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Medium power transistor question

edited February 2014 in Ask an Evil Mad Scientist
I'm looking for some medium-ish power transistors to drive an 8x8 LED matrix I bought from EMS a while back and was thinking I might as well go with something that I could use as replacements for my Peggy 2 should there ever be a need.  However, Digikey doesn't appear to stock the 2STX2220 these days so I was wondering if you had any suggestions re: substitutes.  Also, the 2220 seems like overkill for the Peggy 2 and I was wondering was that to provide flexibility for those wanting to drive high output LEDs, a bunch of them fell off the back of a truck and you got a great price, it was Tuesday when you spec'd them and you always use 2220's on Tuesday, or some other reason?



  • When we designed in the part, it was an active production part, easily found in distribution, and not remotely overkill.  Unfortunately (1) they have now been discontinued in the through-hole package, and (2) most of the comparable parts do not have the same pin order. 

    The actually important spec-- the reason that we were using this type -- is the saturation voltage.  If the board needs to be able to run from 4.5 V (as do the Peggy 2 and Meggy Jr RGB), and the LED drivers drop a certain voltage, and you need to be able to drive blue or white LEDs that take up to 3.6 V to run, then there's only about 0.5 V (or less...) available for the high-side driver to take up.  

    So, your high-side driver needs to be able to handle roughly 25 LEDs * 25 mA = 625 mA (25 LEDs for a Peggy 2 row, or 24 LEDs for a Meggy Jr RGB row), and have low saturation voltage, hopefully spec at < 0.5 V at 1 A.  And, if you search for those two requirements (plus packaged in a through-hole package), you'll find very few options.  One is the "big expensive brother" of the 2stx2220, the stx790A (also now discontinued).  Another is the BC640TA, which might be a good choice for you.

  • Ah... I was looking at the current handling capabilities (hence the overkill comment) and hadn't considered the voltage drop issue.  Now it makes perfect sense how you arrived at that part and what I need to look for.
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