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  • EEVblog #16 – All about CMOS SCR Latchup

    Posted on July 4th, 2009 EEVblog 13 comments

    All about CMOS SCR Latchup

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    • Eric

      very interesting info,

      Dave, I received my oscilloscope (DS1052E), do you have any good advise as far as how I can learn to properly use this thing? a course or a book perhaps?

      You seem to know your stuff (possible understatement), any advise would be great!

      First time scope owner, and I’m a cheapskate too.

      Thanks! Eric

      • http://www.alternatezone.com/eevblog admin

        That’s a tough one, don’t know any good beginner tutorials off-hand. Plenty of info on the web of course, googling “oscilloscope tutorial” brings up plenty of stuff, some good, some bad I’m sure – but how to wade through it all?
        This looks pretty comprehensive at first glance:
        http://www.hobbyprojects.com/oscilloscope_tutorial.html

    • Anthony

      Excellent post as per usual David. As much as I love the wonderful equipment reviews, it is especially nice seeing some very relevant theory and applications. Please keep it coming!

    • Andrew

      Eric, the biggest issue you’ll face when learning to work with an oscilloscope is where to get some signals from. Without some input signals it gets rather boring, quickly. And the built-in probe calibration signal generator is not very interesting.

      So my advice is that you grap a couple of transistors, OpAmps, 555th, MCUs, or whatever you like, and build a few signal generators. The web is full of schematics for building simple sawtooth, rectangular, and sine wave generators, with a couple of the aforementioned components.

    • Brian Hoskins

      Learning to drive scope is a little bit like learning to drive a car. I’m sure you could find some theoretical stuff on it which would arm you with a good background in the basics, but in the end the only *real* way to become a good driver is to get out and learn for yourself. And, as with all things, practice makes perfect.

      So yeah, read some stuff – find out what timebase is, find out what a trigger is, etc. Then after that the only way forwards is to use it on some real life applications.

      Brian

    • Eric

      Great advice guys, My roommate has a special motor he built that i hooked it up to, looks pretty cool on the scope!!! :)

      Also built a simple 555 circuit and played with it.

      Thanks for all the help guys,

      And Dave, the website you linked was very helpfull, Thanks again.

      Eric

    • http://paulandsabrinasevstuff.com Sabrina Holmes

      I would suggest using a fast schottky diode (0.3v drop) to stay clear of the -0.6v or 5.6v.

    • Sean

      SCR Latchup = inadvertent creation of a crowbar circuit between power and ground.

    • walter delbono
    • Wil Easton

      Many thanks Dave for these wonderful videos. As an ex-professional electronics/avionics engineer I can’t get enough… As well as being knowledgeable, your method of presenting is totally addictive in the same way that first delving into Horowitz & Hill was all those years ago!!

      On the topic of CMOS latch-up I enjoyed your explanation on the whiteboard but think you may have overlooked one vital point, unless of course I wasn’t paying enough attention (which is quite possible!). The key point is this – even when a short-lived initial stimulus is removed, the latch-up *continues* to short-circuit the device because Q1 turns-on Q2 and vice-versa, forming an everlasting closed-loop, and it is this internal state which disables device operation and can lead to permanent damage to the device.

      I don’t know if you want ideas for future episodes, but what about covering signals with fast rising & falling edges, ground-bounce, and the need for controlled impedances & terminations? Regards and best wishes.

    • ac

      Wil: The word “latch” implies the “everlasting” effect.

      Now what I’d like to see is, lets say I just got my first PIC/Atmel whatever and want to put it to a breadboard. What’s the tidiest way of implementing all these ESD protections and possible terminations for incoming data? I heard there’s technology to build the chips so they don’t have these ESD issues, why aren’t Microchip etc using that technology? Or if they are, well let me know!

      I haven’t even got started with the micros I got in my deskdrawer because I’m afraid just taking them out of the bag will kill them! (Another note: I notice some shops don’t ship their chips etc in the static shielding bags!)

    • pierre

      > I heard there’s technology to build the
      > chips so they don’t have these ESD issues,
      > why aren’t Microchip etc using that
      > technology?

      If you wanted to protect all IO pins with clamp diodes, those would have to be large enough to absorb all possible parasitic currents…

      This max current is unspecified, but even with just a few 10’s milliamps rated diodes, on your average chip the diodes would take up more silicon than the chip itself… and add parasitic capacitance, slowing things down, increasing power draw, leakage, etc.

      Besides if you could put a perfect clamping diode, well if you put a 5V IC’s input to 6V, the diode would dump the input current into the Vcc rail… which becomes 6V and the chip dies.

      Most chips have (very small) clamp diodes anyway, to protect against ESD. But you shouldn’t count on them for anything else…

      Better leave it to the designer who can choose to put the right king of protection (clamp diodes, maybe a zener, etc) only on the pins that need them.

    • Kurt L.

      You can find great oscilloscope tutorials on the Tektronix website:

      http://www.tek.com/learning/oscilloscope-tutorial

      Disclaimer: I am not affiliated with Tek, though I have fond memories of how much I learned working there.

      Kurt