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Linear 3.3V power supply anywhere?

Discussion in 'Electronic Design' started by Joerg, Apr 26, 2007.

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  1. Joerg

    Joerg Guest

    Tried Digikey, Mouser, Newark and some others, could not find any: A
    linear 3.3V/2A (or more) power supply. No switchers.

    What we really need is 3.3V/2A, 5V/2A, 12V/4A and -12V/4A but I can
    piece that together as needed. The 5V and 12V is, of course, no big
    deal. Condor has some nice ones with the good old uA723 on there, so low
    in noise. But for some reason no 3.3V. I want to avoid modifying them
    but maybe we just have to :-(
  2. D from BC

    D from BC Guest

    I can guess at why you're not finding one....
    1) This is near the drop out voltage of typical regulators
    2) Switchers may rule this area
    3) At 3.3V ..%temp drift maybe a concern. The internal reference in a
    hot linear reg may have too much annoying drift.
    4) Low customer demand
    5) low dropout linear reg + power transistors = any current you want

    Here's some quirky wisdom...
    If you can't find're wrong! :)

    (No offense meant..)
    D from BC
  3. Joerg,
    I guess this ist the domain of controllers and
    logic, so the manufacturers assume a switcher to
    be sufficient for all applications.
    Though there are no sole 3.3V switchers available
    here either.

  4. Joerg

    Joerg Guest

    Drop-out doesn't matter. You just feed in enough volts. It's easy to buy
    2V linear supplies, even 0.5V duals. But no 3.3V.

    They sure do :-(

    No problem. Designed many low voltage regulators with ye olde uA723 or
    semi-discrete. Rock stable. I just don't want to design another one,
    this project has enough other work and the missus ain't going to be
    happy if I tell her that instead of taking her out to our favorite Thai
    place I'll design a power supply.

    Looks like it. Nobody is concerned about noise no more...

    No more LDOs in this here consulting office. They've given us too much
    grief, undocumented pathology mode, explosions etc.

    Aw, that hurts. It just piles on to John Larkin's accusation of heresy
    because we are swinging a VME backplane at 3.3V. Plus we chopped up the
    bus and run SPI over it, but shhhht, don't tell.
  5. Joerg

    Joerg Guest

    If you ever need any of these Digikey has tons of 3.3V single modules.
    AFAIK from Astec but as you said they are all switchers. Digikey lists
    some of them as linear but they aren't.

    Thing is, yes, we have to supply logic but that sits in a card cage that
    must remain super duper whisper quiet. Even the bus must be stopped
    during signal acquisition.

    So, I guess I'll roll my own. Again. Dang. Or modify a 5V but their
    crude SCR crowbars aren't easy to fix at such low voltages.
  6. John Larkin

    John Larkin Guest

    I heard that! What idiot suggested that you use a VME backplane in the
    first place?

    Do you want me to design a 5v to 3.3v linear regulator for you? So you
    don't miss the pad thai and twice-cooked duck?

    4 discretes should do it, maybe 3.

  7. Joerg

    Joerg Guest

    Guilty. We needed a quick prototype, I spotted a fancy Schroff box with
    all the bling on EBay same day, the client's engineer pokered by bidding
    one buck above min and, bingo. I promise, in the final version we'll
    call it something other than VME. But might still use a VME backplane.
    If it's any comfort the really unorthodox matrix stuff happens on P2 :)

    Thanks, but I've already butchered a PowerOne 5V supply. The only pain
    is the crowbar. They use a SCR plus zener. It strays from 0.6-1.5V and
    the trigger current from 3-32mA. And zeners in the 3V range aren't
    exactly that great either. Maybe I'll just use a string of diodes.

    The nice thing about these PowerOne supplies is that you can write a
    nice mod procedure for production. Two screws and the circuit board
    comes off. But tape the transistor in place beforehand.

    This Thai restaurant is a tiny place that only the locals can find. But
    quite genuine and always busy. If I order medium it drives the sweat
    beads onto my scalp although I am used to very spicy food. Then there is
    two more grades, hot, and Thai-hot. Oh, and the Singha beer...
  8. D from BC

    D from BC Guest


    Ohh come on.... it does matter a bit..
    Let's say the drop out voltage is 3Volts..
    Regulator dissipation is 2A*3V=6 watts (or more)
    A monolithic regulator might have some yucky drift too at 6Watts..
    That's nearly = to the load dissipation at 6.6Watts.

    I'm saying that a large dropout voltage results in a less efficient
    linear regulator.
    D from BC
  9. Joerg

    Joerg Guest

    Sure but we live in Caleefohniah where the power can be anywhere between
    110V and 130V, or sometimes 0V. So you need dropout just to accommodate
    that. Dissipation doesn't matter right now and I am not a friend of
    monolithic regulators for anything above a few hundred mA. The LM317 is
    pretty much as far as I will go in that domain. I like the uA723 a lot,
    plus some nice fat pass transistors.
  10. Rich Grise

    Rich Grise Guest

    I'd like to find someone who imports Oscar "Port-Wine-type" "Champagne". ;-)

    It tastes like soda pop, but works like Thunderbird. ;-)

  11. John Larkin

    John Larkin Guest

    Oh. I thought it might have been me.
    There's a lot of cheap VME on ebay.
    Singha Gold is wonderful beer, but hard to find. The regular Singha is
    pretty good, too.

    We like Regent Thai on Church Street, right where they filmed "Sister
    Act." It's run by all women who wear gorgeous silk Thai dresses. Great
    food and cheap.

  12. Joerg

    Joerg Guest

    Yeah, Sister Mary Clarence, a.k.a. Whoopi Goldberg. One of the best
    movies. Often critics call it just a "comedy" but that doesn't do it
    justice. There are a lot of civic (and religious) lessons in there.
    Everyone can do great things, they just have to be "motivated".

    Hey, why are places like Digikey out of stock on so much stuff these
    days? Couldn't even get a decent paired fuse holder there, for crying
    out loud. Even for blue 24AWG hookup wire there is a whopping supply of
    one lone spool left. Arrgh.
  13. D from BC

    D from BC Guest

    The latest linear regulator I've played with is the LP2951.
    Datasheet says a 3.3V version is available.
    Of course pass transistors will be needed for 2A.
    For example:
    Digikey LP2950ACZ-3.3-ND is in stock.
    D from BC
  14. Tam/WB2TT

    Tam/WB2TT Guest

    We ended up using an LDO regulator off the 5V. Board mounted. Not sure if
    the 2 Amps will be a problem.

  15. Joerg

    Joerg Guest

    The National LDO series is where I got my first big black eye. I was
    already prejudiced against LDO but a client really liked it so they said
    we should use it. The client is king, so I had to. Fired up the
    prototype, the output voltage came up sluggishly and a "high amperage
    smell" developed. Hung a scope onto the rail and it was singing the
    blues, oscillating like crazy. Turned out it didn't like it when the
    source impedance got too high but we had no choice. Naturally, that was
    not mentioned in the data sheet. App Engineering told us more or less
    "You are right, it cannot handle hi-Z, sorry about that". Great. I
    forgot the exact part number. IIRC its wasn't LP but something like
    LM2931. I forgot because I vowed never to design one in again. And I
    never did, threw away the datasheet.

    Not that I want to diss one particular vendor here. I've come across
    more than one hardcore LDO problem. For example a TPS71550 that "didn't
    like it" when the supply came up too fast. Phsssst ... BANG. Was that
    mentioned anywhere? Oh no. TI wanted scope plots and what not. I had
    sent them the rather simple schematic but they were not willing to throw
    it onto SPICE. Would have been easy but not for the customer since they
    do not release the innards anymore. So I voted that one off the island
    as well ;-)
  16. D from BC

    D from BC Guest

    What's the difference between an audio amplifier and a linear

    (I'll be working on the punch line...)
    D from BC

  17. There are no "Regulatorphooles"?

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  18. Tam/WB2TT

    Tam/WB2TT Guest

    I don't think it is as bad as all that. We had two National LP3964
    regulators on the board, with no problems. There were 10 uF Tantalums across
    the input and output, plus a .22 on the output. In the one case, there was
    even an RF choke in series with the 5 V feed, on the other side of the input
    cap, of course. They do tell you that you need a bypass cap with an SER of
    very close to 1 Ohm.

  19. John Larkin

    John Larkin Guest

    But tantalums explode. And the capacitor you see on the schematic,
    right next to the ldo output pin, isn't usually the only capacitor on
    that node. There could well be another bunch of microfarads of ceramic
    bypasses all over the board, with a net esr in the milliohms.

    And if you use an aluminum cap, and the temperature drops, the esr
    goes to hell.

    Any not-insane regulator should tolerate esr's down to zero.

  20. Just how much design is a 3.3V linear regulator? How about a couple of
    paralleled 317's?

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