Texas LM3886TF insulated case check

[little wisp]

1.The Texas datasheet for this device refers to sub reference codes to determine case insulated/not insulated.
"See Package Number NDJ0011B for Staggered Lead Non-Isolated Package or NDA0011B for Staggered Lead Isolated Package"
Mine have LM3886TF JM76RU, nothing else.
The chips look to be plastic all over including 'in the holes' - insulated?
PS After reading Rod Elloitts heatsinks article I am keen to get mounting as good as it can be. Can anybody guide me here please?
Photo needed?

2. Various thread comments seem to confer these chips are not quite so good for 4 ohm load (in my case 3.9ohm full range drive units from a kaput Tannoy I30 system.)
Any advice on which 'non class D' chip amp is ideally suited to a 4ohm load would be great.
I understand these thing hinge around supply rail voltages etc but generally those in the know seem to hint that 8 ohm loads get the best out of the chip.
PS I also have LM1875 chips but the same comments seem to apply with the 4ohm load.

 

[picowallspeaker]

...the problem is with the protection circuits that activate earlier on 4 Ω load because there's more current flowing.
Just keep the voltage rails low enough as suggested in the datasheet (s )

 

[little wisp]

By keeping supply rails lower I presume power is limited to within amounts that (at 4ohm load) cannot trigger protection circuits?

 

[Pjotr]

I've running LM3886TF's from 24V/120VA transformers (35V + and - DC rails) without problems into 4 ohm. Spitting out just over 80W RMS into 4 ohms before clipping.

Most important is cooling of the chip! Use a thick enough heat sink (at least 12mm solid alu behind the chip) so the heat can be drained fast enough and use high quality thermal grease. Then the chip stays cool enough to avoid the Spike protection and overtemp protection to come in.

For 4 ohms better don't go higher than 22V for the transformer. But sufficient cooling stays important.

 

[tomchr]

The LM3886 is rated for 7 A peak output current, worst case. So the max output voltage with a 4 Ω load is: Vout_max = 7 * 4 = 28 V.

If you keep the rail voltage limited to ±28 to ±30 V (allowing 2 V drop across the LM3886), you'll be fine and the LM3886 will not hit the current limit. To get a rail voltage in this range, use a transformer with a 2 x 22 V RMS secondary as Pjotr said.

Also: Keep the power dissipated in the LM3886TF (insulated package) below 40 W to avoid tripping the thermal protection circuit. With the LM3886T (non-insulated, metal back package) you can go a little higher.

The best thermal grease I've found is Wakefield 120. You can get it at Mouser, Digikey, et al. Wakefield 126 is good as well. The difference between the two is that the 126 handles the solvents used to clean the boards in manufacturing better than 120. For the hobbyist there's no practical difference between the two.

For the details on how to size the heat sink, I suggest taking a look at my Taming the LM3886 - Thermal Design page.

Tom

 

[little wisp]

Many thanks