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Right now I am creating the schematic and the final pcb layout of my circuit in Diptrace so that I can etch the PCB myself.

I am going to use LM2985-3.3 SMT LDO from Texas Instruments in my circuit to step down the 5V supply. This IC is available on SOT23-5 package. Since I could not find this component in the default libraries, I am attempting to create my own.

Couple of questions

  1. In SOT23-5, what does 23 and 5 stand for?

  2. The chip dimensions in the datasheet are not exact, but a pair of min/max. Why is this so ? While drawing the component, which one do i take into account? enter image description here

  3. Lastly, any good link to a good tutorial for creating custom component/pattern in Diptrace?

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  • \$\begingroup\$ SOT23-5 is often known as SOT25. The -5 refers to the number of pins on an SOT23 package, which is a perfectly standard package which I would be incredibly surprised if it wasn't there. \$\endgroup\$
    – Majenko
    Oct 26, 2014 at 9:19

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The "SOT" is a "Small Outline Transistor", with 2mm distance between outer pins on one edge.

The "3" portion refers to the number of pins on the package, so an SOT23 is a 2mm small outline transistor with 3 pins.

The SOT23-5 is actually an incorrect designation, and should really be SOT25, though SOT23-5 is commonly used.

In this case the -5 refers to a 5-pin version of the SOT23, which would be a 2mm small outline transistor with 5 pins, which by the above numbering scheme should be SOT25.

Chances are you will have a generic SOT23-5 or SOT25 footprint in Diptrace since it is one of the most common size of surface mount transistors / small devices around.

If you do need to make a new footprint you shouldn't be working from the chip's dimensions, but from the provided recommended footprint or "landing pattern" in the data sheet. If a footprint isn't given, and it's a standard size like SOT25, then they assume it will fit any generic SOT25 footprint.

By the way, I use TI's SOT25 LM(something) regulators all the time (I forget the number off hand, might be the same ones), and just use the default SOT25 footprint in my PCB software and they work perfectly.

I haven't used Diptrace, so I don't know how to create footprints in it.

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  • \$\begingroup\$ Thank you for the awesome explanation. Was indeed able to find the generic SOT25 footprint in Diptrace. \$\endgroup\$
    – Ankit
    Oct 27, 2014 at 6:31

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