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.
