High power LED driver with brightness control

Discussion in 'The Projects Forum' started by harsha.ub, May 25, 2009.

  1. harsha.ub

    Thread Starter New Member

    May 25, 2009
    5
    0
    SgtWookie>> suggests using http://ww1.microchip.com/downloads/en/AppNotes/01138A.pdf or http://ww1.microchip.com/downloads/en/AppNotes/00874C.pdf.
    I am not able to comprehend the definite way amongst all these deluge of info.....kindly help me out!">Dear All

    Application I am intending to build - high power white LEDs for illumination in a lab envoirnment. I am using 4 Seoul P7 LEDs (data sheet attached), 4 AA batteries(1.5V each)/ 6V battery pack.

    I want to have a driver for this along with a dimmer circuit such that I can control it using a pushbutton(similar to OSRAM DOT it).
    The push button must be circularly moving between states -
    1st state - OFF
    2nd state - 25% brightness
    3rd state - 50% brightness
    4th state - 75% brightness
    5th state - 95-100% FULL brightness
    constraints include formfactor of the final PCB!

    I have searched through this forum and in general on the internet and from what I have learned I could/ should/ would
    > Use STMicroelectronics L5973 LED driver.
    > Or some other ideas presented in the threads
    http://forum.allaboutcircuits.com/showthread.php?t=13549 where the guy is using the same LEDs as me.
    http://forum.allaboutcircuits.com/showthread.php?t=18860 also provides some good insight into how to go about designing it.

    I need help on -
    Deciding whats the best way to go about it and some handholding when I get stuck up....
    I understand that PWM is the best way to acheive brightness control but then not usre how to go about combining the LED driver chip, PWM and pushbutton interface! also in one of the threads I found links to these app notes where << SgtWookie>> suggests using http://ww1.microchip.com/downloads/en/AppNotes/01138A.pdf or http://ww1.microchip.com/downloads/en/AppNotes/00874C.pdf.
    I am not able to comprehend the definite way amongst all these deluge of info.....kindly help me out!
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The LEDs can pass 2.8A EACH!
    Assuming that you don't need to blind everybody, at 1A each the total current from the battery is 4A.

    Four AA alkaline cells in series are 6.0V with no load and the voltage will drop to 4.2V in a few seconds at 4A. You will see the LEDs dim as the battery dies.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Like AudioGuru wrote, it's just not practical to try to power those LEDs from a supply as puny as AA batteries. You would either need a large and heavy sealed lead-acid battery (that won't last very long) or a mains-powered supply.
     
  4. Mike2545

    Active Member

    Mar 26, 2009
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  5. harsha.ub

    Thread Starter New Member

    May 25, 2009
    5
    0
    Thanks for yoth your opinions, currently the set up is powered using 4 AA rechargables. The reason I feel that its sufficient is....I don't need to use it continously on .

    Currently, a third party driver board is being used which uses ST driver chips.....what I want to do is add a push button interface with brightness control so that battery life may be extended.

    Also the driver board is so small 15 x 15 mm that I can't make out schematics!!!

    Hence I thought its better to design it from scratch.
     
  6. harsha.ub

    Thread Starter New Member

    May 25, 2009
    5
    0
    I am sure that I need to use them in parallel, also aware that heat sink should be capable of dissipating 10 W of power.

    The following graphs from data sheets reveal enough for me to decide that one can scrape through with current constraints.

    The current driver is 1.4A (http://ledtreiber.de/ - Mini SMD 1400mA - 2.7V~4.5V )

    Now, I can either build something from scratch like this or just add a push button interface to the existing driver.

    Worst case would be to add a push button interface with brightness control.....can you please throw more light on this aspect.
     
  7. harsha.ub

    Thread Starter New Member

    May 25, 2009
    5
    0
    Hello All

    I have some more update, basically the LEDs in question are '4 LED die in one' and together they need about 3.5V and max 2.8A but workable with just about 1.4 also!

    Now I just need a pushbutton interface to implement circular switching, along with suitable driver......hope this puts every thing in perspective.
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    1.4 Amps? Given the brightness of these units you can always use less amps, it will extend the battery life.
     
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