Application Note MEASURING THE PERFORMANCE OF POWER SUPPLY REJECTION FEEDBACK AS USED IN CS44800/44600 DIGITAL CLASS-D PWM AMPLIFIERS

Transcription

1 pplication Note MESURING THE PERFORMNCE OF POWER SUPPLY REJECTION FEEDCK S USED IN CS44800/44600 DIGITL CLSS-D PWM MPLIFIERS by Rany oueaux an Sean Davis 1. Intouction Most cuently available igital Class-D amplifies ae teme open-loop systems since no mechanisms have been evelope to povie eal-time feeback to eject unwante tones (ipple) geneate by the powe supply, o the noise couple onto the powe ail ue to ipple cuents flowing though lage ecoupling capacitos. Since the high sies of the powe MOSFETs ae connecte iectly to the powe supply ail, all tones an associate hamonics geneate on the powe supply ail ae couple onto the output of the auio amplifie channel. Costly etection an feeback cicuity must be ae to the powe supply to account fo output voltage vaiations ue to loa cuent fluctuations. In aition, the output voltage ail must be esigne to emain stable as the input C voltage eviates fom the nominal 120 V. ny ift fom the nominal esign point will be iectly eflecte in the output DC voltage ail causing auible volume changes uing playback. To ovecome these inheent limitations of conventional openloop Class-D amplifies, tightly egulate powe supplies ae commonly employe. Utilizing the integate PSR Feeback, it is shown that a igital Class-D PWM amplifie implemente with the CS44800/44600 PWM Contolle is now an effective powe supply close-loop system by employing eal-time feeback of the voltage ail. Real-time voltage feeback now enables this igital Class-D PWM amplifie to opeate with any type of powe supply, with any measue of output egulation. To emonstate the ejection/compensation fo the above-mentione eficiencies, a poceue is escibe below that uses the CD44800 evelopment boa to emulate a system which contains a lage amount of tones an hamonics on the powe supply ail. Powe supply costs can now be euce an othe powe supply technologies use. Open-loop igital Class-D PWM amplifies pohibit the use of taitional low-cost, epenable, low-emi-aiate-noise, unegulate, linea powe supplies. Publishe application notes fo cuent Class- D amplifie systems on the maket ecommen the use of switch moe powe supplies with vey low output voltage ipple an tight egulation fo both loa an line vaiations, in conjunction with lage, expensive, low-esr electolytic capacitos. The CS44800/44600 PSR cicuity eliminates these epenencies an allows fo inexpensive powe supply altenatives to be use. Copyight Cius Logic, Inc (ll Rights Reseve) FE 05 N257REV1 1

2 2. PSR Feeback Measuement Poceue 2.1 Powe Supply Rejection Pefomance Test The test platfom use fo the pefomance measuement plots below consists of the CD44800 (half-bige channels), with Vpowe = +40 V, an a esistive loa of 6 Ω. Channel of the input auio steam was set to channel 3 on the evelopment boa an channel was set to channel 8. 1) Use an unegulate powe supply if available. The PSR ejection pefomance can be seen using any type of powe supply, howeve the actual amount of ejection ue to PSR feeback will be moe visible with a lightly egulate powe supply than with a highly egulate powe supply. PSR calibation shoul be one befoe any of the channels ae enable so an accuate epesentation of the nominal voltage ail can be captue. See the CS44800/600 atasheet fo the PSR calibation outine. 2) Set up the boa une test fo 2-channel opeation with 6-Ω esistive loas an execute the appopiate scipt file such that amplifie auio is playing fom the boa an PSR is calibate. Veify that PSR Feeback is isable (CS44800/44600 bit 5 in egiste 34h set to 0b). The two channels being use shoul be in sepaate powe packages, such as channel 3 an channel 8 on the CD44800, to minimize the effects of switching noise. 3) Using a igital auio souce (such as an uio Pecision), set channel to be a -1-FS, 1-k sine wave an set channel to be a 0-, 60- sine wave. Channel is being use to emulate the hamonics geneate by an inexpensive, pooly egulate powe supply. The 60- switching output of channel will geneate a coesponing ipple voltage on the ail, which will couple into the channel une test, in this case channel. 4) With only the channel PWM output tune on, use an analog analyze (such as an uio Pecision) an take an FFT of amplitue vesus fequency fo channel. The 1-k tone shoul be pesent with an amplitue of -1 FS. Figue 1 shows the esults with only channel enable as a baseline Re Soli 1 Fft.Ch.1 mpl Left Channel = isable, PSR feeback isable Figue 1. FFT mplitue vs. Fequency, Channel = 1 k, -1 FS, Channel = isable, PSR feeback isable 2 N257REV1

3 5) Tun on the channel PWM output an take an FFT of amplitue vesus fequency fo channel. The 1-k tone shoul be pesent with an amplitue of -1 FS (with moulate sie tones) along with a 60- tone an associate hamonics. Figue 2 shows the FFT of channel with both channel an channel PWM outputs enable. The oiginal 1-k tone is shown at -1-FS, with the couple 60- tone fom channel shown at -50 FS. The full scale, 60 tone being playe back on channel s MOSFET evices causes an associate 60- ipple cuent on the powe voltage ail. This ipple cuent, along with the capacito s equivalent seies esistance (ESR), causes the iscete tones on the powe supply ail. Notice the 2n, 3, 4th, 5th, etc. hamonics at 120, 180, 240, 300, etc. ue to the system non-lineaities. ecause all of these tones ae being moulate onto channel s auio output by the powe MOSFETs switching at a 384-k ate, these iscete tones will also be moulate onto the 1-k tone being playe back (see tones goupe aoun 1 k). These moulate tones appea as symmetical, equiistant tones on each sie of the 1-k tone. The amplitue an fequency of each moulate tone is easily calculate using stana FM moulation fomulas lue Soli -1 Fft.Ch.1 mpl Left Channel = 60, 0 FS, PSR feeback isable Figue 2. FFT mplitue vs. Fequency, Channel = 1 k, -1 FS, Channel = 60, 0 FS, PSR feeback isable 6) Enable PSR Feeback (CS44800/44600 bit 5 in egiste 34h set to 1b). Take an FFT of amplitue vesus fequency on the output of channel. The 1-k tone shoul be pesent with an amplitue of -1 FS, howeve the 60- tone an the moulate sie tones will be geatly iminishe in amplitue. Figue 3 shows the esults of having both channels on an PSR feeback enable. N257REV1 3

4 Magenta Soli -1 Fft.Ch.1 mpl Left Channel = 60, 0 FS, PSR feeback enable Figue 3. FFT mplitue vs. Fequency, Channel = 1 k, -1 FS, Channel = 60, 0 FS, PSR feeback enable Figue 4 shows the esults fom Figue 2 an Figue 3 as an ovelay. 2 1 lue Soli -1 Fft.Ch.1 mpl Left Channel = 60, 0 FS, PSR feeback isable 3 1 Magenta Soli -1 Fft.Ch.1 mpl Left Channel = 60, 0 FS, PSR feeback enable Figue 4. FFT mplitue vs. Fequency, Figue 2 & Figue 3 ovelay 4 N257REV1

5 7) To show the effect of this noise moulation on low-level auio signals, set the amplitue of the 1k tone on channel to FS. Channel emains at 60, 0 fs signal. Figue 5 shows the same tests as above. The blue tace is the FFT of channel s output with PSR tune off. The magenta tace epesents channel s output with PSR feeback enable. 2 1 lue Soli 1 Fft.Ch.1 mpl Left Channel = 60, 0 FS, PSR feeback isable 3 1 Magenta Soli 1 Fft.Ch.1 mpl Left Channel = 60, 0 FS, PSR feeback enable Figue 5. FFT mplitue vs. Fequency, Channel = 1 k, FS, Channel = 60, 0 FS, PSR feeback enable/isable 2.2 PSR Feeback Pefomance Vesus Fequency The following gaph in Figue 6 shows the amount of ejection that PSR Feeback will povie vesus the fequency of the powe supply noise (typically in the low-fequency ange). The fequency on channel, which inuces the ipple voltage on the channel une test (channel ), is vaie fom 20 to 20 k. The plot epesents the amplitue of the couple noise fom the powe supply ail onto channel vs. the fequency of the couple noise. Zeo ata is sent to channel, which is equivalent to muting the amplifie output (not MUTE50/50). Channel s output is not tune off, but athe switches at a moulate nominal 50% uty cycle at a 384-k ate. The e tace in the gaph epesents the amount of ipple voltage which is couple onto channel s output fom the 0-FS signal being playe back on channel. Since the output of channel continues to switch, moulation with anothe channel will still occu. The natual ecay in the amount of ipple voltage pesent on channel s output at highe fequencies is ue to the fequency-epenant eactance of the lage, bulk ecoupling electolytic capacito. N257REV1 5

6 1 1 Re Soli 1 nl.mpl Left PSR feeback isable 2 1 lue Soli 1 nl.mpl Left PSR feeback enable Figue 6. PSR Disable vs. PSR Enable Channel = on, but mute (zeo ata), sweep Channel 0 FS 2.3 Powe Supply Doop Test To emonstation how PSR Feeback can compensate fo oop in the powe supply ail, pefom the following using the CD44800: 1) Using the setup fom above, with PSR calibate an enable, an Vpowe set to +40 V, playback a 1-k tone with an amplitue of appoximately 10 V peak-to-peak (o any amplitue esie). Use one channel of an oscilloscope to monito the sine wave output of channel. Use anothe channel of the oscilloscope to monito the DC voltage on Vpowe. 2) Vay Vpowe own to +30 V. Even though the Vpowe supply will op to +30 V fom +40 V, the peakto-peak level of the sine wave output fom channel will emain constant. This test shows how PSR Feeback will maintain the amplifie auio level even if the powe supply ops in voltage (common when low-fequency auio is playe). The maximum amount of voltage ail oop compensation is limite to 10% of the nominal ail when playing back a fullscale signal. s the signal being playe back is euce in amplitue, moe oop in the voltage ail can be compensate. 6 N257REV1

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