• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 544 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3407 Rsense Current

Dan Cristy
Intellectual 2615 points
Other Parts Discussed in Thread: LM3407

Hi Team –

Posting  this on customer behalf.

 

Currently doing a design using the LM3407.

Vin = 24V

VLED drop = 6V (6 series, 5 parallel strings, 1V drop across each LED)

L1 = 68uH

Rsense = 0.82

Based on Rsense, we expect to see 240mA, but we’re only seeing 140mA. By increasing the voltage drop across the LEDs, then it will eventually hit 240mA (around 8V). The inductor current is continuous.

Is there a minimum Vdrop across the LEDs for proper operation?

Thank you.

  • 0
    TI__Mastermind 25500 points

    Hello Dan,

    What switching frequency are you using? There should not be a Vf limitation, only possibly duty cycle limitations that could limit the Vf range, but usually it goes the other direction in those cases. Could you possibly provide a schematic with component values and a BOM? This one is kind of odd so I'm wondering if maybe the inductor has a saturation current rating too low so it is hitting current limit at the lower output voltage.

    Thanks,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 100 points

    Clint,

    it turns out that the snubber we were using to try and quiet our switching noise was sourcing current into the switch node when the Vdrop across the LEDs was low, causing the current through the LEDs / inductor to drop. We were using a 3 ohm / 5100pF snubber, not sure where those values came from.

    -Ben

  • 0 in reply to tracemhos
    TI__Mastermind 25500 points

    Hi Ben,

    Yeah, I saw the switching waveform. The switching edges were very slow due to the snubber and that limits your minimum effective duty cycle. I'm glad you have figured it out.

    Best regards,

    Clint