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Capacitor Discharging with CMOS Gate

Discussion in 'Electronic Design' started by D from BC, Feb 18, 2008.

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  1. D from BC

    D from BC Guest

    Is this safe to do?

    4093 Cmos Schmitt NAND
    -------| \ <<<discharging
    | |0--|<--+
    -------|_/ |
    --- Initially at 14V
    --- 10uF tantalum

    Repetition rate: 6Hz

    On the datasheet
    The max output current spec is 10mA.

    But I think that spec is continuous current.
    Impulse current and internal impulse power is another thing.
    Could a die wire pop?
    Could the drive mosfet fry?

    I'm trying to dodge including a current limiting resistor.
    Extra part, extra pcb space, extra drafting...that sort of thing.

    D from BC
    British Columbia
  2. John Larkin

    John Larkin Guest

    No problem.

  3. D from BC

    D from BC Guest

    Cool thanks..
    Wonderful...That's one less part for my life support system circuit..
    (Just kidding..)

    D from BC
    British Columbia
  4. JosephKK

    JosephKK Guest

    I am not so cheerful about it. It looks to me that there can be a
    localized heating reliability problem with the CMOS gate output pin.
  5. Winfield

    Winfield Guest

    During the 10uF capacitor's discharge from 14V
    the 4093 gate's n-channel output mosfet has to
    dissipate 1mJ of energy in 5ms (assuming a 25mA
    discharge current). Doing this six times a
    second means the gate's die will dissipate 6mW,
    which is certainly not a problem, but this fails
    to fully settle the question of whether a 350mW
    peak power level will be damaging.

    D can try an acid test, by continuously running
    the output at full current and supply voltage,
    and testing for any reduction in the IC's life.

    4093 CMOS Schmitt NAND - acid test,
    +14 --+--| \
    | | |0--- +14V

    We assume the 4093 is powered from the +14V.

    The IC's datasheet says the Absolute Maximum
    package power dissipation is 500mW, at room temp,
    which implies 350mW will run the die temperature
    near the maximum rated value. That's not a very
    good idea, but it may be tolerated OK for a few
    ms every 167ms.

    I'd add the series resistor myself.

  6. Metal migration induced failures?

    Note: abs. max "Input or Output Current
    (DC or *Transient*) per Pin 10mA)

    (emphasis added)
    Indeed. Particularly with a 15V supply. Sink current is off the gaph
    (> abs max) at less than 2V Vds.
    Best regards,
    Spehro Pefhany
  7. Fred Bloggs

    Fred Bloggs Guest

    I don't think that process has the capability to induce metal migration,
    too weak...20mA never burned up anything.
    May be off the graph, but you can see the (Vgs-Vth)^2 characteristic
    across the Vgs, so that 1.75mA @ 5V => Vth~1.35V and [email protected]=15V, which
    is about in line with the curvatures shown there. This makes for 350mW
    peak and 6msec discharge time for 4% duty. It shouldn't be a problem. He
    gets about the same discharge time with a series 330R, current peak will
    be about the same, but peak power reduces to <150mW.
  8. D from BC

    D from BC Guest

    Yup.. Vdd=14V.

    Mmmm.. I might test.. I'm thinking about it..
    Testing SOIC chips are a pita without a breakout board..

    I think this is one of those problems where the datasheet can't
    provide every detail, the testing is a pita and putting in a current
    limiting resistor is the easy way out.

    I'm still learning that sometimes it's just not worth the time to
    dodge only one resistor in a circuit and accept more risk.
    On the other side, it's artsy to dodge a 'be kind to mosfets'
    resistor. :)

    So yeah say use the resistor ... :)

    D from BC
    British Columbia
  9. D from BC

    D from BC Guest

    I think the absolute max 'transient' note is conservative when it
    equals the DC absolute max current.
    However I'm thinking it's not worth the risk to find out.

    D from BC
    British Columbia
  10. JosephKK

    JosephKK Guest

    Actually, even uA currents induce metal migration. Some 30 years ago
    there was a recognized IC failure mode called Kirkendall voiding.
    Please Google before calling me a fool.
    This is getting interesting enough that i must ask someone who has
    access to decent test equipment to make some measurements. After 13
    years in test labs and 15 years in a job where test instruments are
    irrelevant i am down to a DMM. Se le guerre.
  11. Fred Bloggs

    Fred Bloggs Guest

    Metal migration has to do with extraordinarily large current densities
    and not power dissipation. Those weak little CMOSFETs in the 14000
    process can't do it, even the AC family can't do it.

    Assuming David's comments are valid, if those old parts have 10u wide
    metal, then >10mA and certainly 30mA can do it.
    Best regards,
    Spehro Pefhany
  13. Joerg

    Joerg Guest

    If VCC briefly dips for some reason and the cap had enough charge the
    whole thing might go into latch-up. That would not be cool.
  14. Jim Thompson

    Jim Thompson Guest

    On Thu, 21 Feb 2008 20:14:43 GMT, Joerg

    Actually that WOULD be cool ;-)

    ...Jim Thompson
  15. Jim Thompson

    Jim Thompson Guest

    An open-drain-output would most likely be safe, since their ESD is
    usually only an N-channel device, thus not path to VDD.

    ...Jim Thompson
  16. D from BC

    D from BC Guest

    Huh? Latch up?
    The gate is shunting the charged cap to ground.
    (Charging circuit not shown for clarity)
    Ideally, I think this shouldn't have an effect on the rails.
    What's it called...ground bounce??
    The cap dumps through the gate and blips the Vss rail.

    D from BC
    British Columbia
  17. Dave Platt

    Dave Platt Guest

    Huh? Latch up?
    Driving the output of a logic gate too far outside the rail voltage
    can trigger a latchup. This can happen with as little as one
    diode-drop of excessive voltage. The overvoltage/spike has the effect
    of activating a "parasitic" structure on the die (a set of P/N
    junctions not normally part of the circuit, but created in part as a
    side effect of the way the die is fabricated). If you're unlucky, the
    parasitic junctions form a PNPN structure, a.k.a. a thyristor or SCR.
    The spike "turns on" this thyristor, and it begins acting as a short
    circuit which often can't be reset by any means less drastic than
    turning off the power entirely.

    If the source of the overvoltage has enough charge it and a low enough
    impedance, the resulting current flow through the parasitic can
    destroy the chip.

    I heard a tale some years ago about an incident involving a new,
    prototype RISC CPU. The first silicon had just come back from the
    fab, in one of the special ceramic packages with a quartz lid (to
    allow for inspection of the chip). Chip was carefully powered up, and
    lo and behold it worked! One of the developers was so proud that he
    whipped out a camera and took a photo of the chip in the development

    Bad move. The camera's flash fired. The flash emitted enough UV
    (which went through the flash's lens, and through the clear quartz lid
    of the chip) to cause photoelectric-effect activation of a bunch of
    the exposed gates on the surface of the chip - a bunch of electrons
    got knocked loose by the bright light. A whole bunch of parasitic
    thyristor structures turned on, and (in effect) short-circuited the
    power supply to ground on multiple places on the chip.

    The camera's flash was followed, with no perceptible delay, by a
    bright flash _inside_ the chip's lid. Boom. Completely dead chip.
    Do not pass Go, do not collect $200, no saving throw possible.
  18. Joerg

    Joerg Guest

    Depends on what else is hooked up to that rail. Assume the cap is full.
    Now some big fat load dips that rail a little. Phssst ... POOF.
  19. Joerg

    Joerg Guest

    On CD4000 logic that is usually the case.

    Once the parasitic SCR has been triggered the impedance of the source
    doesn't matter anymore. You could completely remove it and the chip
    would remain in meltdown mode anyhow if the supply rail has enough
    juice. A loud bang, a stench, smoke alarms wailing, and so on.

    What did they do with the guy?
  20. D from BC

    D from BC Guest

    Afterward, did that developer go to the bar and got drunk?
    "ssshhhupid flashh... Friffffn flashh... Yah know..I...I ...I... never
    takn anoother picture in my liffee" :p

    D from BC
    British Columbia
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