Quick question about the LM2576 5v Switching regulator.

Thread Starter

cheezyguy5

Joined Apr 3, 2014
12
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.
 

AnalogKid

Joined Aug 1, 2013
11,038
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
 

Thread Starter

cheezyguy5

Joined Apr 3, 2014
12
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
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.
 

strantor

Joined Oct 3, 2010
6,798
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.
 

THE_RB

Joined Feb 11, 2008
5,438
I would suggest using a "low dropout" linear 5v regulator.

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

Thread Starter

cheezyguy5

Joined Apr 3, 2014
12
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?
 

strantor

Joined Oct 3, 2010
6,798
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?
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.
 

IcedFruits

Joined Jan 15, 2014
97
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.
 

Thread Starter

cheezyguy5

Joined Apr 3, 2014
12
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.
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?!?
 

IcedFruits

Joined Jan 15, 2014
97
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.
 

Thread Starter

cheezyguy5

Joined Apr 3, 2014
12
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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