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How to interpret this package drawing?

Discussion in 'Electronic Design' started by Michael, May 5, 2007.

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  1. Michael

    Michael Guest

    Hi - I'm trying to draw out a TO-263-5 from this datasheet:

    Thing is - it doesn't actually give dimensions - just minimum and
    maximum dimensions. It doesn't even specify the actual width!! What am
    I supposed to do here? Take the average of each value?


  2. John Fields

    John Fields Guest

  3. John Larkin

    John Larkin Guest

    What's stunning is that the nominal lead pitch is not specified. So if
    you guess or average to get the pitch, any error stacks up over all 5
    pins! So get a sample and measure it.

    And the compensation/stability guidelines suck, as is standard for
    LDOs. The "Stability" paragraph is just qualitative blather.

    I wouldn't use this part unless I absolutely had to.

  4. Eeyore

    Eeyore Guest

    Well..... I'd make the outline fit the max dimensions.

    The tolerance on the lead pitch is puzzling though. Maybe check out the Jedec
    standard for the package ? I'd take the average value myself.

  5. Michael

    Michael Guest

    That's what really caught my eye. It's ridiculous - I've never seen
    such a bad drawing. I got something hacked together that should work
    though, by averaging all the values.

    I found another company's drawing for TO-263-5s and they specified the
    pitch as 1.7mm, which is essentially the average of the min and max
    values (1.702mm).
    I'm saving a decent amount of money going with this part though, so
    the extra hassle is worth it, I hope. I don't need a particularly
    smooth supply - I just need an adjustable vreg capable of taking a 5V
    signal down to 3-3.5 V (adjustable) with ~200mv or less ripple, all at
    about 2-3A. (constant load)

  6. MooseFET

    MooseFET Guest

    This looks like a design that was in inches that has been converted to
    metric. Try converting the values to inches and see if a sensable
    sounding number lands right near the middle of the span.

    eg: 0.4" = 10.16mm. This is near the center of 10.05 and 10.668 so
    there is a good chance the design started with a 0.4" dimension. I
    would expect the real units to land most often just about at that
  7. Jeff L

    Jeff L Guest

    I'm short on time for details, but that is a standard 5 pin D-PAK or D-PAK2
    that's been thinned out to reduce the height requirements - the pads should
    be the same. The other is a standard SOIC-8.
  8. Long time ago, when still useful in design, and constantly using metric
    and 'colonial' measuring units we agreed to re-dimention the inch for
    easier conversion between the decimal- binary fragments of partial
    measurements. 1 inch = 25.4 mm { standard definition of inch } we
    defined as 1 inch = 25.6 mm and it made the conversions more practical.
    1/16" = 1.6 mm and so on ( the error is insignificant in practice as
    long as you are under many inches in length dimention, and it is
    _normal_ for many years to specify dimentions in mm's).


  9. It's also the same as IRF's drawing for the TO-262 (1.700 mm nominal),
    as you can see from this package drawing:

    It's laid out with proper GD&T tolerancing-- the numbers in boxes are
    "basic" dimensions that are theoretically exact, and the position of
    each lead is defined off the 'A' datum in the maximum material
    condition so you can easily figure worst case conditions.

    It would be tempting to guess that the nominal dimension is the same
    as the SDIP pitch of 1.778mm nominal (0.070"), but it'snot.

    Best regards,
    Spehro Pefhany
  10. John Larkin

    John Larkin Guest

    In the early days of IC design, when mask patterns were cut from
    rubylith, an inch was defined as 25 mm.

  11. John Fields

    John Fields Guest

    Yeah, it is, but I was wrong about "average"

    For the TO-263 package the pitch is the sum of half of the
    difference between the max and the min and the min of "G". IOW:

    1.829 - 1.575
    pitch = --------------- + 1.575 = 1.702mm
  12. One nice thing about the GD&T (eg. ASME Y14.5M or ISO) method of
    tolerancing (as on the other IRF drawing I linked to) compared to
    coordinate tolerancing is that there is no build-up of tolerances. The
    theoretically exact "basic dimension" is given and the position of
    each pin is specified to lie within a certain tolerance zone. To get
    the same effect with coordinate tolerancing you'd either have to put a
    stupidly tight tolerance on the pitch, dimension every pin, or add
    some kind of note to the drawing.

    Best regards,
    Spehro Pefhany
  13. YD

    YD Guest

    Late at night, by candle light, John Fields
    a - b
    Let's write that as ------- + b which we then can write as

    a - b + 2b a + b
    ---------- = -------
    2 2

    Sure looks like an arithmetic average to me.

    - YD.
  14. Hey pooh pooh, why you going around behind my back looking for software
    to wreck a froup? You know I'll do it for frei!

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