Connect with us

building a first audio amplifier; isolation, heat transfer issues

Discussion in 'Electronic Design' started by [email protected], Sep 7, 2007.

Scroll to continue with content
  1. Guest

    So I found this page:
    http://www3.national.com/mpf/LM/LM3875.html

    .... and I'm confused by what they mean by "isolated TO220". It seems
    to be encased in plastic, which seems a very strange thing to do,
    since heat transfer is critical.

    If I get the ceramic one, is there a need to isolate the back of the
    chip from ground?
     
  2. Bob

    Bob Guest

    So if you look at the data sheet it explains the difference between the
    isolated and non-isolated version. So if you get the isolated version then
    the potential of the heatsink doesn't matter. So you need to learn how to
    calculate what your die temperature will be for your particular application
    and then you'll know what to do.

    Just learn what thetaJC and thetaJA for ICs means, and learn what thetaSA
    for heatsinks is.

    Bob
     
  3. Guest



    Thanks for the hints. Reading closely, yes I see the tab is V-. I
    thought it was ground.

    Interesting, the data sheet for the LM3886 shows a ground, but the
    LM3875 (discontinued?) does not have a ground pin.

    Thanks again,

    Michael
     
  4. Eeyore

    Eeyore Guest

    Isolation is often critical too.

    Isolated TO-220s are very common. They're very useful too. The isolating
    hardware is often a real nuisance. The data sheet states 1C/W thermal resistance
    from junction to case which is absolutely fine for an application like this.

    What are you talking about ? There isn't a 'ceramic one'.

    Graham
     
  5. Eeyore

    Eeyore Guest

    Op-amps don't have ground pins either.

    Graham
     
  6. Guest


    ceramic one = non-isolated one... right?

    http://www.national.com/mpf/LM/LM3875.html
    LM3875T, vs. LM3875TF.

    Or, since National is implying the 3875 will be discontinued, LM3886T
    vs. LM3886TF.
    http://www.national.com/mpf/LM/LM3886.html

    Thanks guys

    Michael
     
  7. Interesting that they appear to rate the thermal resistance identically
    for isolated and non-isolated versions. I'm not sure I believe the
    package material's thermal resistance matches that of the bare metal.
    No. Generally it means bare metal (ceramic is usually an insulator
    after all). In this case it does mean bare metal, note that the
    datasheet refers to the tab on the non-insulated version being live.

    Robert
     
  8. Eeyore

    Eeyore Guest

    No ! Where does NS mention ceramic ?

    Non-isolated simply means the plastic doesn't cover the tab.

    Graham
     
  9. Eeyore

    Eeyore Guest

    Probably too little difference to be worth printing. The tab is quite large, so any
    difference is likely to be in the region of 0.1C/W or less in my estimation.

    Graham
     
  10. An iso pak from IR appears to add about 2 C/W (see
    http://www.irf.com/product-info/datasheets/data/irliz44n.pdf and
    http://www.irf.com/product-info/datasheets/data/irlz44n.pdf) The
    national package appear to be about twice the area so I would have
    expected a contribution of around 1 C/W from the isolation. While they
    might be better tham that, I'm having trouble with them being 10x
    better.

    I've seen TO-220 down to 1 C/W so 0.5 C/W for the National case if the
    die is large would not be shocking but I'd have expected a significant
    difference between the isolated and non-isolated versions.

    Robert
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-