LM3886 Heatsinking

[englandinacan]

Looked through the specs sheet, found the information for working out a heatsink rating - however it's cryptic to someone who hasnt studied heatsinks in depth (doesnt tell me what any of the parts of the equation mean)

Can anyone help out?

Looking to run two LM3886s off +/-37V into a 6ohm load, ambient temp wont be above 25C.

Any other information needed?

Spec sheet here: http://newzealand.rs-online.com/web/...duct&R=5342961

[AndrewT]

Page14 of the data sheet shows a graph of Pd (Watts) against Vcc (total supply voltage) for three alternative load resistances.

Follow the 74V (37+37) up, till it hits the 6r0 resistance curve.
OOPS it's off the end of the curve.
Stop, your proposed voltage does not suit your selected speakers.

For the sake of completing the exercise let's extrapolate the 6r0 curve. I think 74V will intersect at about 44WPd.
Now look left till you find your heatsink ambient temperature.
It seems to show 1.6C/W @ 25degC.
Two 3886 will require twice this so we are up to 0.8C/W.
National's minimum recommendation runs the chipamp junctions to maximum operating temperatures with a T type chip directly attached to a heatsink (no isolator).
My usual recommendation to avoid all the maths and experimentation that National do not give you is to double the heatsink rating again.
Look for a <=0.4C/W sink. Enormous.

That's why National have curtailed the 6r0 curve @ 70Vcc, even this would require a <=0.47C/W sink.

These are for a warm South Island summer with an external heatsink. A hot summer, without air conditioning will require an even bigger sink.
If the sink is inside the chassis, it needs to be bigger again.
Choose 8ohm speakers.

[pacificblue]

... and don't use the TF type you linked to with ±37 V.

If you already own those 6 Ohm speakers, choose smaller rail voltages for the LM3886.

The Overture Design Guide can do the math for you. Type in your rail voltage, highest ambient temperature and speaker impedance. The spreadsheet will recommended a heatsink size. Follow AndrewT's advice and choose a heatsink with half the recommended thermal resistance.

[ttan98]

If you want to save money you can even mount 2*LM3886 on one computer/CPU heatsink(Pentium 3 or 4) with fan.

If you are running 2*LM3886(see note) on 1 heatsink then you must turn on the fan continously(6-7 V fan is OK).

If you use only 1*LM3886 on 1 heatsink you don't have to turn on your fan, it will run cool to warm.

note:
I use 2*LM3886 in parallel driving a 12" woofer, dual LM3886 is capable of pumping out at least 100W, with 34VDC. No problem at all. Just ensure the casing is well ventilated.

[pacificblue]

Originally posted by ttan98
If you are running 2*LM3886(see note) on 1 heatsink then you must turn on the fan continously(6-7 V fan is OK).

If you use only 1*LM3886 on 1 heatsink you don't have to turn on your fan, it will run cool to warm.

That depends on the supply voltage, the load impedance, the ambient temperature, the sound pressure level, speaker efficiency, heatsink size, fan capacity, coupling efficiency from fan to heatsink and the LM3886 configuration (single, parallel, bridged).

A single LM3886 is not recommendable for ±37 V rails and 6 Ohm load.

[AndrewT]

Originally posted by pacificblue
That depends on the supply voltage, the load impedance, the ambient temperature, the sound pressure level, speaker efficiency, heatsink size, fan capacity, coupling efficiency from fan to heatsink and the LM3886 configuration (single, parallel, bridged).

A single LM3886 is not recommendable for ±37 V rails and 6 Ohm load.

Hi Pacific,
you've read the same book as me, "the internet is full of facts, lies and untruths".
The difficult bit is knowing what to believe.

[pacificblue]

Originally posted by AndrewT
The difficult bit is knowing what to believe.

It takes some effort to find out. For some that makes it tiresome, for others fun. Whichever it is, isn't the satisfaction priceless, when you discover that you have learned to believe the right thing every now and then?