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Chip Amps Amplifiers based on integrated circuits

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Old 31st May 2007, 01:54 AM   #11
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The spreadsheet is available on the National Semi website...


7/10
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Old 31st May 2007, 01:58 PM   #12
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that is correct
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Old 31st May 2007, 02:59 PM   #13
AndrewT is offline AndrewT  Scotland
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Hi Backworth,
what's the problem with the spreadsheet?
I can see 4r0 load and 59W @ 1% distortion from +-25.4Vdc.

The orange cell showing 1.04C/W indicates the chip is needing a lot of sink to stay cool.
The N/A in the cell directly below tells you the thermal resistance of the isolated package cannot get rid of the heat in the chip.
Am I correct in interpreting assumption4 that the chip is fixed with thermally conductive compound direct to the sink? What voltage is the sink at?

Add in the reactance of a real speaker and the chip is starting to run into real stress. Drop the impedance to 4ohm and 60degree phase angle for a severe speaker load and the chip will start cutting out if you try to get maximum power out of it on a PA type duty.
Use it for typical domestic duty where average output is below -20db ref maximum output (ie. 0.6W average =600mW)and the chip will stay cool enough.
This will require mid efficiency speakers, try 88db/W to 92db/W.
Use this amp to drive an 84db sub-bass and it will tell you it does not like it.
All that comes from reading between the lines in the datasheet and that orange cell in the spread sheet.

I get criticised for reminding folk that chipamps do not like low impedance loads. Who wants to put the alternative case today?

If you parallel two chips so that each thinks it is driving an 8r load then the heat problem virtually goes away. You still get 60W into the 4ohm load from the parallel pair, but at far less distortion.
Try changing load to 8r and then look at the parallel output.
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Old 31st May 2007, 04:20 PM   #14
AndrewT is offline AndrewT  Scotland
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Hi,
having downloaded that spreadsheet, I tried a few numbers.

Keeping the same 4r load, +-25.4Vdc & 25degC ambient.
I had a look at the graph of power dissipation (per double chip) when output averages -10db ref maximum. Pd=47W @ 6W+6W output.
That 1.04C/W sink will run at about 25+49=74degC.
Using Rth c-s of 0.5C/W, the case temperature will be near 100degC when the output averages 6W into 4ohms. I would suggest a sink with Rth s-a of about double the heat dissipation of the figure quoted in the spreadsheet.

6W is loud into 90db/W speaker, but may be the sort of level that might be attempted during a party (if the neighbours don't complain).
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Old 3rd June 2011, 01:31 AM   #15
w0l is offline w0l
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Hi,

Does anybody still have this spreadsheet?

Cheers,

Ian
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Old 3rd June 2011, 03:19 AM   #16
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overture design guide - Google Search
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Old 3rd June 2011, 03:26 AM   #17
w0l is offline w0l
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Thanks!!

Should have searched for guide rather than spreadsheet!
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