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Any innovative ideas for this LED driving problem?

Discussion in 'Electronic Design' started by [email protected], Nov 29, 2007.

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  1. Guest

    I am sure many of you are familiar with the newer higher-power SMT
    LED's that are out. They generally require larger copper pads on the
    PCB to dissipate heat.

    I am working on an application where I need a lot of brightness, and
    very small size. I also need to have my design be dual color (for
    example red and green). My solution was to put the LEDs in strings of
    three, side by side but oppositely biased to one another. So that if V
    + is on the left and GND on the right, its red, if V+ is on the right
    and GND on the left, its green. Since red and green would never be on
    together, this allows me to use the same copper for power dissipation
    on both strings.

    I was originally using an H-bridge to swap the polarity. But the
    design has turned into 6 strings of 3 for red and 6 stings of 3 for
    green. Six h-bridges... I am trying to keep the board as small as
    possible and I am running out of room doing this with discrete
    components.

    Of course they make motor drivers that are H-bridges built in, but
    they are expensive and overkill for what I need. If I had my wish
    come true, it would be for a small chip with 5 or so pins... power and
    ground in, a control line, and to outputs. If control is low, output
    1 is V+ and output 2 is ground, and if control is high, output 1 is
    ground and output 2 is V+. Does such a beast exist?

    If not, is there any other suggestions on how to tackle this? I could
    do away with the h-bridge idea and just parallel the strings of LEDs,
    mass the cathodes (since that's where heat dissipation is needed) and
    seperate the anodes and control via transistors switched from the
    microcontroller, but I am not sure if that is the best way.

    As a non-professional designer, does anyone have alternate suggestions
    or something I am missing or perhaps a more elegant solution?

    Thank you!
     
  2. John Larkin

    John Larkin Guest

    On Thu, 29 Nov 2007 09:21:21 -0800 (PST),
    How much voltage? How much current?

    John
     
  3. Guest

    John,

    The LED's are in strings of 3, and have an individual Vf of ~2.65V and
    a forward current of 150mA each. So each string would be around 7.95V
    and 150mA. There are 6 strings so a total current draw of 900mA. I
    do need each string of three to be able to be individually turned on
    and off seperately from the other 5, which adds a bit of complexity.
    I am using an Atmel Mega8 microcontroller so I have around 15 outputs
    to work with after the ones I am using elsewhere are accounted for.

    This project will run on 12V and I am trying to keep this all on a PCB
    of around 4 square inches. Not a ton of room to work with, although I
    don't mind moving the power solution to a sub-board if necessary (and
    if that helps me keep this from being a 4-layer board).

    Thanks for any insight you can provide!

    -Rina
     
  4. T

    T Guest

    >,
    says...
    What you are describing sounds very much like this 8x8 matrix:

    http://www.sparkfun.com/commerce/product_info.php?products_id=682

    It's a common cathod setup, so with three wires you can trigger a cell
    to go either red, green, or a yellow-orange color if both are set high.
     
  5. James Arthur

    James Arthur Guest

    Why not just connect like this...

    (View in Courier or fixed font)

    V+
    -+-
    |
    Red LEDs | Green LEDs
    +---------+-----+-----+---------+
    | | | |
    --- --- --- ---
    \ / ~~> \ / ~~> \ / ~~> \ / ~~>
    ----- ----- ----- -----
    | | | |
    | | | |
    --- --- --- ---
    \ / ~~> \ / ~~> \ / ~~> \ / ~~>
    ----- ----- ----- -----
    | | | |
    | | | |
    --- --- --- ---
    \ / ~~> \ / ~~> \ / ~~> \ / ~~>
    ----- ----- ----- -----
    | | | |
    +----+----+ +----+----+
    | |
    | |
    | |
    | |

    ....and drive them with just 2 open collector transistors / MOSFETs +
    current limiting?

    Best,
    James Arthur

    They can still share they same heat sinks for the same reason as
    before: they're not bot on at the same time.
     
  6. Frank Buss

    Frank Buss Guest

    Connecting LEDs in series is no problem with a constant current source, if
    it can provide high enough voltage, but don't connect LEDs in parallel,
    because LEDs are very non-linear. E.g. if some LEDs on one string have a
    bit lower resistance, they'll get a bit hotter and then the resistance
    falls again, which is no problem with the constant current source, but the
    other strings gets less current and will be darker.
     
  7. James Arthur

    James Arthur Guest

    Yes, of course--I was trying to sketch the concept, simplified, which
    is why I didn't show the drivers or the limiting.

    I assumed his LEDs have electrically isolated thermal pads however,
    which they might not.

    Cheers,
    James Arthur
     
  8. James Arthur

    James Arthur Guest

    That's about 7 watts max. dissipation in the LEDs, plus any current
    limiting losses, smd, on a small board. Yikes!

    You could use fewer, bigger LEDs, and/or pipe the heat out with
    thermal vias to a "real" heatsink on the backside of the board.

    But we don't know your constraints, optical requirements, or other
    degrees of freedom.

    For driving your existing scheme there are H-bridge ICs like the
    Vishay/Siliconix Si9986--that's an 8-pin part. Possibly some MOSFET
    gate drivers could handle 150mA continuous.

    Cheers,
    James Arthur
     
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