Heat Sink for a LM4780?

I tried asking this at DIYAudio.com forum and got the typical answer........None. I guess unless your name is Nelson Pass, getting an answer is like pulling teeth.

Anyway.......I'm within about a week or two from ordering a LM4780 kit from AudioSector and the rest of the parts needed to build a 4ch. amp. I have been working on case design and am wondering about the heatsink. I have some 1.5" x 3" aluminum channel I was thinking about using for the sides and mounting the chips to. At about 7" long, what is the opinion about using them as heatsinks? One on either side for each chip. It's just simple channel without fins. But, it is a pretty good size (1.5" X 3" x 7"). If this is enough, I will go with it as I already have the channel.

Alternatively, I could go for a 3" length of the 7.28" wide heatsink from Heatsinks USA. Store Surely this would be enough. If this is the route everyone thinks I should go, what would be a minimum length? I am thinking that the 7.28" would be plenty.

I have tried to calculate the channel to see if it is enough but, The numbers are beyond what I can understand.

Here is a link to the datasheet for the LM4780 (It's to big to attach.)

http://www.ti.com/lit/ds/symlink/lm4780.pdf

 

All I know is that there are specs on page 4 of the datasheet and formulas to calculate heatsinks on page 17. I just don't understand how to read the specs, let alone understanding what answer I may get from the formula.

I had planned on insulating the chip from the heatsink. I am also going to mount it with a strap, as opposed to screws directly on either side of the chip die.

 

Thanks Steve. I got more out of reading your last post a couple times and thinking about it for a few minutes than an hour previously spent on tech that was over my head.

Looking over the data sheet again, in this case 65W would be a closer call on maximum package dissipation (+/-25v @4ohm load). Of course, they will be no where near this load continuously. Each chip will each see the same load. Left chip high and low freq and right chip the same. Of course, the "high" freq side will not require the same power as the "low" freq side of a chip. With that, referencing that last amp I used which had VU meters on the front, typical levels observed where peaking in the range of 5W to 8W on the low freq side. I don't know if they were measuring peak or RMS values but, with the "peak hold" button on the meters were in that range. If they measured RMS, then peak would be in the range of 12W. Of course, some margin above this would be good from a heatsink POV but, as you point out, running continuously at the chips peak output will never happen. A sine wave just before clipping isn't pleasant to listen to.

The part I didn't understand was "1.5 DegC/W budget". If this is a reference to what a heatsink will dissipate, I don't understand where/how you arrived at that figure.

Another part I don't understand is the specs on the heatsinks from HeatSink USA. They seem, to me anyway, to be backwards. The 7.28" section, @ a 3" length says....

"C/W/3": approximately 1.30"

Where as the 3.5" section @ 3" length says...

"C/W/3": approximately 2.50"

I am sure I am interpreting this wrong but, it seems like the smaller section dissipates more heat with the high C/W number. Here are links to the above mentioned heatsink sections.

7.28 section

3.5 section

From what I have read, "C/W" is Celsius/Watt. I am thinking (apparently wrongly) that a higher number cools more.

I have been playing with the HeatSink Calc but I'm not sure what it all means. Attached is a screenshot of what I entered. The bottom of the left hand column asks for an enter named "Watts". I used "30" from looking at the datasheet on page 4 where it says "Thermal Resistance" and says "30 C/W". On this, I have NO idea if it is right or not.

The dimensions I used in the HeatSink Calc are those of the 3" aluminum channel I mentioned before. Looking at the numbers the calc generated, I am clueless as to the channels ability to pull sufficient heat away from the LM4780 chip. Plus, I don't know how to allow for the insulating pad under the chip.

Thanks again for all the help, Steve. Any more info you can pass on, I'm sure, will be enlightening.

 

OK...I think it's starting to come together in my head. Going by the Heatsink calc, the aluminum channel is good for 3.779C/W. And as per the datasheet, the die case is at .8C/W and the thermal compound is at .2C/W. So this gets me to 4.779C/W. Chip protection comes in at 150C. 150 / 4.779 = 31.38watts. In theory, the protection would kick in at a sustained power output of 31.38watts or above.

Now, same numbers but with the 7.280" heatsink sited above, 1.3C/W + .8C/W + .2C/W gives 2.3C/W. 150 / 2.3 = 65.21watts.

Given that the chip really starts to distort at 50watts (+/-25v on a 4ohm load), the 7.28" heatsink from Heatsinks USA would cover the chip and then some, under any conditions.

Real world, the channel would probably work as well. At 31.38watts, that's more than I would push it in normal use. In fact, it would probably about peak wattage when I push it.

This amp will be used to power a pair of near field monitors. They are less than 1meter from my head. Efficiency is listed as 91.5dB/m for the woofer and 90dB/m for the tweeter. Between actual dB measurements, the dB specs for the drivers and the VU meters on the previous amp, I have to believe what the meters have told me. That being, when I push them, the most I will use is about 30-35watts, and 5-8watts under normal use.

So, do I have this sorta figured out or am I still lost? If I am about right on the calcs, what would you do, regarding the aluminum channel vs the proper finned heatsink?

 

Thanks for the insights, Steve. I wasn't aware of number 1 but do know about 2 & 3.

Regarding your last sentence...

"If you're only building a single unit, it's probably easier to design it as if it were going to have a constant load, knowing that it will run a lot cooler than you calculate."

That is the case here. Just a single unit. Since my last post, I have given it some more thought and decided that the 7.280" X 3.5" (2RU high) heatsink from HeatSink USA would be the best route. Not that the channel wouldn't work day in and day out. But why push it? There is no such thing as to much heatsink, where the chip is concerned. I can fit them comfortably on either side of the enclosure and they, I think, will be aesthetically pleasing.

One last question. Thermal paste or insulating pad? I know the sinks will be electrically "hot" with the paste and not with the insulating pad. Would it be a problem if the two chips (and their own heatsink) were tied together electrically? The back of the die is tied to the V- pin out of the chip. (ref.page 3 from the datasheet).

Here is a link to the schematic from the AudioSector kit if it is helpful.

 

Attached is a quick drawing of what I had in mind, as viewed from the top. The heatsinks make up the right and left hand sides of the enclosure. The question about them being tied together comes from the use of what material to use to tie the enclosure all together, aluminum or wood/plastic. The front will be a nice piece of wood. It's the back, top & bottom where I have the choice. For me, aluminum is the easiest to work with and to get.

 

I was just going over the supplied parts list from AudioSector for the LM4780 kit and they do include insulating pads. My plan is to just sit the amp alone on a shelf between the two speakers and behind my computer monitor. Though, I am considering adding removable rack mount ears for the possibility that I may put it in a rack sometime in the future.

 

Thanks for all the help, everyone. I was going to post the build of this amp when I get all the parts but, I guess that would be better for a new thread.

Thanks again