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+5 to -5 charge pump thingie

Discussion in 'Electronic Design' started by John Larkin, Feb 27, 2005.

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  1. John Larkin

    John Larkin Guest

    I have +5 and need -5 at maybe 30 mA. A SOT-23 package would be nice.
    We have a Maxim part in stock, but it's an SO-8 and, of course,

    Any favorites?

  2. LM828 looks nice, not expensive.

    How would you do +12 to -12 at 0.5 or maybe 1.5A with
    no custom magnetics? LM2576 is passable, but a bit pricey.

    Best regards,
    Spehro Pefhany
  3. John, your better than that.
    Take any digital signal in the system between 50KHz and 1MHz. Cap couple
    it with a 100nF cap to the anode of a Shockley diode to gnd and cathode of
    similar diode to the -4.65VDC output. Filter this output to gnd with 10uF.
    But you knew all of this, why am I preaching to the choir? You must have
    some other requirements.
  4. Mike Diack

    Mike Diack Guest

    cheap as chips and works a treat
    ***NOT SHORT CIRCUIT PROTECTED*** (bitter experience)
  5. Ah, that's a little beefier than my initial suggestion, and only a
    nickel more. (0.31 vs. 0.26 in 1K). Fairly high switching frequency

    Best regards,
    Spehro Pefhany
  6. John Larkin

    John Larkin Guest

    The thing I'm doing is a little standalone optical-to-electrical
    converter, in a few cubic inches of metal box, powered by a wall wart.
    It's so simple it has no clocks or anything. I guess I could do a
    schmitt-trigger oscillator and a charge pump, but that's a lot of
    parts. I just need a negative rail for a couple of fast opamps.

    I thought I'd ask the guys to see if anybody has a favorite part, and
    just to make conversation.

  7. John Larkin

    John Larkin Guest

    Yeah, that one should work with ceramic caps.

    Your other problem, in the 1-amp range, is trickier. One of our
    products uses an LM2596 ($1.80 at 1K) in the weird upside-down config,
    where -Vout is the chip ground. We use a standard Talema inductor, 68
    uH, 3A, another $1.33.

    Thanks, everybody.

  8. Ol' Duffer

    Ol' Duffer Guest

    He said 30mA. Which means you would have to source and
    sink >60mA (assuming 50% duty cycle). I doubt you'll get
    that much current from "any digital signal in the system".
    I've seen folks do things like parallel several buffers
    or inverters to get drive for stunts like this. And then
    there's the matter of the diode junction voltage loss.
    He didn't say how close his -5 needed to be...
  9. Ken Smith

    Ken Smith Guest

    How about a 5V zener between the ground and the return of the wallwart?
  10. Jim Thompson

    Jim Thompson Guest

    I have +1.65V to +5.5V and need -1.5V

    Device-level solution required

    BUT, substrate is P-type, so the sneak paths get tricky ;-)

    ...Jim Thompson
  11. Thanks, that's better than the 2576 and compatible- the output cap
    should be a lot smaller with the higher frequency. Good enuf.
    Thank you, I didn't realize the low end ones were that cheap these
    days.. $260 for a reel of 1K of the low current ones. Nice if you need
    a taste of -V for a front end or whatever.

    Best regards,
    Spehro Pefhany
  12. John Larkin

    John Larkin Guest

    Somebody makes a something chip (not very specific, I admit) that has
    a 3 GHz on-chip oscillator used to invert the Vcc for some reason.
    Micrel or somebody with an M I think.

    Isn't the classic bipolar process a p-type substrate? So if the
    substrate is V-, diode isolation works for all the sigs on the chip,
    but nothing can go negative without dire consequences.

    Drams used to (still do?) pump the substrate negative.

  13. Luhan Monat

    Luhan Monat Guest

    Bingo. I've done this, works great since op-amps are very noise-immune
    from the power feeds.
  14. Terry Given

    Terry Given Guest

    ROTFLMAO! all depends on the frequency. 50/60Hz, yes. 100kHz, no. Shit,
    PSRR curves go below 0dB at some frequency. for a TL064 its around
    100kHz, as I discovered the hard way. RTF data sheet would have been
    easier - *if* they gave me a PSRR curve.

  15. John Larkin

    John Larkin Guest

    I do want it to be possible to power a bunch of these from a grounded
    +12 volt supply (if I can persuade anybody to buy a bunch of these) so
    that's my preferred way to do this. Sounds like an LM2664 will invert
    the +5 to give me enough to power the opamps.

  16. Bob Stephens

    Bob Stephens Guest

    LTC1983-5 ?

  17. John Larkin

    John Larkin Guest

    $2.07 each. Yikes! I don't need the regulation, so the LM2664 looks
    better at about $0.31 or something. Maxim and LTC are always way

  18. Jim Thompson

    Jim Thompson Guest

    If you already have a buck regulator in your system, you can steal
    some negative power with this circuit on the S.E.D/Schematics Page of
    my website...


    ...Jim Thompson
  19. Bob Stephens

    Bob Stephens Guest


    My stuff is all low-volume military these days, so component cost isn't
  20. Bob Stephens

    Bob Stephens Guest


    My stuff is all low-volume, military these days so component cost isn't
    much of a concern.

    Nice little part though.

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