Bulk DC-DC converter for battery-powered system - Some tips?

Discussion in 'The Projects Forum' started by esm., Jan 20, 2012.

  1. esm.

    Thread Starter Member

    Jan 20, 2012
    30
    0
    Hello Sirs.
    I working on development of three products who will be powered by batteries.
    I'm thinking to use 3x 1,2V NiMh batteries in series. Considering them connected in series, the voltage of the system will be around 3V (when the batteries are almost fully discharged) and around 4.2V (when they are fully charged). I will also need to use these power supplies on USB ports (range 4.75 to 5.25V).

    So my actual input range will be 3V to 5.25V.
    Considering some tolerances, I will need two kinds of power supplies:

    A) Input range: 2,8V to 5,5V. Output: Fixed at 3,3V (capability of 400mA or more)
    B) Input range: 2,8V to 5,5V. Output: Fixed at 5V (capability of 400mA or more)


    Which IC is recommended to meet these requirements, some ideas? I think some Bulk step-up\step-down converters may do this job fine.
    Remembering that input voltage is adjustable (2,8 to 5,5V) and not a fixed voltage.

    I checked LM2735's datasheet from National but I continue thinking that input voltage must be stable to obtain a stable output voltage with this IC and thats not the objective of this power supplies.

    An easy-to-find IC would be great. From National, Fairchild, Microchip, Texas etc.
    Im looking for an IC in SMT package, like SOT23, SOT223 or something similar.

    Somebody suggest ICs for this purpose?
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The LM2735 regulates its output depending on load and input voltage changes.
     
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  3. esm.

    Thread Starter Member

    Jan 20, 2012
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    Hi. Thanks for the answer.
    So you think it is suitable for both power supplies (A and B)?
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Yes, it is fine since it is adjustable via external feedback resistors.

    If you want you can find a fixed regulator for 3.3V and 5V just to eliminate the external resistors.
     
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  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    LM2731 boost converter might also do it, 2735 looks like it has higher current FEt switch. They brought that one out after I left NSC.
     
    Last edited: Jan 21, 2012
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  6. esm.

    Thread Starter Member

    Jan 20, 2012
    30
    0
    Hello friends.
    Thanks for the help.
    Actually, I need a BUCK-BOOST converter, because the input voltage can be lower or higher than the output voltage.

    A simple BOOST or BUCK controller cannot be used for these power supplies, because they just increase the voltage input (BOOST) or decrease it (BUCK).

    Looking at the picture below, the LM2735 can be configured to act as a SEPIC converter. Vout is fixed at 3.3V and the input range is 2,7V to 5V in the example.
    The question is: Does a SEPIC DC-DC converter works like a Buck-Boost converter? Doing the same function I mean.

    If yes, I could change the feedback resistors of the picture below to obtain a 5V output also (for power supply B).

    [​IMG]

    Uploaded with ImageShack.us
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    Yes a SEPIC converter is a Buck-Boost type converter.

    Yes, you can change the feedback resistors (R1 and R2) to obtain 5V output.

    Note: If you build one you should use the PCB layout shown in Figure 17.
     
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