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Luxeon 5w driver

Discussion in 'Electronic Design' started by Olivier, Oct 19, 2006.

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  1. Olivier

    Olivier Guest

    Hello all.
    Does anyone have an idea on how to build a constent current (1A) driver for such
    a big white led (Vf=3,5v). I need to ligh it from 2 NiMh batteries (2,2v).
    I have already tried a TPS61020 from TI (http://i10.tinypic.com/3yfmno3.gif) but
    I had big thermal problem caused by the small package of the IC.

    Olivier, Italy.
     
  2. ehsjr

    ehsjr Guest

    According to the datasheet, the TPS61020 provides only about
    350 mA. Take a look at the TPS6130, 31 or 32
    http://focus.ti.com/lit/ds/symlink/tps61031.pdf

    Ed
     
  3. GregS

    GregS Guest

    The Luxeon V will drop over 6 volts or do at 700 ma. average, 1 A peak.
    The K2's will peak around 1.5 amps.

    greg
     
  4. Olivier

    Olivier Guest

    I can't find that in the datasheet. At page 2 there is a table that says OUTPUT
    NOMINAL SWITCH CURRENT LIMIT 1500 mA for the TPS61020DRC.

    Olivier
     
  5. Olivier

    Olivier Guest

    I use the K2 white LEDs at 1A for thermal dissipation problems.

    Olivier
     
  6. default

    default Guest

    Have you read the datasheet with respect to thermal considerations?
    Looks like they expect the pad to be mounted against a heat sink.
     
  7. ehsjr

    ehsjr Guest

    The easy answer is here, in TI's literature:
    http://focus.ti.com/lit/ml/slvr259/slvr259.pdf
    See the bottom of page 1 under "CURRENT LIMITATIONS
    AND CAPABILITIES:" It states: "The TPS61020 is capable
    of supplyinmg about 350 mA of I/O current with an input
    voltage of 1.8 V."

    The answer that requires a little more effort is understanding
    the information in the datasheets. I'll try to explain
    what you can find there regarding output current and how
    you can use the information.

    First, read the description on the first page for both
    the 61030 family and the 61020 family. The 30 family
    description states "... is capable of delivering output
    currents of up to 1A at 5V at a supply voltage down to
    1.8 V". The 20 family description states "Output
    currents can go as high as 200 mA while using a single
    cell alkaline, and discharge it down to 0.9 V. It can
    also be used for generating 5 V at 500 mA from a 3.3-V
    rail or a Li-Ion battery." If you interpolate the numbers
    shown, with 2 V input you get about 350 mA out using
    the TPS61020.

    There's more to be found when you look into the circuit
    and the datasheet numbers. You need to consider worst case
    (battery voltage at ~ .9 v per cell) and use that in your
    design, and you need to understand the whole datasheet.
    Look at figure 1 (page 7) on the datasheet. It shows the
    maximum output current vs the input voltage. Note that with
    lower input voltage you get lower max current, and that at
    1.8 volts input, it is no where near 1 amp - you can get a
    maximum of about 400 mA. You want to allow a little extra,
    so that's about 350 mA.

    Another interesting specification is efficiency. The 61030
    is over 80% efficient at 1.8 volts input with a 1 amp
    load, while the 61020 drops below 50 % at that voltage,
    even if the load was only about 350 mA. That means that
    over 50 percent of the energy taken out of the battery
    becomes heat. In your circuit, it is even worse, because
    you are boosting the voltage, while the curve is using
    Vin = Vout. It's no wonder your 61020 is getting hot!

    Ed
     
  8. Guest

     
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