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[IC] Electrostatic charge in switched-capacitor circuit

Discussion in 'Electronic Basics' started by Boki, May 10, 2004.

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  1. Boki

    Boki Guest

    Dear good guys,

    We have a question in switched-capacitor circuit that is

    when we design switched-capacitor circuits, do we have to consider the
    problem of "electrostatic charge" ?

    does it is possible the capacitor was "full of electrostatic charge" before
    circuit connected to power supply?

    does it is possible the full of electrostatic charge capacitor causes IC
    borken?


    Thank you very much!

    Best regards,

    Boki.
     
  2. Boki

    Boki Guest

    Jamie, thank you very much, but I am a little confusing about it.

    You mean there is a switch/delay circuit that perform discharge task first,
    and then while switched-capacitor circuit start work?



    Thank you very much!

    Best regards,

    Boki.
     
  3. Jamie

    Jamie Guest

    yes, it can cause problems like current surges.
    that is one reason you many use a timer to delay
    on a relay mean while a resistor is used to get the
    charge in the cap going so that when the relay
    does switch in it will not surge it.
     
  4. Jamie

    Jamie Guest

    actually the other way around.
    you use a resistor to slowly charge the cap to get it
    out of the region of where the inrush surge would be
    tomuch for the ic. i am not sure exactly what your
    problem is ? maybe you have highvoltage building on the
    cap while it is left unloaded ? if that is the case you could
    put a drain resistor on the cap (always).
     
  5. CFoley1064

    CFoley1064 Guest

    Subject: [IC] Electrostatic charge in switched-capacitor circuit
    I'm assuming you're designing a switched capacitor filter or something of that
    sort, and that you're using a switched-capacitor filter IC to do the job. Any
    additional information you might provide would be helpful.

    Any residual charge on the cap when the power supply is turned off will bleed
    down through the substrate. A couple of points:

    1) Of course, you should always be sure the cap is discharged before inserting
    it in the circuit during assembly.

    2) Especially if you have a switching power supply, look carefully at how fast
    the supply creeps down at turn-off. A lot of inputs won't have much to limit
    input current in the even the cap has a higher voltage than the + rail or lower
    than the - rail. Arrange so the power supplies drop off slowly, and evenly if
    possible to keep that cap bleed-off slow.

    3) If you've got dual supplies, this can be much more of a problem. In dual
    supplies, if the two turn off unequally, sneak paths through the power supply
    load can cause one of the power supply rails to be reversed in its relation to
    GND. This is bad news. If this might be a problem, try two back-biased diodes
    protecting each power supply rail between the power supply output and
    GND/common. This also protects things at turn-on, for the same reasons.

    Power Supply Diode Protection
    ____
    V+ | | +5V
    o-------o------|7805|-----o-------o-----o
    | |____| | |
    | | | |
    --- | --- -
    --- | --- ^
    | | | |
    o-------o--------o--------o-------o-----o
    | | | | GND
    --- | --- |
    --- | --- -
    | _|__ | ^
    | | | | |
    o-------o------|7905|-----o-------o-----o
    V- |____| -5V

    This is general advice which applies to all ICs that have inputs that can be
    connected to a charged cap at turn-off. If power supply turn-on and turn-off
    aren't the problem (that's the biggie), I've found most reliability problems
    with switched capacitor circuits to be with ESD.

    Sorry -- wish I could be more helpful, but you haven't given us a lot to run
    with.

    Good luck
    Chris
     
  6. boki

    boki Guest

    Wow, thank you very much for such clear description.
    It will be very useful.

    When I measured the sc chip, I will report to every body ~ thank you very much~

    Best regards,
    Boki.

     
  7. boki

    boki Guest

    Is this right? : )
    C
    vin -------/ ----||---- \------->OP input
    | | |
    | | |
    / | \
    | | |
    | | |
    GND | GND
    |
    R
    |
    \ Switch_R
    |
    GND


    Regards,
    Boki
     
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