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Op amp brownout circuit

Discussion in 'Electronic Design' started by Steve Morgan, Feb 20, 2005.

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  1. Steve Morgan

    Steve Morgan Guest

    I am designing an Atmel-based circuit with an associated single supply
    op amp. I do not want the op amp Iq to deplete batteries any further
    than down to about 1V after the programmed micro brownout.

    Note that the batteries are not to be manually switched off.

    What is the simplest way of doing this?

    Thank you,

    Steven
     
  2. Fred Bloggs

    Fred Bloggs Guest

    Use an OA with "shutdown":
    http://cache.national.com/ds/LM/LMV341.pdf
     
  3. Joerg

    Joerg Guest

    Hello Steve,

    If Fred's solution is too expensive you could use a pnp transistor in
    the supply that has a Schottky plus series resistor connected between
    base and ground. That'll let go when the voltage drops too low.

    However, this cheap trick will make the 1V cutoff a bit "mushy" and will
    continue to draw a little base current until you are further down the
    Vbe curve.

    Best might be to let the micro monitor the voltage and turn off
    everything if below 1V. You'll also need a pnp to switch the opamp
    supply as FETs won't cut it at these low Vgs levels. Well, a pnp is
    cheaper anyway. Whatever you do make sure that the opamp inputs and
    outputs are not driven into unhealthy regions when its power is turned off.

    Regards, Joerg
     
  4. Tim Shoppa

    Tim Shoppa Guest

    Best might be to let the micro monitor the voltage
    Do the new Atmel microcontrollers go that low? Last I checked their
    brownout stuff kicks in around 1.8V. (That is pretty good compared to
    some others... but I'd still love to hear of something at 1V.)

    Tim.
     
  5. Fred Bloggs

    Fred Bloggs Guest

    I think an even cheaper trick would be to power the OA through an ATMEL
    I/O pin or use a TinyLogic gate to power it.
     
  6. Ken Smith

    Ken Smith Guest

    Yes, IIRC, the ATMEL chip can easily sink enough current to switch the
    ground side of the op-amp power connections.


    If you don't need much performance from the op-amp, there are some that
    draw very little. The have a gain-bandwidth of only several KHz but the
    DC characteristics aren't too bad.
     
  7. Joerg

    Joerg Guest

    Hello Tim,
    Probably not. You are right, 1V is too low for the usual micro. But
    Fred's suggestion of powering the opamp through an IO port is a good
    idea. As long as the code will turn power off to the opamp when the
    micro detects that the voltage gets too low for its own sake. Kind of
    like how a laptop goes from standby to hibernate when the battery begins
    to fade.

    Regards, Joerg
     
  8. Joerg

    Joerg Guest

    Hello Fred,

    That is an excellent way to do it. However, the bypass cap across the
    opamp and maybe across other associated circuitry needs to be taken into
    account. The current peak caused by charging and discharging it might
    exceed abs max limits. Possibly a resistor in series with the I/O can help.

    Regards, Joerg
     
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