Quick question about the LM2576 5v Switching regulator.

Discussion in 'The Projects Forum' started by cheezyguy5, Apr 3, 2014.

  1. cheezyguy5

    Thread Starter New Member

    Apr 3, 2014
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    I'm currently trying to make a 4xAA portable USB charger, because my tablet only has a 2600mah, and lasts 2 1/2 hrs before dying. But I've run into a big problem, 4x1.5v=6v... And I have only been able to find 1 single regulator that may work, but every document I've read about it says it can take in 7v, 8v, 7.5v, but someone asked a similar question on a different forum and he was told the LM2576 would work... :confused: To sum things up, I want to know if the LM2576 (5v) can accept 6v in.
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Maybe, but that seems like a lot of work to me. Putting a diode such as a 1N4004 in series with the battery will decrease the output by 0.7V or 0.8V.

    ak
     
  3. cheezyguy5

    Thread Starter New Member

    Apr 3, 2014
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    According to my multimeter my 4 alkaline batteries reach 6.28v, my tablet's range of acceptance is (supposedly) 4.9v to 5.1v dropping it 0.8v would be get me 5.5v, too high I think, :( If only someone actually HAD one of these things and could test the voltage acceptance.
     
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
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    The LM2576 isn't a linear regulator like the LM7805 or LM317. It's what TI calls a "simple switcher" - basically a simple SMPS controller; simple as in compared to other SMPS controllers, not to be confused with ordinary "simple," like a linear regulator. It won't do anything by itself; you need an inductor, some caps, a schottky diode, possibly some resistors, a way to make legitimate PCBs, and some practical knowledge about SMPS design.
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I would suggest using a "low dropout" linear 5v regulator.

    That will do 6v battery to 5v conversion just fine. :)
     
  6. cheezyguy5

    Thread Starter New Member

    Apr 3, 2014
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    How about an LM1117? Would that work? as with the other regulator, the datasheets all say it accepts different voltages... So WILL it accept 6v? or not? :confused:
     
  7. Johann

    Senior Member

    Nov 27, 2006
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    I agree! If one diode does not give enough drop, use 2. That will bring you spot on.
     
  8. cheezyguy5

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    Apr 3, 2014
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    Do Ohms matter in this case? And is series like how you would normally wire up batteries? Sorry for the stupid question.
     
  9. cheezyguy5

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    Apr 3, 2014
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    The lm1117 won't work... according to this datasheet, It needs a minimum input voltage of +1.224 the desired output voltage, so it'll only work when the batteries are fresh and new. On another note, which diode can I use to drop 0.4v?
     
  10. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Good question. I was recently seeking to use a diode in the same way, to shave off about 1.4v; I looked up the datasheets for the various rectifier diodes that I have, and found that for all of them, the voltage drop is not constant. It follows on a chart in the datasheet, a change as current through it changes. So with 4 diodes in series for me, at zero load, the voltage drop would be perfect, but as I load it above 1 or 2 amps, it will no longer be the proper voltage drop. I was looking for a diode with a fixed voltage drop. My only idea was a reverse biased zener, but I couldnt find one with a zener voltage that low. Maybe someone can come along and teach both of us what kind of diode to use for this application.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    A Schottky diode will drop about 0.4-0.5V.
     
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  12. cheezyguy5

    Thread Starter New Member

    Apr 3, 2014
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    So, 2 Schottky's will drop the voltage around .8v~1.1v? Could I just use 4 AAs and 1 AAA to get the voltage to 7.5v and use a standard 7805 regulator?
     
  13. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    I'd use 5 AAs and call it a day. Don't like mixing battery types.
     
  14. cheezyguy5

    Thread Starter New Member

    Apr 3, 2014
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    I don't like it either, but 5 AAs + a USB port will NOT fit in an altoids tin, I just tried...
     
  15. crutschow

    Expert

    Mar 14, 2008
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    Then you may as well use all AAA's since your total battery capacity is limited by the smallest cell.
     
  16. IcedFruits

    Member

    Jan 15, 2014
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    hi, drop the idea of using buck converter. use a boost converter instead.

    use 2-3x NiMh(2100 -2600 mAh) cells, and have the voltage boosted to 5v with either of these ICs : Max757 / Max756 / Max1701 / LT1302 / LM2623 / MC34063A.

    here is an instructable guide: http://www.instructables.com/id/Adjustable-Voltage-Step-up-07-55V-to-27-55V/

    dont use less than 2 cells, the current draw for USB (5v) may be too much for just 1 cell and make sure not to deep discharge the batteries.
     
  17. cheezyguy5

    Thread Starter New Member

    Apr 3, 2014
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    I've already looked into that, I can't find any inductor that'll work, and the assembly won't fit in an altoids tin at all... WHY CAN'T ANYONE JUST MAKE A 6V TO 5V REGULATOR?!?
     
  18. cheezyguy5

    Thread Starter New Member

    Apr 3, 2014
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    Would this work?
     
  19. IcedFruits

    Member

    Jan 15, 2014
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    yes, thats a ready made solution of the boost conversion. use 3x-4x nimh cells to hav it work.
    but there are many fake products these days, so, dont forget the check it thoroughly after receiving.
     
  20. cheezyguy5

    Thread Starter New Member

    Apr 3, 2014
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    As Strantor said before, the LM2576 IS a simple switcher, and I would need a schematic. well,
    here's
    the schematic. It doesn't use some kind of exotic inductor or other part, just standard pieces that I know RadioShack, has (but very overpriced) and if RadioShack has it EVERY electronic store has it, But, would the schematic work? or blow up in my face and burn down the house?
     
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