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LM3409HV: Visibly Surging output

Mike Jeskins
Prodigy 100 points
Part Number: LM3409HV
Other Parts Discussed in Thread: LM3409

Hi,

I have been working with the LM3409HV for several years.

Initially with a successful 4 channel driver PCB running at 48V with an output of 36V, 700A.

This was designed the be used used with a particular LED a Lumileds 1205 COB.

More recently we have been expanding our range to use a larger variety of COBs which has required new drivers at higher currents.

I have designed one at 1.5A output and one at 2A output which are intended to be used with a variety of forward voltages.

The output current is then intended to be adjusted using the i Adj pin and a digital pot.

During testing we have noticed the output seems to surge up on a long fade.

In order to investigate this i have stripped the circuit back to pulling up the EN pin and using a Shunt fet PWM dimming at 300Hz.

Is this something that has been reported before?

Do you have any advice as to what might be causing this?

I have tried various switching frequencies for the driver and re calculated the inductor etc.

I can provide more information if required.

Thanks in advance for your help.

Mike Jeskins

  • 0
    Prodigy 100 points
    That should be 700mA not 700A.
  • 0
    TI__Mastermind 28655 points
    Hello Mike,

    The LM3409 is fairly straight forward in how it works. I'd like to find out more of what you are seeing. Basically the LM3409 turns on the MOSFET until the peak current threshold is met. This can be adjusted via the iadj pin. The ripple is controlled by the off-time and output voltage. Unless there is noise coupling the part will just do what it's told to do. It's hysteretic so no control loop to worry about.

    Does the LM3409 work correct once it has gone through power up?

    Is it possible the voltage source to the LMM3409 is dipping?

    What is the power up sequence causing this?

    Not sure I understand "In order to investigate this I have stripped the circuit back to pulling up the EN pin and using a Shunt fet PWM dimming at 300 Hz". Does this correct the problem? And please explain what you are doing instead of adjusting iadj.

    Is Roff connected to the LED stack?

    If you have an oscilloscope and current probe you can watch the LED current, Iadj voltage, output voltage and input voltage to see what could be causing this.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 100 points

    Hi Irwin,

    thanks for getting back to me.

    It does look like the circuit is powering up correctly and working.

    And I can't see any dipping on the input voltage.

    Sorry for the confusion on the stripping back, i don;t think i explained myself properly.

    We had added a large output cap to smooth out some steppy dimming at the bottom end of a fade.

    And we were also using the a digital pot on the iadj pin and had coupled the PWM input to the shunt fet and the EN pin.

    This at the time appeared to give us the best dimming response.

    The circuit i am currently using has the EN pin pulled high so the output is always on and then htis is shunted to ground via a Mosfet.

    This was done to eliminate any other potential causes of the problem.

    The problem which we are seeing is that the light output seems to stall for a moment when we fade over a longer period of time, usually 20 seconds.

    It is not something we have been able to view by looking at the output current or voltage on a scope. it is just something subtle which we can see.

    I really am just looking to see if this is something anyone has reported before?

    Essentially our market is theatrical lighting so smooth LED dimming from 0-100% is critical.

    We have been using this chip for several years. do you have any new products which may be an improvement?

    Thanks

    Mike

  • 0 in reply to Mike Jeskins
    TI__Mastermind 28655 points
    Hello Mike,

    It would be best if you could look at waveforms to see what is going on. "The problem which we are seeing is that the light output seems to stall for a moment when we fade over a longer period of time, usually 20 seconds." If this means the light doesn't appear to be dimming right away it's possible since the eye perceives light not linear but in log form. Also, the way the LM3409 dims is not linear at the bottom of the dim range since it is controlling peak current. This can actually be a plus since it slows down the dim rate at very low light levels but makes in non-linear. With PWM dimming it can be fairly linear.

    I guess I'd like to know what 'seems to stall' means, a delay in dimming.

    Again, looking at waveforms will show you what is going on. The LM3409 is very straight forward in how it works. Off-time (if connected via a resistor to the LED stack) controls peak to peak ripple. Iadj controls where the LM3409 shuts off (where it regulates the peak current). If the peak to peak ripple gets below 2X the average LED current the LED current becomes discontinuous (wjhere dimming becomes non-linear). It should work as you want it to, just have to figure out what's going on.

    Regards,