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Strength of CD4000 substrate diodes

Discussion in 'Electronic Design' started by Joerg, Dec 7, 2006.

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  1. Joerg

    Joerg Guest

    Usually I am easy on them. This time I need to jam it pretty good and
    have to drive a few CD40106 inputs below GND at 1mA-2mA. TI's datasheet
    says 10mA is the abs max. Does anyone remember a manufacturer that might
    spec less than 10mA?
     
  2. Jim Thompson

    Jim Thompson Guest

    Joerg! Joerg! Joerg! Joerg! ;-)

    ...Jim Thompson
     
  3. Joerg

    Joerg Guest

    I know, I know. The topper was a design (no, not by me) where a whole uC
    plus periphery was supplied via one (!) port pin substrate diode. Guess
    they needed to save that one cent for a rectifier diode. Now that was a
    real white knuckle design.
     
  4. Jim Thompson

    Jim Thompson Guest

    What is your concern, finding a CD40106 that can't take 10mA, or are
    you looking for a preventative measure?

    Post your schematic ;-)

    ...Jim Thompson
     
  5. John  Larkin

    John Larkin Guest

    The old A series parts would SCR latch if you looked at them wrong.
    Pulling an input below ground would take out a power supply or some
    wirebonds. A few mA should be OK for a B-series part.

    Can you use an HC40106? They're a lot better.

    John
     
  6. Zak

    Zak Guest

    I 'designed by acident' a sync amp. The scenario: monitor wants
    composite sync at TTL level, computer delivers sync at video signal
    level. Monitor input is at 0 volts, no pull up resistor.

    Thus, I built a little amp with a CMOS inverter. This worked but the
    extra power supply bothered me. Then, I switch off the power, but the
    thing keeps working. Hmm, there was no voltage on teh input and teh sync
    output is insufficient as well.

    I decide to use just a decoupling cap, no power connection. Doesn't work.

    I take the decoupling cap off and bing the thing works.

    Must be the most ugly analog CMOS amp in existence. I suppose the
    monitor outputs some pull-up voltage whenever it detects a signal on the
    sync input. Never tried to figure it out; it worked and it was a one-off.


    Thomas
     
  7. Genome

    Genome Guest

    I'm not sure I get this stuff......

    Is that.......

    Joerg! Joerg! Joerg! Joerg! ;-)

    In a cheering sort of 'You Are The Man' sort of way?

    Or is it a

    Joerg! Joerg! Joerg! Joerg! ;-)

    In a kind of 'You are such a child, let me explain it to you' sort of way?

    DNA
     
  8. Jim Thompson

    Jim Thompson Guest

    The latter ;-)

    ...Jim Thompson
     
  9. Genome

    Genome Guest

    Oh.... OK.......

    There you go Jeorg, e-mail Jim, but don't be a bottom boy, Texans have a
    problem with that stuff.

    Between yourselves.

    On a scale of 1 to 10 where 1 is not very cuddly and 10 is 'We are trying to
    explain to the wives about the lesbien type stuff'....

    Something else

    DNA
     
  10. Joerg

    Joerg Guest

    No, just wondering if there is a brand that specs much less. You know
    how that goes, purchasing finds a cheaper source and then it's like
    popcorn at final board test.

    Since I wrote the contract myself it doesn't specify the gun type with
    which I'd be shot. But it kind of says that I would be :-(
     
  11. Jim Thompson

    Jim Thompson Guest

    Can't you show us the source impedance equivalent? Then we can solve
    it for you ;-)

    ...Jim Thompson
     
  12. Joerg

    Joerg Guest

    The uC I mentioned was a lot scarier. AFAIR the supply came from a
    higher voltage AC source via a resistor, could even have been 117V,
    don't remember. Smack dab into the port pin. Anyhow, the "regulation" so
    that it doesn't exceed abs max worked something like this: Bypass cap
    was the minimum they could get away with but large enough so the boot
    procedure could get a hold. Then the uC executed lots of dummy code just
    to burn a fairly consistent amount of milliwatts. Supposedly the dummy
    code was adjusted on the fly so that the MIPS levels kind of balanced
    out slow workload phases. Woe to the firmware programmer who didn't know
    this.

    The way I found out was that I had to probe the oscillator. The probe's
    capacitance stalled it for a brief time. KAPOOF! Big crater in the uC
    and a puff of smoke wafting off.
     
  13. Joerg

    Joerg Guest

    Yes, I remember the first generation chips. Those were the ones that
    gave the 4000 series such a bad rap and scared a lot of people towards TTL.

    I don't remember if the 74C series was as fickle. I never had one die on
    me but others weren't very happy with them.

    Unfortunately not here. Got to work with 12V.
     
  14. Mike Monett

    Mike Monett Guest

    Joerg, how could it blow a hole in the IC? That takes more than a
    few watts.

    If the uC only needed milliwatts to regulate the voltage, where did
    the extra power come from? Especially since the bypass cap was the
    minimum they could get away with.

    There has to be some other explanation.

    Regards,

    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    http://silversol.freewebpage.org/index.htm
    SPICE Analysis of Crystal Oscillators:
    http://silversol.freewebpage.org/spice/xtal/clapp.htm
    Noise-Rejecting Wideband Sampler:
    http://www3.sympatico.ca/add.automation/sampler/intro.htm
     
  15. Joerg

    Joerg Guest

    It's a 10uF cap on the input that is whipped around. During one phase it
    is pulled to -12V via a 5K resistor. I could hang a largish resistor
    between the cap and the CD input but I'd rather not.

    It should be fine as long as the CD40106 all have the same specs.
     
  16. Joerg

    Joerg Guest

    No idea. All I know is that I had a little crater in there. The bypass
    wasn't that small since there was a whole lot of other stuff that rode
    on this VCC rail. It was an electrolytic and they didn't spring an extra
    cent for a ceramic in parallel. Later I looked at the supply rail on a
    working unit and the ripple on there was nauseating.

    What possibly happened is that some PWM stopped, shedding the whole
    load. Then the voltage crept up. This was a really old uC back from the
    80's. The AC drop resistor was a big fella, maybe a watt or so. It got
    quite toasty.
     
  17. Joerg wrote...
    I imagine all modern parts can take at least 20mA, or more, whatever
    spec they might put out. It's easy to test representative production
    parts to test reality... SCR supply breakdown is easy to observe.
    A limiting resistor can protect your test sample for further analysis.
     
  18. Joerg

    Joerg Guest

    Yes, Win, I did such tests a while ago. They could stomach more than
    20mA. But that always leaves the nagging uncertainty about the next
    batch or chips from another manufacturer. Bottomline all designs here
    need to comply with respect to the datasheet limits. Except, of course,
    when doing scientific stuff like avalanching.

    BTW, just read a laser article (Elsevier) from Rowland. Vollmer,
    Fischer: "Frequency-domain displacement sensing with a fiber
    ring-resonator containing a variable gap". Wow, interesting things that
    you guys are doing over there.
     
  19. Andy

    Andy Guest

    The CD40106BC from National does not have this maximum current specified
    in the datasheet at all. Ok, it's marked "discontinued", but it's still
    in stock at some places.

    MM74C14 from Fairchild does not have it specified neither.

    -- Andy
     
  20. Allowing for an external Schottky diode clamp on
    the pcb layout would seem prudent.

    App notes on 4000-series astables and monostables
    may give some insight into the maximum recommended
    negative input current whilst working.
     
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