looking for a good dc-dc step up solution_no real estate..

Discussion in 'The Projects Forum' started by jaydag77, Jan 14, 2008.

  1. jaydag77

    Thread Starter Member

    May 28, 2007
    17
    0
    Hullo again all- today I am again in need of some experience. I have been trying to find a preferably integrated solution that can step up both voltage and current? I am definatly not familiar with building power supplies but have been doing quite a bit of reading about it and also looking at various manufacturer offerings- I am hoping that someone can guide me better than I have been doing!

    I need to step up a N-cell or those rough specs- 1.2vdc @ ?mA to about 3.6vdc @~300mA or better. I also am looking for a better battery choice- should be rechargable and at least as small as an n-cell. If I could find one at 3.3-3.6vdc 300-500mA then I would be in great shape, but haven't been able to find anything that small...

    Am I being unrealistic here? Any advicewould be appreciated, I can provide additional info if needed, but must say that alot of the parts I am using are salvaged, like ir leds so I don't know much about them, which is also causing some headaches....

    The solution I need has to be as small as they get, the pcb itself is only 21 x 19mm and is already packed, I was thinking about making the pow supply a tiny plug-in in series with the batt leads, like a micro dongle or something? I dunno, I just was farting around with the pcb layout late last night and it is increasingly obvious that there are to many parts and not enough space, so I may need to cut back on a couple of functs still...

    thanks in advance--!me
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,145
    1,791
    Are you being unrealistic? Yes I think so. In a DC-DC converter with a boost technology you can raise the voltage but the available current is reduced.

    The input power to a converter is volts times amperes with units of watts. Call it Wi. A typical DC-DC converter might be 80% efficient. So the available output power is 0.8 * Wi, call it Wo. Divide Wo by the desired output voltage and what you have left is the available output current.

    So if you need 3.6 V * 300 mA that is 1.08 Watts.
    Input power is 1.08 / 0.80 is 1.35 Watts
    Input Current is 1.35 / 1.2 = 1.125 Amps

    With that kind of current draw your batteries should collapse in the twinking of an eye.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    My cell phone has a small lithium battery. It is 3.0V to 4.2V (3.6V nominal) and will supply 300mA for 2 hours. It charges in 2 hours.
     
  4. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Power density is definitely one of the most challenging aspects of creating small projects. You should find a small prismatic lithium polymer in roughly the same dimensions as the PCB itself. I believe that they can happily deliver 3.6V@300mA, but you need to be wary of the heat.

    When I was younger and maybe a bit inspired by a recent Mission Impossible film, I tried to make a RF bug device as small as possible. In the end, I had it down to roughly 1cm x 1cm x 2cm. It operated at 900MHz and ran off of a small and expensive prismatic lithium polymer battery. I never built it though, since I didn't know that LTCC circuits cost as much as they did...

    Steve
     
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