PMIC "bypass" Q

Discussion in 'General Electronics Chat' started by s_mack, Jun 9, 2012.

  1. s_mack

    Thread Starter Member

    Dec 17, 2011
    187
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    Hi. I'm attempting to do a retrofit on an existing product. My "add-on" draws too much power for the host device's wimpy voltage regulator. So I'm wondering if the following "bypass" is OK. Black lines are the existing circuit and blue are proposed. Goal is to be the same as existing but with more current handling ability. How is the dropout affected? If the original Vreg has a d/o of 1.7v and the new one has a d/o of 0.2v, are we looking at a 1.7v? 0.2v or something else? Thanks.


    - Steven

    ps.

    The feasibility of my retrofit should be looked at in terms of mid-scale production run. Ie. its not a one-off that we can expect the end user to do any soldering, component removals, etc. So I guess I'm saying... please answer my question as its asked, rather than try to guess or suggest alternative means because I only have access to certain parts of the circuit and I'm not providing the whole picture here.
     
    Last edited: Jun 9, 2012
  2. s_mack

    Thread Starter Member

    Dec 17, 2011
    187
    5
    Oops. I guess my attachment won't work. I'll post it in a few minutes, sorry.
     
  3. s_mack

    Thread Starter Member

    Dec 17, 2011
    187
    5
    OK, picture is up in the original post now. In case I wasn't clear... basically my "idea" (if you can call it that) is that the original Vreg would remain and a new one would operate parallel to it. Is this OK? Is the regulated voltage still 5v? What happens if a current greater than what can be handled than the original is demanded? Does the original crap out? Or does the new one just handle it?

    Since this isn't something that would normally be done, I really have no idea how to even research this (other than experimentation I suppose).
     
  4. s_mack

    Thread Starter Member

    Dec 17, 2011
    187
    5
    Just more clarity:

    I've seen a lot of posts about using vreg in parallel... but their goal is always the same: to get around the power limit of an individual vreg by adding one or more vreg. Its not the same as I'm trying to do. My total power use will be below the replacement vreg's capability, but above the existing one. I'm not trying to get a combined greater power handling capability.
     
  5. #12

    Expert

    Nov 30, 2010
    16,338
    6,824
    They will fight.
    Whichever chip has a slightly higher voltage will supply all the current until it starts to "safety out". Only then will the other chip start helping.
    If you could add a resistor to each output, they would share better.
    The best answer is to just change the 78L05 to a 78M05.

    It's not the answer you wanted, right?
    Maybe somebody else will have a good idea.
     
  6. s_mack

    Thread Starter Member

    Dec 17, 2011
    187
    5
    Changing the circuit is out of the question. I can only add to it, and only at certain points.

    In the "fight" scenario you mention, do we care? I mean... let's say the replacement has the slightly higher voltage so it takes the current. Then we're fine, because it can handle it. Now let's say the original has the slightly higher voltage so it takes all the current, which it can't handle then the replacement takes over.

    Am I missing something? Because that sounds perfectly fine to me.

    Or... does the "fight" have some repercussions?

    My primary goal is that 5v is outputted and the current (which is within the replacement's specs) is handled. My secondary goal is that the drop-out voltage is defined by the replacement.

    - Steven
     
  7. s_mack

    Thread Starter Member

    Dec 17, 2011
    187
    5
    I had a dumb mistake in my schematic. voltage source in wrong area. Its been fixed.
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,045
    3,244
    If either regulator can supply its full load without overheating (both have an adequate heat sink), then it probably is ok to parallel them.

    If the input voltage drops below the dropout voltage of one regulator, then the other will take over the full load.
     
  9. s_mack

    Thread Starter Member

    Dec 17, 2011
    187
    5
    Ok let's say the original is poorly designed and does not have adequate heat sink (it probably doesn't). what's the worst case scenario here? Does it just crap out and leave the bypass one to do its job, and that's kind of what I want anyway? Or is there something I'm not seeing?
     
  10. #12

    Expert

    Nov 30, 2010
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    Can you just disable the weak one and let the better one take all the load?

    Scenario: The better regulator has a tiny bit higher voltage.
    The better regulator takes all the load and the lesser regulator never works for a living.
    Not a problem.

    Scenario: The weaker regulator has the higher voltage.
    The weaker regulator works as hard as it can until it overheats. Then its voltage lowers in self protection mode.
    Then the larger regulator takes over whatever partial current the lesser regulator can't do.
    Not a problem..except for the heat. If the heat doesn't bother anything nearby, you can get away with it...I think.

    All this while assuming you keep the regulators close to their capacitors so they don't start oscillating.
    It's just difficult to figure out a circuit that the chips were never intended to do.

    Try a couple and see if it works in this manner.
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    Assuming "Vcc" is the same point...

    There should be no problem with this scheme. Linear regs such as the 78xx series are tough little beasties and can survive a lot of abuse such as over current. Their life may be shortened if they run in power limit mode, but they will work OK.

    When you put two regs in parallel like this, the one with the larger output voltage wins the battle and supplies all the current. If this is the new regulator then the old one will coast. If it is the old regulator and it is unable to supply all that current it reduces its output till the new reg takes over some of the current.

    You may not need the extra caps as the old caps are still on the new ones output.
     
    #12 likes this.
  12. s_mack

    Thread Starter Member

    Dec 17, 2011
    187
    5
    thanks everyone.

    if I choose an adjustable VR for the new one, then could I not deliberately set it to something like 5.05V to assure it is the one that "wins"?
     
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