5v reulated supply for bear bone arduino board

Hi,

This question might have been asked previosuly and i had searched a lot on Google.But i think i can get a more satisfactory nswer over here.

I am building my own Arduino based board using Atmega348 and want to use it for a low power based product.What i read about arduino that due to use of LM7805, i consumes a lot of power even in ideal conditons and if we want a battery back up of some what about 6 months with some components attached to it like 3 sensors, A GPRS/GSM module and a LCD, it’s not possible with 6AA batteries with 1500mAh current.Device will be in power down mode and wake up after every 20 minutes and do some reading, send data and then again go to sleep.We may even use internal oscillator to reduce the power consumption.

So what i though why not remove this 7805 and have some voltage regulator that power up 3.3 v to 5v.I came across ceratin links in net like this
http://www.rocketnumbernine.com/2009/04/10/5v-33v-bidirectional-level-converter

Is that a right approach or what i am thinking of getting a 6 month back-up is not possible.I also went through these links but didn’t get any workable answer

so i will be greatfull if some one can enlighten me more on this.

Can you run everything from 4.5V and skip the regulators?
Use P-channel MOSFETs to turn off current to the 3 sensors, GPRS/GSM module, and LCD, bring all outputs to those devices low (so you don’t power them thru '328P High IO pins feeding power thru IO clamp diodes), go into power sleep mode in between. On wake up, bring up power, bring up IO as needed, do whatever, then power it all down again.

Do you even need to go as high as 4.5V? The ATMega will run happily from 3.3V - how about a small 3.3V LDO regulator to stabilize the output of a 3.7V Lithium Ion cell and run everything at that voltage?

And ditto on the P-channel MOSFETs.

Or just run @ 8 MHz from LiPo/LiIon directly. Add a charge control chip to recharge the battery.

Its not often I actually can contribute but oddly enough I purchased one of these today http://www.pololu.com/product/2561 it outputs 3.3v from 5.5 down to 0.5v and has a shutdown feature to turns off all power to the load.

My question is what would be more efficient, 4AA in parallel at 1.5v at 4500mAh or two in parallel and those two in series at 3V @ 3000mAh?

Wouldn't 4 in parallel be 6000mAH? Going no-regulator would be the most efficient. That design shows better efficiency at 2.4V than 1.5V for most current draw ranges.

Wouldn't 4 in parallel be 6000mAH?

very right you are. I sure do have some fantastic math skills