5V boost circuit: 1AA or 2AAA? 12V boost possible?

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The application is a tiny robot which will hopefully draw in the neighborhood of 50mA undle normal conditions and maybe 100mA in a worst case scenario.

5V boost chip: MAX856 ... good choice? I like it on paper, low parts count.
Power source: One 2500 mAh NiMH cell... good choice? Sounds better than two 750 mAh AAA cells.
Would it be a wise idea to "gang" a 12V boost regulator on the output of the 5V boost circuit? Or maybe someone knows of a 12V boost regulator that can run on only one or two cells?

Thanks for any feedback 8)

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I build a device that has "ganged" power supplies. The first switcher boosts 3 volts to 3.3, then a second
boosts the 3.3 to 100 volts. I'm using the SOT-23 MAX1522 controller for the hv supply. This chip wants
at least 2.5 volts to operate on, so you might, for example, power the 1522 from the 5 volt supply, but
feed the inductor directly from the battery so the 12 volt load is not a burden on the 5 volt supply.

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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Max1760 for 5V boost, lm2621 for 12V

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MAX771 for 12V, u can pull upto 0.5A from it, i found it to be pritty stable and consistantly puts out 12V ( even when the input drops to <4V).

http://pdfserv.maxim-ic.com/en/d...

nesh

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Wow that LM2621 is a very cool part....

Any comments about 1AA vs 2AAA? Thanks for all the advice guys!

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One Li+ AA size accu plus MAX1811 charger, or 2 AA NiMH with external charger.
Alexander,

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Quote:
Would it be a wise idea to "gang" a 12V boost regulator on the output of the 5V boost circuit? Or maybe someone knows of a 12V boost regulator that can run on only one or two cells?

If you use custom magnetics then geting higher voltages is not a problem,
A Maxim Apnote covers this: Autotransformer Boosts Maximum VOUT - MAX857
http://pdfserv.maxim-ic.com/en/a...

A centre tapped inductor may just work.

Peter

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Why do you need the 12V ? Is there no way to use 5V for this instead ? Would be more efficient

P

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1.2v to 12v step up is not really practical for the kind of currents you are going to draw. With 100mA output, the cell is going to be subjected to a current draw of well over 1 Amp. Not many switchers will be capable of providing this, and you'd have to be very carefull about voltage drops when you have so little to start with.

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The 12V is for two tiny Maxon motors. They draw hardly any current at all. I believe the majority of my power consumption will come from the optics (color sensor and various IR sensors). But, the PCB layout is getting pretty full anyway, and I agree with the points made thus far, so I will just hope the little motors can move the robot at 5V instead of 12V. Still open to any ideas and suggestions since I'm still early in the design phase...

Thanks for all the input guys ;)

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If the 12V is for two motors only, then you might consider an adjustable supply for them. Many of the single-cell switchers (such as LM2621) can be set to any voltage by changing a resistance.It can also be shut down when you aren't using the motors, saving even more current.

The software for controlling the motors "independently" will get a lot more complex.

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I just visited WEBENCH, where it seems that their line of ICs doesn't have a practical solution for stepping 1.2V up to 12V.

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Linear Technology has the best selection for this kind of forward convertor. LT1930 will work down to 2.6 volts, LT1615 will work down to 1V. These are pretty low-power devices, though. The switch current is the limit. LT1930 has a 1A switch, so (ignoring inefficiency) you can get at most 250mA at 12V from a 3V input (3/12 * 1A). The LT1615 has a 350mA switch, so it can only deliver 35mA at 12V from a 1.2V input. LT has many other parts, these are just two that I know.

http://www.linear-tech.com

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t's a shame that national hasn't much to offer when it comes to boost or flyback regulators for low input voltages. I've used their line of simple switchers and flyback regulators in many applications and have found them to be very easy to implement and reliable, especially when designing in conjunction with web-bench. I have also found pricing and availability of these parts to be good also, which is something I certainly cannot say for Maxim and LT.

Assumption is the mother of all f#ckups!

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Hi all
i am using LM2621.i am getting the output same as input voltage.I tried the circuit which is mentioned in datasheet.can any one tell me how to test it.t

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If you get the same output as input, then simply the chip isn't working and the input is just passing through without boost. Make sure the enable pin (pin 2) is connected to pin 6 and that the diode from pin 8 to pin 7 is a schottky.

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I have used the LM2621 in the past and found it rather finicky, more plug and pray than plug and play...

But I should mention it might have been the alligators, I bought a bunch from eBay that turned out to be copper coated steal with a few ohms resistance...