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LM3409: AVERAGE led current

Jonathan Navor
Expert 7845 points
Part Number: LM3409

Hi Guys,

Good day. We need your confirmation about the average led current for LM3409. Is it right to say that the average led current is only determined by RSNS with 100% duty?

Kindly enlighten us please. Thank you in advance. 

Best regards,

Jonathan

  • 0
    TI__Mastermind 28655 points

    Hello Jonathan,

    If you are just talking about switching frequency the average current is constant over most duty cycles (there are limitations of minimum on-time and also near maximum duty cycle/maximum duty cycle).  The current sense resistor does not see the average current, it sees the on-time current.  The LEDs see the inductor current which is continuous. 

    After Coff charges (the off-time) the MOSFET turns on and current ramps up until the peak current threshold is met.  The MOSFET turns off and Coff charges via Roff.  Since the current through Roff is proportional to Vled the falling di stays constant.  This makes the average current constant independent of input voltage or LED stack voltage.  Average current is Ipeak (the peak current trip threshold) minus (1/2) of the peak to peak current ripple.

    The current sense does not need to see the inductor current, when the MOSFET is off the current ramps down to the valley so current can be calculated from just the on-time (the average current during the on-time is the same as the average current during the off-time).

    Figure 23 in the datasheet shows what the current sense resistor current would look like (when the MOSFET is on.  The actual inductor current looks like figure 19 (the LED current looks the same unless there is an output capacitor).

    Best Regards,