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Calculating Rsen for the LM3466 Smart Linear LED Driver for Multi-Channel LED Systems

Cory Brown
Prodigy 20 points
Other Parts Discussed in Thread: LM3466

I want to power separate LED strings with 700mA each (X3) from a single 2100mA constant current source. However, I Rtotal is not defined in the datasheet. Here is the formula I am trying to manipulate:

ILED,k = ISRTOTAL

                 RSEN,k

Where: ILED,k = 700mA, Is=2100mA.

What is Rtotal?

Thanks!

 

  • 0
    Prodigy 5 points

    Cory, Rtotal is the sum of the Rsense, each LM3466 drives a string, has its own Rsense.

    In the case below from the data sheet, you have 5 LM3466, each with a 1 ohm, so 1/Rtotal = 1/5

     

     

    I(s) is the current needed to be provided by the main power supply source.

     

     

  • 0 in reply to Ed Walker
    Prodigy 20 points

    Thanks Ed,

    My design utilizes 3 LM3466 ICs. Is is 2.1A and Iled is 700mA. I have tried to calculate  the value of Rsense using the formulas but cannot. The ratio of Rtotal/Rsense should be .33, but that would require Rsense being greater than Rtotal. What am I missing here?

  • 0 in reply to Cory Brown
    Prodigy 5 points

    Cory,


    From the data sheet page 12:
    To set the LED current of all 5 LED strings equal, the sensing resistors corresponding to all 5 LM3466 are equal.
    It is recommended that the nominal voltage of the SEN pin VSEN should be around 0.3V.
    Therefore,    RSEN is selected to be 1Ω. As a result, VSEN should be 0.35V if the LED current is 0.35A.

    So for yours all 3 would be equal value.
    For 0.3V at SEN and 700mA, R=0.35/.7= 0.5 ohms.
    Makes sense for 2X 350mA you have 1/2 the R.

    This goes as resistors in parallel. That's what your are missing, each individual R will be greater than the parallel total.