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Opamp w/ Vsupply >36V

Discussion in 'Electronic Design' started by George Herold, Jul 15, 2013.

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  1. Opamp Vsupply >36V

    Hi all, I’ve got a circuit with that uses an opa2134. I find myself pushing the 36V supply limit and I’m a little worried. So I went looking for a similar opamp but with a bit more headroom.
    So specs might read,

    Dual opamp
    unity gain stable
    4(+) Mhz BW (8MHz or more would be nicer)
    10 V/us slew (again more is better)
    20mA current (positive, again more would be nicer)
    small input C ( <5pF would be nice.)
    8 pin dip preferred

    Trolling digikey I found a few possible candidates.

    TLE2142 (6 MHz GBW)
    TLE2072 (only 38 V supply, 11 pF input C)
    OPA2604 (nice but 10 pF input C)
    (I’ll order a few of each.)

    There was also the LF412A, but a bit lacking is positive supply current.

    Any others that I might have missed?

    Thanks,
    George H.
     
  2. Frank Miles

    Frank Miles Guest

    TI lists some opamps with varying maximum voltage limits. Try:
    http://www.ti.com/lsds/ti/amplifier...mplifier-greater-than-equal-30v-products.page

    HTH..
     
  3. Well I might be using 'headroom' the wrong way. I'm splitting a 48 V wallwart,
    ground is a few volts above the bottom rail and then ~35 volts down from the top for the positive rail. But 35 volts designed and 1% resistors that'sabout a 4% max error... 36.4 volts. I'm feeling squeezed.
    Thanks I'll look at those.
    (I rejected the MC33072 already.)
    I've got some TCA0372's in stock, but haven't played with them yet.
    (too slow for this job.)

    George H.
     
  4. Make sure there are no transients possible on the power supplies etc.
    if you do this. I once had a really horrible experience using 16V TI
    CMOS op-amps at the system supplies of +/-8V (almost 1% failure rate).
     
  5. miso

    miso Guest

    You can get a lot of soft failures due to overstress, but the part keeps
    "working." A soft failure in a chip is something like the shutdown
    current out of spec or the input impedance is low, etc. Often chips have
    soft failures and nobody notices. For instance, a bit of increased
    shutdown current can be lost if there are a lot of parts on the board. A
    lower impedance on an input may not matter if the rest of the components
    are low impedance.

    But your problem sheds light on an interesting test problem, namely
    absmax. There is no standard on how to test parts beyond their datasheet
    limits. Some companies only QA absmax. They stress the part and see if
    it still meets spec when used at stated voltages. Others do a real
    absmax test, but not at strict electrical limits. But then there is the
    question of should you be stressing your parts at all?

    But to get 99% failure when operated at the datasheet limits implies to
    me a part that has other problems rather than your supply being the
    culprit. There is a lot of margin in absmax.

    That said, transorb diodes are highly recommended!
     
  6. Grin, the first time I did the circuit I had big caps (10uF) from the input 'grounds'* to the regulated voltages. (LM317/337's) The positive supply was fine, but the negative supply would 'turn on' to ~1/2 the supply voltage.. (Caps basically split the supply to start with) and then went down as the regulator took over. This took out a voltage reference downstream ~50% of the time. (Bad cap!)

    George H.
    *'ground' for the LM337 was the positive supply input.
     
  7. Guest

    Fix your power supply and use readily available, and commonly used, OAs.
     
  8. Ahh, well there's nothing wrong with the supply. I just need 30V of stable bias. The LM317's had too much drift and so I've now got a 10 Volt reference gained up to 30V.

    George H.
     
  9. Phil Hobbs

    Phil Hobbs Guest

    Might be cheaper to use the 317 inside an op amp loop. The op amp can
    run at low voltage then.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  10. Ohh, that's a nice idea.. can I still power the opamp from the LM317 output?
    In this case I wanted a new voltage reference after the 'ground' and V(+) were defined by the regulators. I gain up the 1.25V's by factors of 20-30 and any change in the 1.25 V gets magnified. Once I get the 30 volt bias I'd like it stable to ~1mV or so. (For maybe 10 minutes to an hour.)
    So anyway the current circuit works fine... (the pcb is spun and in the box..)
    I don't want to redo it. I guess I should think of all the gotcha's beforedoing the design.

    George H.
     
  11. Got it.. that's good to know.
    I've used the LM395 as an opamp power booster.. I think Jim T. said that the guts are similar to a 317. Hmm I don't need any power I could have just stuck almost any transistor on the output. OK maybe next time.

    George H.
     
  12. 99% non-failure rate. Still terrible.
    A TVS that is suitable for 16V nominal (wg. SA16A or SA17A) can havemuch of a solution for a marginal situation. It's great if the chip
    can actually stand 30V momentarily.
     
  13. Guest

    Well what is this stuff about exceeding 36V??? Is it some kind of start-up overshoot?
     
  14. miso

    miso Guest

    A TVS that is suitable for 16V nominal (wg. SA16A or SA17A) can have
    I had a burn-in oven issue on a 5V part and 8V transorbs were fine. A
    lot depends on the chip design. Parts with large devices tend to do
    worse since the field created due to the excess voltage "seeks" a
    crystal defect. Lots of active area means more chances to find that
    defect. Most electromigration rules are generous enough that you don't
    see metal failure due to overvoltage. So the routing area doesn't count.
     
  15. miso

    miso Guest

    That part is bipolar on SOI. You would think the active circuitry would
    benefit from the insulator, but I have no first hand knowledge of that.
    [Usually a reversed bias diode is what fails in overvoltage.] But the
    part is cutting edge, so I can see the tolerances being tight.
     
  16. Yeah I was going to add to the original thread that
    Apex opamp's need not apply :^)
    (too big and too expensive)
    There's that nice little LTC6090 that Fred Bartoli pointed to.
    (I should order some of them, to replace an Apex part.)

    George H.
     
  17. Well I'm not exceeding 36 V. Just feeling that the design is a bit tight..30 Volt out with ~2V of head room at the top and the minus supply 2V belowground... And then worrying about the variation in parts. If I had, say 40 V, then I could relax everything.
    Not to worry I've got opamps ordered. I think the TLE2142 may be the winner. It's even got a bit less input C.

    George H.
     
  18. Glenn

    Glenn Guest

    Hi George

    Ask John Larkin if he can "dig" the schematic up again:

    https://groups.google.com/forum/?_e...i.electronics.design/Q_w6eQEc3ec/-4XBtDF23xEJ

    /Glenn
     
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