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LM3409: LM3409 Shunt FET Topology Inductor Selection for TOF

Ben Alsin
Prodigy 60 points
Part Number: LM3409

Hi,

I'm trying to to design a high powered LED driver for a TOF sensor. I will be driving 4 high powered LEDs in series at 2Amps and modulating them at frequencies up to 20MHz. I'm planning on using the LM3409 with a shunt FET topology. The voltage input range is 18V-20V and the voltage across the 4 LEDs is 14.8V (3.7V x 4).

First off, is the LM3409 suitable for this application? Is there a better suited driver?

I've reviewed the TI resources: SBAA209A document and the 3D Time of Flight Sensors TI Training, but I am having trouble figuring out what inductor I need to use. I am slightly confused because I am not sure if I need to be following the LM3409 datasheet, the TI Time of Flight resources, or both... 

Video #2.6 in the Time of Flight Sensors TI training states the inductor should meet the following requirements:

  1. The ripple current should usually be kept to less than 10% of Imax. (Since my Imax=2A, ripple current should be < 200mA)
  2. Self resonance frequency should be greater than the modulation frequency (20MHz in this case)
  3. Saturation should be ~40% greater than Imax (So rated for >2.8A in my case).

Info from the LM3409 datasheet:

  1. The ripple current should be greater than 24mV/Rsns. So if Rsns ~0.1Ohm then ripple current should be >240mA (This seems to conflict with requirement #1 above, which one do I follow?)
  2. Using the datasheet equations and playing with the variables I always calculate an Inductor between 10uH or more which is quite a bit larger than the 510nH inductor used in the shunt FET topology Time of Flight evaluation. 

Are these requirements are for the same inductor? I am assuming there is only one inductor used in the LM3409 with a shunt FET topology, not two. Should I be trying to find an inductor that satisfies all these conditions? Any help in pointing me in the right direction would be appreciated. Thanks!

Ben

  • 0
    TI__Mastermind 28655 points
    Hello Ben,

    Just want to make sure, you want to shunt FET dim at 20 MHz? Most likely this will not be the correct part for this if you want to shunt FET dim at 20 MHz.

    What % dimming do you plan on using, example 5% to 100% duty cycle?

    Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 60 points

    Hi Irwin,

    Yes, my plan is to modulate the LEDs at 20MHz using shunt FET dim for illuminatiom for a time of flight sensor. I was planning on using shunt FET for this as it seems TI recommends this over series switching because of the high LED modulation frequency.

    The TI time of flight eval kit looks to be using this topology to modulate lasers at 80MHz. I will be modulating the LEDs at only a 50% duty cycle. No other duty cycle is required.

    Thanks!

  • 0 in reply to Ben Alsin
    TI__Mastermind 28655 points
    Hello Ben,

    This is theoretically possible but we, in this LED group, have not built or tested anything like this. You do want the inductor current ripple to be small.

    Regards,