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OPA551 Op Amp w/Single Supply?

Discussion in 'Electronic Design' started by Rileyesi, May 16, 2004.

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  1. Rileyesi

    Rileyesi Guest

    I would like to use the Texas Instruments' op amp OPA551 (Burr Brown, actually)
    with a single supply rather than the dual that seems to be called for in the
    data sheet.

    Here is a link to the data sheet http://focus.ti.com/lit/ds/symlink/opa551.pdf

    However, I am using the op amp in unity gain to boost the current capacity of a
    voltage reference to about 150 mA max (the capacity of the op amp is 200 mA).
    The reference voltage is a steady 5 VDC. The OPA551 data sheet says that the
    signal must be at least 2 volts from the rail. I could not find any specifics
    in the data sheet, but couldn't I use a 0-12 VDC supply and deliver a 5 volt
    output? That would make the output at least 5 volts from the supply rail.

    Makes sense to me...but I just want to be sure. (Worked well in bench testing,
    by the way). I tried contacting TI directly, but with no answer as of yet.

    As a side note, I am also using the OPA547 that has a capacity of 500 mA
    continuous output for another application. That data sheet does address the
    use of a single supply (i.e. you can use one), but the chip is overkill for my
    needs and is about twice the cost of the OPA551.

    Thanks.
     
  2. Tim Wescott

    Tim Wescott Guest

    Operational amplifiers in general don't know ground from Adam. They
    have their common-mode input range that you can't violate without weird
    things happening, and they have their overhead voltages beyond which
    they won't drive. As long as the thing is getting sufficient voltage
    and you're feeding it the right input voltages it should work fine.

    The only thing that makes an op-amp "single-supply" is that it will take
    common-mode to one rail or another and will drive close to the same rail.
     
  3. Tim Wescott wrote...
    Right. Rileyesi, you may run into a bit of trouble pulling 150mA out
    of the OPA551, dropping 12V to 5V, creating 1W of heat dissipation.
    The junction temp will increase by an unsafe 100C over nearby warmed-up
    air in the miniDIP package, and the SO-8 is little better. The DDPak-7
    version (OPA551FA) may work, but only if you have a rather large PCB
    heat-sink area, and only if you can find them stocked someplace.

    Thanks,
    - Win

    (email: use hill_at_rowland-dot-org for now)
     
  4. Fred Bloggs

    Fred Bloggs Guest

    That's called a power supply, you friggin MORON-
     
  5. Tim Wescott

    Tim Wescott Guest

    Could you use a pass transistor following the op-amp, or do you need to
    sink current as well as source it? If you use an emitter follower after
    your op-amp you can significantly more current out of the thing -- using
    a power MOSFET is even better except that you'll need to really reduce
    your bandwidth and pay close attention to overhead.
     
  6. Tim Wescott

    Tim Wescott Guest

    So if I use a 78L05 instead of a high-precision reference followed by a
    power amp it'll be able to sink current and source it while holding the
    output steady at 5.00V, _and_ it'll be as precise as a precision reference?

    Wow. I never knew.
     
  7. Rileyesi

    Rileyesi Guest

    However, I am using the op amp in unity gain to boost the current

    Thanks, Tim. Your reply was ALOT more polite than what I had in mind!

    By the way, you hit my problem right on the head. I can't rely on the
    precision of an off the shelf power supply for my application.
     
  8. The SO-8 and miniDIP packages of the OPA551/2 are not capable of
    getting useful current & voltage capabilities out of this IC. I wonder
    why manufactures sometimes design IC with unrealistic power
    capabilities that are severely limited by package dissipation. The
    DDPak-7 will get you into safe temperatures provided its heat sinked
    in a non-standard way. How does one clamp aluminum heatsinks to a SMD
    device like this ?

    Adam
     
  9. Tim Wescott

    Tim Wescott Guest

    Did you see my reply to Win's post? It's quite common to use an emitter
    follower after an op-amp if you just need a 1-quadrant regulator.
    Emitter followers have very good frequency response and a voltage gain
    slightly less than one, so it's hard to upset the stability of your op
    amp with one. Furthermore with the right pass transistor you can use a
    fairly wimpy op-amp and still get 200mA out of your pass transistor.
    Zetex advertises some transistors with very nice specs, but you could
    probably get by with just about anything that'll dissipate the power.
     
  10. Rileyesi

    Rileyesi Guest

    Tim,

    Yes I did see your reply and I think that is what I am doing currently. I got
    the circuit out of the datasheet for the reference (LT1460-5 by Linear
    Technologies go to

    http://www.linear.com/pdf/14605fs.pdf

    for the datasheet). I am using the circuit listed on page 12 of 12 that shows
    a boosted current output. The problem is that I'm finding in the field that I
    need to deliver more than 100 mA, even though my bench testing shows otherwise.
    Also, there is the little problem of two techs that reversed the positive and
    ground on the output of the supply (i.e. downstream of the transistor) and
    cooked it! My thought is that the op-amp may be able to survive that condition
    better than the transistor.

    Thanks for all the advice. I;m a mechanical engineer by degree and am self
    taught in electronics.
     
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