ESD Zener diode shunt to ground or power rail?

Discussion in 'Electronic Design' started by kean, Dec 28, 2004.

  1. kean

    kean Guest

    Hi,
    I normally connect Zener diodes for ESD protection to ground so that
    the ESD event will be shunted to ground. However i have noticed a
    design that actually connects the ESD zener diode to the power rail
    (3.3V).
    Does anyone know why it is done this way instead of shunting it to
    ground? What would be the advantages and disadvantages of doing it
    this way?

    Thanks in advance,
    Kean
    kean, Dec 28, 2004
    #1
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  2. kean

    Robert Baer Guest

    kean wrote:
    >
    > Hi,
    > I normally connect Zener diodes for ESD protection to ground so that
    > the ESD event will be shunted to ground. However i have noticed a
    > design that actually connects the ESD zener diode to the power rail
    > (3.3V).
    > Does anyone know why it is done this way instead of shunting it to
    > ground? What would be the advantages and disadvantages of doing it
    > this way?
    >
    > Thanks in advance,
    > Kean


    1) you said "zener". Reasonably abrupt knee zeners are above 5.6V
    which is not too good for such low voltage devices.
    2) tied to the supply line, the forward region would prevent the
    voltage from going about 0.6V more positive, and the negative zener
    region would prevent the input from going (6.0V-3.3V =) 2.7V below
    ground (ASSuMEing the power is on).
    3) i am assuming the devices can withstand short negative input energy
    bursts without damage. CMOS cannot withstand energy bursts above the
    supply rail.
    Robert Baer, Dec 28, 2004
    #2
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  3. kean

    James Meyer Guest

    On 27 Dec 2004 23:55:14 -0800, (kean) wroth:

    >Hi,
    >I normally connect Zener diodes for ESD protection to ground so that
    >the ESD event will be shunted to ground. However i have noticed a
    >design that actually connects the ESD zener diode to the power rail
    >(3.3V).
    >Does anyone know why it is done this way instead of shunting it to
    >ground? What would be the advantages and disadvantages of doing it
    >this way?
    >
    >Thanks in advance,
    >Kean


    Voltages are always two terminal sources. In other words, a voltage at
    one point must be specified with respect to another point somewhere.

    If you expect ESD voltages to be referenced to ground, then it makes
    sense to return the diode to ground. If, on the other hand, the ESD is expected
    to be referenced to the power rail, it makes sense to return the diode to the
    rail.

    ESD protection design is no more or less important and worthy of careful
    consideration than any other part of a design.

    Jim
    James Meyer, Dec 28, 2004
    #3
  4. kean

    nospam Guest

    (kean) wrote:

    And another

    >Hi,


    Solve my problem

    >Thanks in advance,


    post from a google information leech.
    nospam, Dec 28, 2004
    #4
  5. kean

    Rich Grise Guest

    On Tue, 28 Dec 2004 12:22:10 +0000, James Meyer wrote:

    > On 27 Dec 2004 23:55:14 -0800, (kean) wroth:
    >
    >>Hi,
    >>I normally connect Zener diodes for ESD protection to ground so that
    >>the ESD event will be shunted to ground. However i have noticed a
    >>design that actually connects the ESD zener diode to the power rail
    >>(3.3V).
    >>Does anyone know why it is done this way instead of shunting it to
    >>ground? What would be the advantages and disadvantages of doing it
    >>this way?
    >>
    >>Thanks in advance,
    >>Kean

    >
    > Voltages are always two terminal sources. In other words, a voltage at
    > one point must be specified with respect to another point somewhere.
    >
    > If you expect ESD voltages to be referenced to ground, then it makes
    > sense to return the diode to ground. If, on the other hand, the ESD is expected
    > to be referenced to the power rail, it makes sense to return the diode to the
    > rail.
    >
    > ESD protection design is no more or less important and worthy of careful
    > consideration than any other part of a design.
    >


    Whenever I've ever done inputs, I just use a couple of ordinary 1N4148s or
    1N914s, one to each rail. Sometimes a 10R resistor in series, or so,
    depending.

    Cheers!
    Rich
    Rich Grise, Dec 28, 2004
    #5
  6. kean

    Pooh Bear Guest

    kean wrote:

    > Hi,
    > I normally connect Zener diodes for ESD protection to ground so that
    > the ESD event will be shunted to ground. However i have noticed a
    > design that actually connects the ESD zener diode to the power rail
    > (3.3V).
    > Does anyone know why it is done this way instead of shunting it to
    > ground? What would be the advantages and disadvantages of doing it
    > this way?


    Well..... it would avoid 'ground noise' that might cause malfunctioning.

    As others have noted, zeners aren't really good at this. Small signal
    diodes to ground and supply make more sense. You'll need a smallish
    value limiting resistor before them too. Most chips now actually have
    ESD specs for their I/O but the above gives extra 'security'.

    Graham
    Pooh Bear, Dec 29, 2004
    #6
  7. kean

    Guest

    Thanks for the answers. They were really helpful.

    I was wondering also another possible problem with connecting a zener
    diode to ground would be that if the ground is too noisy, the diode may
    actually turn on causing noise signals to travel from ground into the
    signal line.
    , Dec 30, 2004
    #7
  8. kean

    R.Lewis Guest

    <> wrote in message
    news:...
    > Thanks for the answers. They were really helpful.
    >
    > I was wondering also another possible problem with connecting a zener
    > diode to ground would be that if the ground is too noisy, the diode may
    > actually turn on causing noise signals to travel from ground into the
    > signal line.
    >


    Noisy with respect to what?

    If the signal ground is noisy the noise is already injected.
    R.Lewis, Dec 30, 2004
    #8
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