Pin4 Description of LM3886

[Jay]

I have several National LM3886T. Most of them have pin4 (-Vcc) connected to the body, as if it was designed for single supply circuit. I have been long confused with this Anyone has the explanation?

[MWP]

Why does that make you think that its for single supply?
Not unusual for a tab not to be ground.

[Jay]

Honestly I guess have never found an IC of this size with mica washer. I didn't find precaution about this either in the pdf documentation, while we know that this is a serious issue. I hardly believed my eyes when I noticed that the pin was connected to the body. The suspicious thing was that there are 4 LM3886T that do not have their pin4 connected to the body! And these 4 chips have exactly the same batch numbering.

[MWP]

You sure?
Ive used LOTS of LM3886T's all have the tab connected to -ve power.
The ony ones that dont are the TF's which obviously have the tab insulated.

[Jay]

So I'm using the abnormal one? I thought the majority of my LM3886T were abnormal or something

BTW, how do you isolate the pin from ground? By isolating the heatsink from chasis? Hmm... lately I have been thinking to find a good circuit for single power supply design. But my best results with amps have been with symmetrical power supply???

[MWP]

The normal way, mica or silicon impregnated rubber insulators.

[Jay]

Correction: my best results have been with single supply!

Having been working in QA Department, I'm confused with the fact that my chips have different properties. No, not confused but curious. Same as my curiousity on the availability of SK135/J50 nowadays.

I have heard other people crescendos built using "new" Hitachis and I just think that the character of the sound is not right (it is difficult because I use a lot better components, and speaker too). The body of the transistor is very clean, contrast with my new old stocks.

Bottom line, I don't trust my chips

[peranders]

Most silicone chips have their most negative electrode at the bottom of the chip

=

case or lug plus an additional pin for soldering.

[janneman]

Traditionally in power chips/transistors the tab was always connected to some internal node for the purpose of getting the heat out of the power die out as efficient as possible, not specifically for electrical connection.

It is only in later years that improved manufacturing processes have allowed isolated tabs (which lowers manufacturing costs) while still keeping up the efficient transfer of heat to the outside world.

I would not rely on the tab for electrical connection, use the provided pin to be sure of the best and intended connection.

Jan Didden

[peranders]

Originally posted by janneman
I would not rely on the tab for electrical connection, use the provided pin to be sure of the best and intended connection.

Most of the time one pin is in one piece with the tab. For TO220 and similar packages the center pin is always in contact with the tab. One easy way to detemermine that is the measure the resistance. If it's milliohms, it's probably the same metal piece.... but it doesn't hurt to read datasheets first.