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LM3409HV: Issue with Current Fluctuation and Intermittent Behavior in LM3409 Driver

Ruben Mendez
Prodigy 10 points
Part Number: LM3409HV


Tool/software:

Hi everyone,

I am using the LM3409HV driver to control LED current. The system generally works fine, but I am encountering an issue when I attempt to adjust the PWM signal (either increasing or decreasing the duty cycle). During these adjustments, the current output fluctuates instead of smoothly tracking the PWM signal. In some cases, the current changes erratically in steps and not fluently like the PWM.

The PWM signal itself appears to be stable and functioning as expected, but the current behavior does not consistently follow the PWM adjustments smoothly.

Could you provide any insights as to why the current might behave this way when adjusting the PWM signal? Is there something in the circuit design or configuration I might be missing that could cause this intermittent behavior? Find attach my schematic and a video of the signal, where in yellow you can see the PWM signal and in purple the current output.

Thank you for your assistance.

  • 0
    TI__Expert 3670 points

    Hi Ruben, 

    What is your switching frequency and dimming frequency? In general, dimming frequency should be at least one order of magnitude lower than the steady state switching frequency to prevent aliasing. However, for good linear response across the entire dimming range, the dimming frequency may need to be even lower.

    Thanks,

    Jaron

  • 0 in reply to Jaron Wilson
    Prodigy 10 points

    Hi Jaron,

    I think the switching frequency comes directly from my Coff = 470pF and Roff =  42kohm and my dimming frequency is 20 kHz, for our application we have to record light images at 2500 fps and we need to know if this driver is appropriate for this this application. If not, can you please recommend as an alternative driver?

    Best regards

  • 0 in reply to Ruben Mendez
    TI__Expert 3670 points

    Hi Ruben, 

    Your switching frequency also depends on your output voltage. Please share either the frequency or the output voltage so I can calculate it.

    Thanks,

    Jaron

  • 0 in reply to Jaron Wilson
    Prodigy 10 points

    Hi Jaron,

    the output voltage can change from 15V to 22V and the voltage input is 50V constant.

    Thank you for your help.

  • 0 in reply to Ruben Mendez
    TI__Expert 3670 points

    Hi Ruben, 

    Sorry for the delay in response, I was out last week for Thanksgiving holiday in the US. 

    Okay it looks like your dimming frequency is sufficiently low compared to your switching frequency so you shouldn't be seeing any aliasing. Can you be more specific in describing this erratic output current? Perhaps provide timestamps in the video that illustrate your point or take screenshots and annotate. From the video it looks like the output current is tracking the PWM signal quite smoothly. Maybe I'm missing something here?

    Thanks,
    Jaron