Lesson Learned … or Not

MC39100 pin out I’ve written quite a number of times about the perils of CAD software land patterns. Especially if you don’t have an exact match and need to adapt something close.

Recently, I was looking in my Eagle library for a low-drop out regulator, MC39100 is SOT223. It’s just a standard, run of the mill 7805 replacement. Nothing special. A million other parts share the same pin-out. Shouldn’t be a problem. Shouldn’t …

If I were to follow my own advice, it wouldn’t have been a problem. But did I follow my own advice? Well, not this part of it. I took for granted that all three terminal regulators follow the 78XX pin-out. Most do, but the LD1117A (below) does not. This isn’t the first time I’ve used a non-standard regulator, so I really don’t have an excuse.LD1117A pin out

Naturally, I assumed that the pin-out matched what I needed and I didn’t hunt down an LD1117A data sheet to verify their pin-out. Well, at least I didn’t do so until trying to get my new PCB to power up. Very sad.

So, is there a moral to this story? Probably. Most likely it would be twofold. One, if you’re repurposing a land pattern from a part that’s close, but not exact, double-check your work. Get both datasheets out and compare the pin-out.

The second part of the moral is, if you give advice … follow it yourself. Duh.

Duane Benson
Help! I’m blinded by the obvious.

http://blog.screamingcircuits.com/

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