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LM3409: Vadj to LED current transfer function

David Naviaux
Intellectual 590 points
Part Number: LM3409

Hi,

After some more analysis, it seems that if I fix the off-time of the LM3409 by applying a fixed voltage to the Coff/Roff RC circuit instead of the output voltage the transfer function from Vadj (voltage at the iAdj pin) to the LED current will depend on the LED voltage and whether it will be in DCM or CCM.

Before I spend a lot of time working out the equations that might be wrong, do you have this transfer equation?

In my case, I need to adjust the LED current from 250mA to 4A with LED voltages that can vary between 2.5V to 6.2V (depending on its wavelength and current).

Thank you,

David

  • 0
    TI__Mastermind 28655 points

    Hello David,

    We don't have that relationship since the idea of the off-time generated from the LED voltage is to keep the current ripple constant with a varying LED stack voltage.  This way the average set-point is more accurate over temperature and LED variation.  This works will at higher voltages at lower voltages you run into the error introduced of the off-time capacitor voltage as it is charging.

    If you fix the voltage feeding Roff the off-time becomes constant.  The current ripple now depends on V=L*(di/dt) of the inductor.  If you double 'V' you double 'di' since the other two variables are fixed.  Also note that at low output voltage the freewheel diode starts becoming a percentage as well.  The math on this shouldn't be to difficult.  Note that the average current will be based only on the off-time and V=L*(di/dt) since the LM3409 regulates peak current.  The on-time is how long it takes to get back to the peak, it will have nothing to do with the math to figure out average current.

    Best Regards,