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Old 1st May 2004, 02:59 PM   #1
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Default LM3875 heatsink

I'm searching for a suitable heatsink for the LM3875.
Can you help me with this?
I understand there are variables to be considered.
Can I assume an average Pd, W of 20?
Can I assume a max air temp in the enclosure of 50 C or 122 F?
Can I assume |V| +/- is 40 or less?

Looking at page 8 of the data sheet leads me to believe that a heatsink thermal resistance of 3.8 C/W or less is fine.

Is this correct? And are my assumptions reasonable?

Thanks in advance.
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Old 2nd May 2004, 12:39 AM   #2
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Default Re: LM3875 heatsink

Quote:
Originally posted by bg40403
I'm searching for a suitable heatsink for the LM3875.
Can you help me with this?
I understand there are variables to be considered.
Can I assume an average Pd, W of 20?
Can I assume a max air temp in the enclosure of 50 C or 122 F?
Can I assume |V| +/- is 40 or less?

Looking at page 8 of the data sheet leads me to believe that a heatsink thermal resistance of 3.8 C/W or less is fine.

Is this correct? And are my assumptions reasonable?

Thanks in advance.
Well, the power dissipation per channel wouldn't even get close to 20W unless you were feeding it a continuous test tone at maximum power, and unless your room is extremely hot, or you plan on having the chips in a virtually air-tight enclosure, 50°C would be a much higher air temperature than I would expect. I don't know what your rail voltages are, but in most cases, a "real" heatsink isn't necessary, and large (1/2-1 pound) plate of 1/2" think aluminum would work very well. If you can't easily get acess to that, I think one of the best and most economical solutions is to use an old slot 1 heatsink. Compusa's
part number 50077044 with the fan removed should be well suited for two LM3875's, but it's sold out, and I don't know if that's temporary or permanent. Part number 50077045 is identical, except that it comes with two fans and costs about twice as much. I think it's still a fairly good deal, especially if you have a Compusa nearby and can just go pick it up without paying for shipping.

I have my chips mounted on a wider PII heatsink with shallower fins, and they never get noticably warmer than room temperature at comfortable listening levels. They only get warm with a continuous sine wave input just below clipping (to measure power output), but it's unlikely that you enjoy listening to very loud sine waves, so any decent-sized pentium two heatsink should be just fine.
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Old 2nd May 2004, 12:47 AM   #3
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