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LM3414: lm3414 LED driver initial rush current when load conected

vikas salvi
Prodigy 90 points
Part Number: LM3414
Other Parts Discussed in Thread: LM3409, TPS92515

Hello , 

i am using LM3414 for LED string (4 LED in series ) driver . Power supply 18~ 24V . LED VF = 2.82 V/ LED 

please see attached schematics  . when switch B closed (2LED in CIRCUIT  ) , When Switch A is close from open condition , rush ciurrent peak occur which is more than 1.5A .

but when switch B open (4 LED in circuit ) and switch A open to close no much inrush peak occure ( less than 1.5A )

Please help how to reduce that peak above 1.5A ???

  • 0
    TI__Mastermind 28655 points
    Hello,

    If you open the LED string the error amplifier will try and compensate to get current flowing to the output, railing high. When the switch is closed the error amplifier needs to readjust to the load returning causing an overshoot until the error amplifier recovers. I would recommend using the DIM pin when doing this. You could also used the iadj pin to soft-start the turn-on. You could also look at the LM3409 or TPS92515 which are hysteretic parts that will not do this.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 90 points

    Hello Irwin , 

    thank you  for your explanation on issue , can you please let me know how i can use DIM pin to slove this issue ? current design i have left Dim pin open . 

    so please let me know also how soft start Iadj pin ?? 

    thank you .

  • 0 in reply to vikas salvi
    TI__Mastermind 28655 points

    Hello,

    I would start by using DIM by pulling it low when you open the LED string, however, see below.  This way it won't try to drive current to an open circuit.  You may find that when switching from four LEDs to two that you have to lower the LED current before disabling with DIM, I would try the following before doing that. 

    I'm not sure how you are planning on running this (sequencing).  If you use the DIM pin you can remove switch 'A' since Dim will control the on/off of the LED string, that will leave only switch 'B'.  You can try closing 'B' and see if the overshoot is acceptable, if it isn't you may need to use Iadj to lower the current before switching 'B' closed.  Again, TPS92515 and 3409 would have no problem doing this because they are hysteretic parts, no control loop, they just regulate peak current.

    Best Regards,