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Need some help with LTC4425

Discussion in 'General Electronics Discussion' started by pityocamptes, Sep 6, 2012.

  1. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    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 series) 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. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    It is all in the datasheet. You program the voltage to charge to with this equation:

    VOUT = VFB • (1 + RFB1/RFB2)
    where VFB is 1.2V.

    i.e. divide Vout by 1.2 to get the ration to make the Rfb1 Rfb2 divider.

    Example: To charge to 3.6V use a divider that will divide the voltage by 3, like 2M and 1M.

    And the charging current with this equation:

    RPROG = 1000 • (1V/ICHRG)

    i.e. the resistor is 1K / <charge current in amps>

    Example: to charge a 2 amps, use 1K / 2 = 500 Ohm.


    Bob
     
    Last edited: Sep 6, 2012
  3. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012


    OK, thanks. I will look at it again. The sheet had several different schematics and I was not sure which one to go by. Perhaps you can help on that? Kind of new to this whole thing. Thanks again.
     
  4. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    Explain more about what you are trying to do. The purpose of this chip is to charge up a couple of supercapacitors so that they can provide more current in a short burst than the battery or power supply can supply. Is this what you want? If so, choose the sample application circuit closest to what you want and adjust the 3 resistors to program the voltage and current you want.

    Looking at your OP, you said you wanted to charge supercapacitors from a 2.7 to 5.5V (or is it 4.5 to 5V?). This chip will not charge to a higher voltage than the input if that is what you are thinking.

    What is your input source, (USB port, maybe?) And what is it providing power for?


    Bob
     
  5. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    Another question, what is your experience? This chip is a surface mount chip with a thermal pad that must be soldered to a ground plane of a PC board, which pretty much means that it must be soldered by reflowing. If you did not understand any of that, you probably cannot use this chip.

    Bob
     
    Last edited: Sep 6, 2012
  6. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012

    Thanks. No I realize that. I have done reflowing before. I am pretty good at soldering, and have the equipment, I just suck at trying to figure out schematics that are not set in stone for me to follow. Thanks again!
     
  7. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    Bump? For help?
     
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