Chip amp temperature

I put together a small amp with the LM 3886 chip and it is currently powering my AR3A's which are 4 ohms. The power supply is putting out 28 volts +- and the unit operates without noise or any issues so far. I do not listen at very high levels and wanted to find out the current temp on the chip and the heatsink. The laser temp gauge indicates 95 F on the chip about 100 on the heatsink after about 3 hours of normal to moderate volume levels

My concern, after reading some threads that running 4 ohm speakers is not a good thing with these chips, is are these temps too high and will there be issues and possible melt down.?. I do have 8 ohm speakers to substitute and will do that to see the difference in operating temps.

Anyone have any advice on this? Thanks for the time Cheers Wayne
 
That doesn't seem excessive to me, not at all in fact. The maximum junction temperature (*) is 150 degrees Celsius or 302 degrees Fahrenheit, so there is still quite some margin for temperature differences between chip and heatsink.

(*): At this chip temperature, it doesn't go into thermal shutdown and it still meets the manufacturer's reliability targets, but it gets more reliable when you stay well below it.
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
The LM3886 will power down once its thermal limit is reached. Before that happens, you'll find that the SPiKe (SOA protection) will engage, which causes the THD to degrade significantly and harsh "chipamp" sound to be emitted by the speakers as result. With a 4 Ω load, the SPiKe system will engage sooner due to the higher output current.

I suggest reading: Taming the LM3886 Chip Amplifier – Neurochrome
Specifically the sections about output power and thermal design will be of value to you.

Tom
 
After TI took over Nat Semi they eliminated the "Overture Design Guide" spreadsheet. You will need a heat sink with thermal impedance less than 2 degC/Watt. Here ya go:
 

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The laser temp gauge indicates 95 F on the chip about 100 on the heatsink after about 3 hours of normal to moderate volume levels
Then that measurement is wrong.
If true it would imply heat is flowing from heatsink towards chip amp, which of course is not the case.

I am always dismayed when suppossedly "accurate measurements* fail to show the *basic concept* behind them, meaning these are not really understood.

Truth behind this, of course, is that your meter is measuring not *chip* temperature, by any means, but that of "a black plastic surface around it", quite a different thing.

What´s important is actual Chip temperature, which looks like a tiny blob of etched metal inside that plastic case, and soldered to a flat copper heat spreader.

If anything, you might pop that black epoxy over off without damaging the real chip (easier said than done).
 

christiang

Member
2009-12-19 12:27 am
I am always dismayed when suppossedly "accurate measurements* fail to show the *basic concept* behind them, meaning these are not really understood.


Hi all.


Well said. I know this is an old topic, but I figure some info might be useful to others (and I'm bored). So-called 'laser' thermometers are (unless I am wrong) just infrared sensors. The laser just shows the user what the sensor is looking at.


The important part is that these devices operate using some kind of assumption of blackbody radiation. A perfectly absorbing (thus perfectly emitting) object emits power proportionnal to the 4th power of the absolute temperature. But a non-perfect absorber emits a fraction of the blackbody power, at the same temperature. As a result, for instance, a gold brick (reflects IR) would be measured cooler by the infrared sensor than, say, a graphite brick--even if they were exactly at the same T.



I suspect the higher temperature of the OP's heat sink is just this kind of measurement artifact. Accurate temperature measurements should be done via thermocouples. Cheers!


--Christian