Need some help with the LTC4425?

Discussion in 'The Projects Forum' started by pityocamptes, Sep 5, 2012.

  1. pityocamptes

    Thread Starter Member

    Jul 25, 2012
    75
    0
    I'm not sure if this is the chip that I actually need, and if so, what the program values I would need to meet my goals. Here is what I am trying to do. I would like to auto charge two supercaps (in PARALLEL) from an input voltage of between 2.7-5.5 volts (should be stable at about 4.5-5.0 volts at 3amps max) - the super caps have a max voltage of 2.7V. I would like to use the maximum amount of amps from the power source to the supercaps. Anyways, I would like to build a circuit that charges the super caps, but also allows me to power a small device. Basically, use the small device from the input voltage, but when the input voltage is terminated, the supercaps would kick in (sort of like a backup batt system). I was looking at the LTC4425, but I am not sure if this would work for what I need. If so, could someone help me figure out how I would "program" it for my needs? I was looking at the datasheet, and it would seem that if the vout does not equal vin then I won't get the max current? Maybe I am reading this wrong. If this is not what I need, can someone suggest something simple that may work? Thanks again!

    http://cds.linear.com/docs/Datasheet/4425f.pdf
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You should be charging the supercaps in SERIES instead of in parallel. That's what the LTC4425 was designed to do; and it will automatically balance the charge between the caps.

    If you charge them in parallel to <= 2.7v, their voltage will quickly drop when you connect them to a load.
     
  3. pityocamptes

    Thread Starter Member

    Jul 25, 2012
    75
    0

    Thanks! Will take this advice. How about the configuration? I am not sure about the chip or programming resistor, or if I need it, etc. Thanks again!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Give us some more information.

    Are these two supercaps identical?
    You say they're rated to 2.7v; what's their capacity in Farads?
    Do you have a manufacturer name?
    What is the part number?

    What is the load that you're planning on powering? What are its' voltage and current requirements?
     
  5. pityocamptes

    Thread Starter Member

    Jul 25, 2012
    75
    0
    Yes, two identical super caps, 2.7v @ 700 farads each. I got them off ebay, so probably chinese made.

    The loads would be small devices like to recharge a phone, mp3 player, stuff like that. Thanks.
     
  6. pityocamptes

    Thread Starter Member

    Jul 25, 2012
    75
    0
    Anyone? Bump! Thanks.
     
  7. pityocamptes

    Thread Starter Member

    Jul 25, 2012
    75
    0
    Bump? anyone?
     
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