increasing LED drive current with PWM

Discussion in 'The Projects Forum' started by vahidasadi, May 21, 2010.

  1. vahidasadi

    Thread Starter New Member

    May 21, 2010
    11
    0
    hi everyone
    first, i'm sorry if my question is noob
    i want to drive my infrared LEDs with pulsed high current(mine are SFH485P) using simple 555. i problem is that i can't fully understand it's datasheet information here is it's pulsating curve [​IMG]
    and voltage drop table
    [​IMG]

    could anyone tell me what 1 A & tp=100us means? is this means that if i want to power the LED with 1 A , i don't have permission to drive them more than 10e-4 second duration?
    and from curve max duty cycle could be 5%? and it means that it will be powered for 10e-4 second and for 0.0019 second will be off.
    am i right?
    my aim is to drive LEDs with high frequency pulsed current and sync it with camera to use it in my project (s.th like night vision cameras)
    thank you in advance
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If=1A,Tp=100uS means a pulse of 1 Ampere for 100 microseconds, or 0.1 milliseconds, or 0.0001 seconds.

    If you're going to blast it with 1A current, you'll only have a 0.5% duty cycle or risk frying the LED(s).
    So, 100uS on, and 9.95mS (9,950uS) off.
     
  3. vahidasadi

    Thread Starter New Member

    May 21, 2010
    11
    0
    but why duty cycle is 0.5%. from curve tp=10e-4 hits 1A axis with 5 %.
    and another point is if you choose D=0.5% so T will be tp/D=(10e-4/0.005)=0.02 second=20 ms not 10 ms
    maybe I'm wrong
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Doesn't your data sheet (I assume that's what the numbers are) show 100µS for 1A?

    [​IMG][​IMG]

    This is the max duration this LED can be on. Add the 5% shown in the chart and it is 100µS on / 2ms off.

    You might get by with it, but don't be surprised if the LEDs die. It is never a good idea to push components to the edge of their ratings.

    The gain isn't that great either, since the average power is actually less, much less.
     
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