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LM3409HV IADJ analogue dimming issue

Rene Jansen
Prodigy 70 points
Other Parts Discussed in Thread: LM3409HV, LM3409

For a custumor we plan to use the LM3409HV LED driver. To investigate the optimal values for the components, we used the evaluation board. To simulate the analogue dimming, we lower the voltage at the IADJ pin by placing a resistor to ground. Placing a 220k resistor does indeed have a minimal effect of the output current. However, when connecting a 100k resistor between the IADJ input and ground the output is diisabled, instead of reduced.

What is the correct use of the IADJ pin of the LM3409?

  • 0
    TI__Mastermind 25500 points
    To dim you need to apply a voltage between 0V and 1.24V to the IADJ pin. On the EVM you can apply this voltage to the VADJ test point. 0V will give you minimum current and 1.24V will give you maximum current.
  • 0 in reply to Clinton Jensen
    Prodigy 70 points
    Problem is that this works for voltages between 1.24V and 0.98V. When a lower voltages is applied, the regulator shuts down.
    Note that we use a 3 Ohm dummy load resistor instead of a LED to simplify cooling and prevent eye-damage from to much optical exposure. Thus that trigger some kind of safety inside the driver?
  • 0 in reply to Rene Jansen
    TI__Mastermind 28655 points
    Hello,

    I would guess what is happening is the output voltage is dropping too low for the Coff to work correct. You could try and use a fixed voltage for the Coff resistor connection or change you load to allow a higher Vout if this is the issue. Check the output voltage when it is running full current and check it as you lower the output current via Iadj. Coff needs to charge to 1.24 volts. If you have a one amp output you will have three volts for the load with a three ohm resistor.

    As you use Iadj to lower the current the off time will get longer than intended because Coff will charge more like an RC time constant then if Vout was high (it will be almost linear with a higher Vout).

    Thanks,
  • 0 in reply to Irwin Nederbragt
    Prodigy 70 points

    The output voltage was indeed was too low. When using a LED load, the current regulation works better.