EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: nathanpc on July 15, 2013, 01:42:02 am
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I'm currently designing a battery powered, 2xAA in this case, project. The problem that I have is very simple, I want to power a DS1307 (http://datasheets.maximintegrated.com/en/ds/DS1307.pdf) (the only part in my circuit which requires 5V). I've looked into similar parts like the DS1337 (http://datasheets.maximintegrated.com/en/ds/DS1337-DS1337C.pdf) and the MCP7940 (http://ww1.microchip.com/downloads/en/DeviceDoc/25010A.pdf), which operate under 3V, but they don't have a battery backup pin like the DS1307, which is something I want.
So, the first thing that I thought was to build a step-up DC/DC converter circuit to power the RTC (that's the first thing I thought, so I'm open for suggestions). Since I never used a switchmode converter before I decided to watch Dave's video about the MC34063 (http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF):
(https://www.youtube.com/watch?v=qGp82xhybs4#ws)
But the problem with it IC is that its minimum input voltage is 3V. So I would like to know some tips on how to choose a suitable switchmode converter for this kind of project.
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There are plenty of boost converters, it's enough to just go on digikey.com and play with the filters, for example here's some suggestions : http://goo.gl/0THCB (http://goo.gl/0THCB)
But the power consumption of that ds1307 is low enough that you may get away with a voltage doubler/tripler and then getting it stable with a low drop linear regulator like mcp1801, mcp1702 etc .
Dave did a video on voltage doublers just a few weeks ago if I remember correctly.
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Is it always powered from batteries or are there also time when it is wall powered? If always from battery, I don't see much advantage in requiring the vbatt pin. I would consider selecting something operating at a lower voltage if possible.
As far as boost converters, look at Semtech SC120, very easy to use (just copy the application circuit in the datasheet). It has decent Iq and low Vin.
Personally I prefer using a low, single alkaline cell so that the PS is always a boost configuration over the voltage range of the battery. (Assuming mostly 3.3v circuits. For 2 cells there is an interesting boost converter from TI that is a boost but also does some step down conversion for fresh batteries. Part number escapes me at the moment...
Voltage multipliers work but they don't have very great efficiently. They also don't regulate voltage so you would end up with more loss in the circuit for regulation.
I have been doing an experiment with SC120 where I'm powering a Hall effect sensor with the SC120 and a single AAA battery, it's been running non stop since march and still works like a charm.
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Use a DS1338-18? It has a separate VBAT supply and works down to 1.8V.
http://www.maximintegrated.com/datasheet/index.mvp/id/3696 (http://www.maximintegrated.com/datasheet/index.mvp/id/3696)