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OPT8241: LED Illumination Driver

Ben Alsin
Prodigy 60 points
Part Number: OPT8241
Other Parts Discussed in Thread: LM3409, TPS92515, , CSD16301Q2

Hi,

I am looking for some guidance on designing a LED driver for Time of Flight Illumination. I have tested the OPT8241-CDK-EVM and gone over the SBAA209A document and 3D Time of Flight TI Training Videos. I am using the HexLED design from the evaluation kit as my reference. The requirements for the LED driver are:

  • Voltage in = 18-20V
  • Drive 4 LEDs at 2-3Amps (voltage across all 4 LEDs would be 14-15V total)
  • LEDs have a 9ns rise and 13ns fall time
  • Up to 24MHz modulation 
  • 50% duty cycle

I was planning on using shunt switching to drive the LEDs as TI states it is recommended "as it enables higher frequency performance". My questions are:

  1. Is shunt switching the better choice over series switching for my requirements?
  2. Is the LM3409 suitable for my application or would a different current source work better? (I landed on LM3409 as is it mentioned in the SBAA209A doc to have worked well in experiments)

Side note: I posted a related design question in the LED Driver forum here but was told the LM3409 was most likely not the right part and they have not designed anything like this. I figured I'd post here instead as there seems to be more specific knowledge about LED Drivers for TOF.  

Thanks!

  • 0
    TI__Expert 3800 points

    Hi Ben,

    LM3409 would work well for this application. You cold also consider using TPS92515 as LED current is only 2A to 3A. I read through your post in the other forum and I understand why they don't think it can be done.

    LM3409 would not dim at 20MHz in the proposed circuit. As 20MHz is an exceptionally high frequency for any power electronics system, we made a separate app note to ensure customers are able to design these systems successfully. As you can see in the app note, LM3409 would only function as a constant current source which is drawn in this circuit diagram.

    LM3409 is the current source and the recommended N MOSFET is CSD16301Q2. You would have to connect ILLUM_N signal from OPT8241 to a buffer and use that buffer to drive the N MOS gate. LM3409 will continue to provide current continuously as the MOSFET switches at 20MHz modulation frequency. You should connect ILLUM_EN output of  OPT8241 to the enable input of LM3409 to save power while the LEDs are not illuminating. This turns off the power circuitry when the camera is not under exposure. Same comments apply for TPS92515.

    The app note covers how to design the  high frequency modulation at such high currents. Along with this post, you should be well covered on how to use DCDC converters to make the illumination driver. Please let me know if this answers your question.

    Regards,
    Subhash

  • 0 in reply to Subhash Chandra Sadhu
    Prodigy 60 points

    Thank you for the response. Knowing that the LM3409 and the TPS92515 are good choices for my application is helpful. I am unable to see the diagram in your post (not sure if it didn't link properly, or if the issue is on my end).

    I have just a few more questions...

    1. What is the max current output for TPS92515 in the shunt FET topology. The datasheet only says 2A, but you said if can handle my requirements of 3A. Can it go even higher like 4A?
    2. Do I use 2 inductors? One spec'd according to the drivers datasheet, and the other being a high frequency inductor spec'd according to the SBAA209A app note? For example, like the schematic below with L1 and L2...

    Thanks!

  • 0 in reply to Ben Alsin
    TI__Expert 3800 points
    Hi Ben,

    I think one of the constraints limiting maximum current on TPS92515 is thermal dissipation. As you would be using TPS92515 in a duty cycled mode, you should be able to use higher peak currents than the datasheet specifies. The official maximum specified limit would continue to be 2A but you should be able to use it for higher currents.
    You would have to use 2 inductors. 1 high inductance part to cater to the spec of the DCDC converter, and another high frequency inductor to allow the high speed switching to happen. It also serves to have a high frequency capacitor connecting the node between the 2 inductors to ground. This reduces the amount of EMI by grounding the high frequency pulses that the high speed switching generates.

    Hope this helps.

    Regards,
    Subhash
  • 0 in reply to Subhash Chandra Sadhu
    Prodigy 60 points

    Great! Thank you very much for the helpful responses Subhash.