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LM3464 Dimming issue

Other Parts Discussed in Thread: LM3464

My customer is asking the following.  Please review and let me know what you recommend.LM3464.pdf

Part 1:

I’m having some issues with dimming on this LM3464 driver. 

I have thus far been unable to use PWM dimming or the analog dimming without the driver throwing a fault and disabling all the channels. I haven’t been able to observe any of the 3 fault conditions listed in the data sheet – over voltage, open circuit, or short circuit – at least not in terms of the voltage levels it references. 

I have my PWM from the CPLD at 250Hz and the analog dimming PWM (set by Cthm) running about 530Hz. For the analog dimming I’m using a voltage divider on the Thermal feedback pin and on the DMIN pin to switch to the “Minimum duty cycle” thermal foldback mode. 

The datasheet makes no mention of a preferred range of PWM frequencies, nor does it give a minimum or maximum value. 

I am running “full bright” mode fine, with a constant 3.3V signal on the DIM pin, and I have not had an issue with a 1Hz pulse. I need a “half bright” setting as well though. 

The 250Hz PWM seems to operate a little longer before faulting than the 530Hz internal PWM. This all leads me to believe that a slower PWM frequency will be more stable… 

Any help you could provide here would be great!

Part 2:

When I saw these issues on my hardware I actually went back to the Eval card for a sanity check. I made some modifications to match my hardware, I’ve attached a schematic with notes detailing the changes I have made. 

My LEDs are Cree XHP35, 12V. 

In summary: 

I can drive “DIM” from my CPLD at a constant 3.3V or a 1Hz pulse @ 3.3V, and everything is ok. 

If I drive “DIM” @ a 250Hz PWM, LEDs come on for a short time then FAULTb is pulled low and they turn off. 

If I drive “DIM” @ constant 3.3V and pull THM+ to ground, LEDs come on for VERY short period of time and then fault and turn off.

  • Hello,

    I have limited knowledge of this part but will try to help out.
    What are the values for CDHC, CTHM and CFLT?
    Is the system stable when PWM dimming, including the power supply sourcing the LM3464?
    How is the LM3464 set up to feed back to the power source?
    After looking through the datasheet I don't see an issue with pwm dimming at 250 Hz.

    Not sure if you have an oscilloscope but it may be helpful to know what the output of the power source is doing during dimming as well as some of the other signals on the LM3464.
  • Here are the answers from my customer:

    CDHC = 0.22uF
    CTHM = 68nF
    CFLT = 0.1uF (I have tried values from 33nF-2uF)

    I am not using the Dynamic Headroom control feature, primary power supply output is a constant 12.7V. System seems stable, based on everything I have observed.

    I have attached scope shots: Primary supply output, DRx pin, and Sex pin… all are triggered on the FAULTb pin falling to zero. I have tried watching the CFLT voltage on the scope but it seems the 1M load from the scope probe keeps it from charging up and the fault is never thrown.

    I do have it working on the eval card right now @ 100Hz, still working on modifying my hardware to test. I would like to be able to use a faster PWM frequency though, if possible.

    Just as a reminder, to get it to work on the eval card I increased CFLT to 0.1uF and decreased my PWM to 100Hz.
  • One thing we have not found in the datasheet is the min/max PWM specification.