1 cell alkaline to 5v & 12V

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I am (attempting) to create a device the size of a credit card, which will need 5v and 12v from battery power. The power requirements for the 5v will need to run an avr + a couple led's (20-30ma max I would say), and for 12v 1-2ma should be plenty. The device will normally only run for 5-10 seconds at a time, and probably just be turned on with a momentary switch (powered as long as switch is on). Usage would not be very frequent.

Cheap as possible is good. Low parts count is good. Through-hole parts is good.

After much messing around with coin cell lithium batteries, button cells and small alkalines like A23's (~12v), switched capacitor doublers, voltage triplers, reading battery datasheets, etc., I have finally decided (I think) to just use a single AAA alkaline and a dc switcher. The AAA has enough power to run a switcher, is common and inexpensive, the battery holders are cheap enough, and it will nicely fit on the credit card sized pcb.

Since I want through-hole parts, that eliminates lots of switchers. Since I want cheap, that eliminates lots more (I don't want to pay for a switcher that costs more than the avr, and sometimes its double the price of an avr). Since I want to run from 1.5v and below, that eliminates about the remaining ones left.

Somehow I ended up finding the TL499A from TI (cheap, and available in pdip, down to 1.1v input). It seems to be working as planned so far (have not powered an avr with it yet). This is what I have wired up so far (this was from memory, so it may not be exactly correct)-
http://www.mtcnet.net/~henryvm/T...
I can power led's from both outputs (15ma/2ma) for at least several hours from unkown AAA's just laying around, so at least I'm heading in the right direction.

Its all standard datasheet values/connections, except for my added D1/D2/C3/C4 doubler and C2 being 100uf instead of 470uf. I have R3 adjusted (a 100k pot) so I can have ~12 at V2 (V1 is then 6.? volts), so R3 value in schematic is not actual value. The zener business at V2 output is just something I tacked on the schematic but have not tried (limit to 12v/2ma with 14v in if I am thinking correctly).

Anyway, the plan for now is to have the TL499 ref from the V1 output so as to make V2 ~12 volts. Then drop the ~6.? volts down to 4.5-5.5 via a diode or two. I will also need to control V2 on/off, so that may change what I need at V2 (and change what I need to do to drop V1).

If anyone has suggestions, comments, improvements, hints, tips, etc., fire away. I could be doing something obviously dumb here, and would like to be kept on the straight and narrow path.

(I'm not thrilled at what happens to the voltage output for 100-200ns when the switching takes place, but maybe that's normal, or maybe I just put my scope away and just use a multimeter)

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> Since I want through-hole parts

Why that if, on the other hand, you have size limitations? I don't
really understand that restriction then.

TI has just announced sampling for switchers that can go down to 0.3 V...
I built a white LED driver with one of the (now older) TI parts that
easily goes down to 0.9 V, it literally sucks up everything it can get
from a single Alkaline cell, driving the LED full current until the
cell is really empty. But agreed, it's a DFN case which is not too
easy to handle, but I managed soldering it.

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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Quote:

Anyway, the plan for now is to have the TL499 ref from the V1 output so as to make V2 ~12 volts. Then drop the ~6.? volts down to 4.5-5.5 via a diode or two. I will also need to control V2 on/off, so that may change what I need at V2 (and change what I need to do to drop V1).

Let me get this straight, you want to output 5v and 12v from a single AAA 1.5v battery?

You need a DC-DC converter. The advantage of DC-DC converters is their high efficiency (low wasted power), and little space needed on board, but your DIP restrictions kinda blow this away.

I would recommend you use either two 1.5v AAA's in series, or a 9v battery instead, it will make your chip selection easier. This will make your battery last longer, you will be able to output more current, and you will probably save space on your board... I would also suggest you use a SOIC package, they are easy to manage and solder, and use much less space than a DIP package...

http://search.digikey.com/script...

This comes in SOIC-8, and outputs 5v to 12v from an input 2v to 30v... All you need is an external inductor, and a couple of caps...

If you are resolved to use a single 1.5v cell, you can also use the LTC1110, but output power will be greatly limited...

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Quote:
Why that if, on the other hand, you have size limitations? I don't really understand that restriction then.
All the limitations are just self-imposed (the size limitation has the side effect of keeping the project from growing beyond what's really needed).

I also have a TPS61070 in tsot-6, but I don't enjoy working with these small parts. It won't be long until there is no choice, but for now, there is.

The AAA will give plenty of life for this thing even when just running down to 1.1v as opposed to .9v (which it seems is where there's nothing left).

I've gone back and forth on the smd/dip question enough times that I can easily go back and forth a few more.

Quote:
Let me get this straight, you want to output 5v and 12v from a single AAA 1.5v battery?
I'm already doing it. With 1984 technology.
Quote:
You need a DC-DC converter.
Thats what a TL499 is.
http://search.digikey.com/script...

$.75/100 vs. $2.80/100 for the Linear parts mentioned. TL499 also available in SO if I need.

Quote:
I would recommend you use either two 1.5v AAA's in series, or a 9v battery instead
One AAA, a TL499 in DIP, and the few other parts take up less space than either of these.

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> I don't enjoy working with these small parts.

OK, if that's the point, I think you've already got the best
solution anyway. (Unlike you, I /do/ enjoy working with small
parts. It took me some time, too, but now I don't go back to
DIP except for repairing historic stuff.)

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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This might work with a little modification:
http://elm-chan.org/works/led1/report_e.html

KISS - Keep It Simple Stupid!

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It's completely unstabilized, so the output voltage will
vary widely depending on the battery voltage level. This
is acceptable to drive a LED, but not to power some
electronics.

Been there, done that. It works “well enough” for the LED,
but my TPS61028 does a lot better due to its current control
(LED has same brightness down to battery level of 0.9 V),
and that with just one or two parts more than the cheap
flyback converter.

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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I'm actually getting something done in EAGLE (after trying it many times, for some reason I'm starting to 'get it')
http://www.mtcnet.net/~henryvm/T...

I am now using the max232 to get my 12v and letting the tl499 just provide 5v. Seems to work good. I am also using 2 AAA's instead of 1 (can 'extract' more from the batteries).

Now I'm deciding on how to turn this on (and off). Right now I have an N channel mosfet on the battery high side with the gate getting 9v from the max232. To 'jump start' this, I just short the gate/source. It seems to work ok with very little loss from the mosfet.

The question I have, is using a cheap little button switch to do the jump start going to cause problems from running too much current through it? and the other is since I have +/-9v for use from the max232, which is better- high or low side, N or P channel? (it seems N channels are cheaper, and I could run it on the low or high side I think with the +9v)

The tl499 will need probably 500ma max through the inductor (sort of a guess right now). Not finding small switches that can handle the current is why I'm thinking of using the mosfet.

Any suggestions would be welcome on this subject (low voltage, relatively high current switching).

Thanks.

edit-
perhaps I'm making this more complicated than it actually is, as I now find something like this (after expanding my search for packages I know nothing about)-
http://search.digikey.com/script...
which is relatively cheap, is through-hole, has low Rds, and Vgs of 1.5v typ (2.5v max), which means I only have to initially get the gate going (not short drain/source). Which means I could use any switch to do that.

Or something.

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Don't forget that making 12V from 1.5V multiplies the current; its approximately a constant power conversion.

So, a 10ma load from the 12V supply would turn into 10ma *(12V/1.5V) = 80ma load for the battery. And, it gets worse as the battery discharges! And that does not count for the 80% or so conversion efficiency.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Quote:
Don't forget that making 12V from 1.5V multiplies the current; its approximately a constant power conversion.
Which is why I'm heading in the mosfet direction to turn on/off as most 'little' switches are rated to 100ma at most (maybe I'll still find a little slide switch that can handle the current- something the size of the switch on the stk500).

I'll have to do more experimenting, but maybe the part I linked to above will work good on the low side of the battery. Since the battery voltage will be higher when it is off, there may be enough voltage to get the mosfet going even when the batteries are low.

Maybe using a low current switch to short the drain/source to get it going would be fine, too. It may decrease the lifespan of the switch, but maybe it will still last a lot longer than needed.

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Using a MOSFET at only 3 V ist tricky, because there are few MOSFETS that work with such a low gatevoltage. It gets even more difficult since with older alkaline cells you are more at 2.4 V.
It may be easier to use a bipolar transistor.
If you realy need a FET, look at some datasheets of logic level MOSFETs in the 2-30 A range.

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curtvm wrote:
Which is why I'm heading in the mosfet direction to turn on/off as most 'little' switches are rated to 100ma at most (maybe I'll still find a little slide switch that can handle the current- something the size of the switch on the stk500).

It's too bad that you won't give in to the dark side and do SMT. Zetex makes some really nice little SOT23 FETs (http://www.zetex.com/3.0/pdf/ZXMN2F30FH.pdf) which can handle 4 amps with a gate voltage of just 2.5V. Pretty pimp.

Wow. I just looked at at the FDC604P that jayjay mentioned... even more pimp. A larger package, but still... 1.8V, man.

With the 80+ or 100mA draw on a AAA, you might find the voltage depressed prematurely, which with a cutoff of 1.1V wouldn't be ideal.

If you're going to use a DC-DC converter, most of the through-hole units have a relatively low switching frequency, which means large inductors and caps. If you can bring yourself to do even just three SMT parts (two caps and the IC), the small switchers in the 500kHz-1mHz range can use some TINY inductors and caps, and keep your overall project size way down.

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That FD604P looks nice (I have been excluding smd in my searches)

Quote:
With the 80+ or 100mA draw on a AAA, you might find the voltage depressed prematurely, which with a cutoff of 1.1V wouldn't be ideal.
I'm now using 2 AAA to make life easier.
Quote:
If you can bring yourself to do even just three SMT parts
It seems all the 'good stuff' is smd. The TL499 does work pretty good though, and a 47uh/~800ma through hole inductor is not that large. I may still end up in smd land, though.

I did find a small dpdt slide switch that can handle .3a, so if I use both sides .6a (I think). Maybe that will work ok.

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That FDC604P will fit nicely on 2x2 std 0.100" pitch pads.