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LM3421 LED Driver: Setting the limit current

Benny Smith
Prodigy 225 points
Other Parts Discussed in Thread: LM3421, LM3423

I am using the LM3421 as a boost controller to drive a high-current LED array.

I refer to the LM3421/LM3423 data sheet, which includes several pages of application information, formulas, and sample designs.

The average LED current I have chosen, set by the value of the RSNS resistor in series with the LED array, is 3.75A. (see page 12 of the data sheet.)

The current-limit for each cycle of the controller is set with another resistor, RLIM, which is located in the source lead of the power-FET switch.

The formula for calculating the current-limit resistor (see page 13) does not involve the choice made for average LED current.

I assumed that there would be some link between the choice of the average LED current desired and the current-limit placed on the controller.

I assumed that if I set the current limit too low, then the controller will not be able to deliver the desired average LED current.  But, how low is low?

Or, is the current limit chosen for some other reason, for example, to protect the boost inductor, or some other component?

How to choose the limit current?

  • 0
    TI__Mastermind 25296 points

    there is not a definitive equation: it depends on led p/n (how much overcurrent can manage), on device, on thermal stability of the parts involved and so on...

    normally (rule of thumb) in a "well designed power converter" the current limit is set 1.3x to 2x the average/nominal current (typ. 1.5x)

    V.