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Understanding the LM3409 ratings

Other Parts Discussed in Thread: LM3409, LM3401

Hey all, this is my first post here.

I'm building an LED lighting circuit around the LM3409 controller using Cree XM-L LEDs. I am using the WEBENCH tool which is very helpful to help me select components. I hope to use the 3409 in multiple designs of varying LED strings and currents, including: 3S, 3S2P, and 3S3P at 3A/string (total 3, 6, and 9A respectively), as well as other configurations.


My question is - the datasheet specifies the maximum I_out for the 3409 is 5A. However, my understanding is that this controller (or any asynchronous buck with external FET) simply controls the gate for a power FET, which should be rated for the proper current. So, what does the 5A rating refer to?

Thanks in advance

  • Hello Mitchell,
    You are correct, as a controller it can do more than 5A. I spoke with the original systems engineer for the device and he said it was more a marketing thing to put that in the datasheet along with keeping some folks out of trouble. The main issue, other than power dissipation, is that you need to select a very good FET to do higher currents. It's all in the ability of the LM3409 VCC regulator to drive it hard enough that you do not destroy efficiency (or the FET), so the lowest gate charge you can find is a must. This probably is even more important than finding the lowest Rds(on) device as long as the package can still dissipate the heat generated. At higher currents noise also becomes an issue, so very careful layout is a must.
    You can always create an external totem pole driver as well if you need the extra gate drive, but that requires a few extra components.
  • Hi Clinton,
    Thanks for looking into that! It makes more sense now. My knowledge is a bit limited since this is my first switching regulator build. Can you possibly point me in the right direction for learning what the terms Rds(on) and the gate charge mean? I have a basic understanding, but how they relate to designing an LED driver capable of higher current still beyond me!
  • Hello Mitchell,

    There is some good information regarding this in the PFET selection section of the LM3409 datasheet, and maybe even a more helpful one on pages 11 and 12 of the LM3401 datasheet (they are similar, both PFET controllers).

    But the Rds(on) in the ON resistance of the FET chosen (determines power dissipation during the ON time) and the gate charge determines how much current the VCC of the device must supply to drive it effectively without excessive turn on/off losses.

  • Hi Clinton,

    I have a similar question and since you've talked to the designer already, perhaps you have a quick answer.  I haven't throughly read through the datasheet yet (doing so right now) but would it be theoretically possible to drive a 20A LED if I find a suitable PFET?  Or would this be nigh impossible?

    Thanks!

  • 20A is possible, but unlikely. I haven't researched PFETs in a little while, but if you can find one big enough to do 20A that still has a low enough gate charge for the LM3409 to drive (you will likely need to use a low switching frequency as well) then it is possible.
  • After a quick look, I found: STL42P4LLF6 is a 40V, 42A PFET with an rdson=18mohm and Qg around 20nC @ Vgs=4.5V.

    This appears to meet the rule of thumb in the datasheet to keep Qg < 30nC.  Is there more math somewhere to explain this? Or the comment that "if the switching frequency remains below 300kHz then a larger Qg can be considered".

  • The LM3409 will drive that fine, the math you need is in the two datasheets I mentioned. Just keep in mind the power dissipation (also explained in those datasheets). 18mohm isn't a lot, but 20A is. The FET will dissipate significant power so heatsinking will most likely be needed.