Hottest part of LM3875

alleycat · 30th July 2006, 02:56 AM

Does anybody know what part of the LM3875 gets hottest? I assume it's not going to be where the mounting hole is. I'd like to centre the hottest part of the chip in the middle of the heatsink. It probably doesn't matter much, but given a choice...

Nuuk · 30th July 2006, 08:35 AM

Are you talking about a heatsink that is vertical or horizontal?

alleycat · 30th July 2006, 11:19 AM

I was planning on using a heatsink with vertical fins, with the chip attached so that its legs are pointing downwards. A similar arrangement to Vikash's TATCA.

Nuuk · 30th July 2006, 11:42 AM

Well I would mount the chip slightly below the center of the heatsink. I know that the heat is conducted by the aluminium but the question is does it rise as it does in air?

Factually, it probably doesn't make the slightest bit of difference though with the (comparatively) small amount of heat produced by a GC!

alleycat · 30th July 2006, 02:28 PM

OK, makes sense. Thanks!

AndrewT · 30th July 2006, 10:32 PM

Hi,
when mounting a device for maximum cooling on a passive sink the centre of the device should be about 40% up from the bottom and 50% in from either side.

The back plate of the sink should be thicker than 10% of the radius from the device to an extreme corner of the sink backplate.

pinkmouse · 30th July 2006, 10:40 PM

Quote:

Originally posted by Nuuk
I know that the heat is conducted by the aluminium but the question is does it rise as it does in air?

Only if the aluminium is molten, so convection currents get chance to set in. That might be pushing the chip a little out of it's operating range though...

AndrewT · 30th July 2006, 10:44 PM

alleycat · 31st July 2006, 03:57 AM

Quote:

Originally posted by AndrewT
...the centre of the device should be about 40% up from the bottom...

On a sink with 75mm fin height, and with the arrangement as described above, having the mounting hole located in the centre of the sink should mostly take care of this?

sangram · 31st July 2006, 10:10 AM

Yup, should. The hottest part is somewhere on the back behind the plastic top. A clamping bar across the chip face should be the best way to mount the chip, better than using the tab.

Normally the center of the die is used for the hottest parts. This may not be an exception.

30th July 2006, 02:56 AM	#1
alleycat diyAudio Member Join Date: May 2005 Location: Melbourne	Hottest part of LM3875 Does anybody know what part of the LM3875 gets hottest? I assume it's not going to be where the mounting hole is. I'd like to centre the hottest part of the chip in the middle of the heatsink. It probably doesn't matter much, but given a choice...

30th July 2006, 08:35 AM	#2
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	Are you talking about a heatsink that is vertical or horizontal? __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

30th July 2006, 11:42 AM	#4
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	Well I would mount the chip slightly below the center of the heatsink. I know that the heat is conducted by the aluminium but the question is does it rise as it does in air? Factually, it probably doesn't make the slightest bit of difference though with the (comparatively) small amount of heat produced by a GC! __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

30th July 2006, 10:32 PM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, when mounting a device for maximum cooling on a passive sink the centre of the device should be about 40% up from the bottom and 50% in from either side. The back plate of the sink should be thicker than 10% of the radius from the device to an extreme corner of the sink backplate. __________________ regards Andrew T.

30th July 2006, 10:44 PM	#8
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	__________________ regards Andrew T.


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30th July 2006, 11:19 AM	#3
alleycat diyAudio Member Join Date: May 2005 Location: Melbourne	I was planning on using a heatsink with vertical fins, with the chip attached so that its legs are pointing downwards. A similar arrangement to Vikash's TATCA.

30th July 2006, 02:28 PM	#5
alleycat diyAudio Member Join Date: May 2005 Location: Melbourne	OK, makes sense. Thanks!

31st July 2006, 10:10 AM	#10
sangram diyAudio Member Join Date: Sep 2002 Location: India	Yup, should. The hottest part is somewhere on the back behind the plastic top. A clamping bar across the chip face should be the best way to mount the chip, better than using the tab. Normally the center of the die is used for the hottest parts. This may not be an exception.

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