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Adjustable current on buck-boost IC designed for adjustable voltage

Discussion in 'The Projects Forum' started by Obi_Kwiet, Mar 14, 2012.

  1. Obi_Kwiet

    Thread Starter New Member

    Apr 4, 2011
    I am trying to design a buck boost current regular that could be used to drive LEDs or laser diodes from a Li-I battery. I've been looking over datasheets for several days, and I really like the TI TPS63020. The trouble is that it's designed to regulate voltage, and I need fairly precise, adjustable current regulation. Looking at the datasheet, it seems like I should be able to modify the voltage divider on the FB pin and use a current sense resistor and an amplifier to achieve current regulation. I was considering the National Semiconductor LMP8645.

    However, I'm worried about controls issues. I've on had one fairly poor control systems class, and I'm trying to teach myself power electronics, so I don't really have a feel for whether this approach is going to introduce instability into the control loop. I just know that I really don't know what I'm doing in this respect, and I'm not really sure how to know other than to brute force my way through some power electronic and control systems textbooks. I considered this, but when I do this, I get bogged down and finish the project. I would be grateful for some insight or at least a hint in the right direction.

    Note that in addition to getting a 2A output current between ~2 and ~6V, I really want to be able to get the whole package as small as possible to use in flashlights. I want to get the size of the package as close to 200mm^2 as possible, and smaller if I can.
  2. crutschow


    Mar 14, 2008
    Linear Technology and TI make switching regulators especially designed for powering LEDs with a constant current. I suggest you look at those. As you say the control issues of converting a constant voltage converter to a constant current require a considerable expertise in close loop control system design.
  3. DickCappels


    Aug 21, 2008
    With the National Semiconductor part, you should be able to use their Web Bench online (Java, I think) application, which should provide you with a complete parts list.

    Failing that, just build and application circuit and see what happens. Nothing teaches an power supply engineer like a few 2nd degree burns on his fingers (seriously).
  4. Obi_Kwiet

    Thread Starter New Member

    Apr 4, 2011
    Thanks! I'll give it a shot and see what blows up.