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LM3414 without regulation?

David Dean
Expert 2305 points
Other Parts Discussed in Thread: LM3414

I am currently using the LM3414 to drive some LEDs.  However, the need for the LM3414 to actually regulate the current has disappeared, and I will be controlling the LED current with a PWM signal from a microcontroller.  However, there are still a lot of nice features in the LM3414 - a 65V MOSFET, gate driver, thermal shutdown, etc.  I'm leaning towards continuing to use the LM3414, set the current to higher than I need, and drive the DIM pin with my PWM signal.


I'd basically be using the LM3414 as a combined MOSFET and gate driver, in one tiny package.  I could use a separate gate driver and MOSFET but I have serious size constraints.  Are you familiar with any devices that do this - combine a MOSFET and gate driver into one package, but without the extra circuitry of the current regulation?

  • 0
    TI__Mastermind 25500 points
    Whether an IC like that exists or not I honestly am not sure. But the LM3414 seems like it would work just fine, it's quite versatile. But could you explain to me exactly how you need to dim? The LM3414 is designed specifically so it can regulate the on time current so you can PWM dim with no wavelength shift if that is your intention. In either case it can easily run open loop out of regulation and do what you are trying to do if I understand it correctly. But I would be interested in exactly what you are trying to do (Vin, Vout, LED current, how exactly you want/need to dim, etc...) and I would be happy to provide any advice.
  • 0 in reply to Clinton Jensen
    Expert 2305 points

    My application is more or less a MPPT circuit, although it's not a solar panel.  I have found that, rather than trying to regulate to a certain current, I am better off adjusting the PWM duty cycle and measuring the resulting current and voltage.  I could do this with a FET, but that brings me back to my original issues about all the nice features of the LM3414 that would be missing with a simple FET.

    Can you tell me more about using the LM3414 to run open-loop?  I'm not sure I'm following that.

    Also - while the LM3414 has a maximum output current of 1A, it looks like it would be possible to use a value of Riadj that would set a higher current.  What would happen in that case?  Is there an internal current limit?  Would I be risking damaging the part?  Would regulation accuracy degrade?  Etc, etc...

  • 0 in reply to David Dean
    TI__Mastermind 25500 points

    When I mean open loop I mean in cases like you might be interested in where you are basically running in current limit (the 1.2A plateau you see in the graph) or running in 100% duty cycle. At times they can be the same thing. In either case the part will behave fine and respond to PWM dimming. You can run it up to the 1.2A current limit just fine and it will run open loop. The only concern is thermal, but if PWM dimming that will cut down on power dissipation. Regulation accuracy in this case is dependent on your measuring method since it is open loop. The LM3414 only regulates on current during closed loop switching operation.

  • 0 in reply to Clinton Jensen
    Expert 2305 points

    Got it - so, as I am setting the current to 1A but only using about 500mA, I am currently using the LM3414 in "open loop", correct?

    If I were to use Riadj to set the current to something like 2A, I'd still be limited by the internal 1.2A switch current limit, right?

  • 0 in reply to David Dean
    TI__Mastermind 25500 points

    For the first that depends. If you are in 100% duty cycle mode you are. But if you set the current at 1A and PWM dim at 50% duty cycle for an average of 500mA the converter is in closed loop and in control.

    Yes, if you try and set the current above 1.2A it will limit the current at 1.2A as shown in the datasheet graph. Let me know if you have any more questions.