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 12th April 2010, 11:35 PM #1 jamey0824   diyAudio Member   Join Date: Apr 2010 ti 5709 evm I am trying to understand why ti did certain things in there evm design linked here http://focus.ti.com/general/docs/lit...0&fileType=pdf 1) Like i don't understand why the common mode resistors are 4.7kohm, I use R9=R10=R11=R12 = 1/(√2 • 2πfCC1) this formula i got off this page Understanding output filters for Class-D amplifiers | Audio DesignLine in which case it is much lower for this design more like 3-6ohms, depending on fc. Can someone explain the rational 4.7kohms? Thanks
Neil Davis
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Join Date: Dec 2004
Location: Reston, Virginia
Quote:
 Originally Posted by jamey0824 I am trying to understand why ti did certain things in there evm design linked here http://focus.ti.com/general/docs/lit...0&fileType=pdf 1) Like i don't understand why the common mode resistors are 4.7kohm, I use R9=R10=R11=R12 = 1/(√2 • 2πfCC1) this formula i got off this page Understanding output filters for Class-D amplifiers | Audio DesignLine in which case it is much lower for this design more like 3-6ohms, depending on fc. Can someone explain the rational 4.7kohms? Thanks
It's looks like some of the TI chips in this family have internal pull-down resistors for the bootstrap capacitors (see Figure 1 in the TAS5709 data sheet), and maybe some of the chips don't. So R9-R13 are pulldowns for chips that don't have them internally. I'm guessing...but they are the wrong size and value to be part of the output filter.

 13th April 2010, 04:23 AM #3 jamey0824   diyAudio Member   Join Date: Apr 2010 yeah good call, you would think ti would mention something about that in the 5706 datasheet.
theAnonymous1
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Join Date: Feb 2004
Location: Anonymityville
TPA3122D2 CLASS D AUDIO POWER - Audio Amplifiers Forum - Audio Amplifiers - TI E2E Community

Quote:
 The 4k7 ohm resistors are used to discharge the dc blocking cap when the amp is turned off, either by removing the power supply, or putting it in shutdown. This helps with pop performance, so the absolute value is not critical.

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