used for the performance measurement

Strona 3 - -1-dBFS, with the coupled 60

AN257AN257REV1 35) Turn on the channel B PWM output and take an FFT of amplitude versus frequency for channel A. The1-kHz tone should be present with

Strona 4 - 4 AN257REV1

AN2574 AN257REV1Figure 4 shows the results from Figure 2 and Figure 3 as an overlay. ColorSweep Trace Line Style Thick Data Axis Comment3 1 Magenta So

Strona 5 - AN257REV1 5

AN257AN257REV1 57) To show the effect of this noise modulation on low-level audio signals, set the amplitude of the 1kHztone on channel A to -60 dBFS.

Strona 6 - 6 AN257REV1

AN2576 AN257REV12.3 Power Supply Droop TestTo demonstration how PSR Feedback can compensate for droop in the power supply rail, per-form the following

Strona 7 - 3. Revision History

AN257AN257REV1 73. Revision History Release Date ChangesRev 1 Febuary 2005 Initial ReleaseTable 1. Revision HistoryContacting Cirrus Logic Supp

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Cirrus-logic AN257 Instrukcja Użytkownika

Przeglądaj online lub pobierz Instrukcja Użytkownika dla Sprzęt komputerowy Cirrus-logic AN257. Cirrus Logic AN257 User Manual Instrukcja obsługi

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