Modifying Output of DC2345A for 40V, 5.5 A

Good evening,

I purchased a DC2345A evaluation board for the LT8391A. I modified the board to output 40V by changing R6 to 25KOhms and 5.5A by modifying the sense resistor to 18 mOhms. This works great when running the LEDs at 100% duty cycle.

However, when PWM dimming is introduced (internal or external) the lights flash at 7 Hz with modulation active while the lights are actually on during each of the 7 times per second. It seems like there is another component which needs swapping, but I need some help looking in the right direction of what to change.

Things I've tried:

  • External Sync and Spread Sprectrum off for all scenarios
  • Internal dimming with JP1 and JP3 set to INT and adjusting the potentiometer. In this case, the lights were either 100% on or 100% off.
  • External PWM dimming with JP1 and JP3 set to EXT. See attached waveform. Cycled through frequencies from 120 Hz to 300 KHz with no major changes in the output. Also cycled through duty cycles from 5-95% and output waveform looked similar to that attached.
  • Modify input voltage to use both BOOST and BUCK stages of the design. I've varied from 10V - 55V. The BOOST stage did seem too behave slightly differently, but both were still flashing the LEDs 7 times per second.

Any input is greatly appreciated! I know the chip is capable of this load so it has to be some of the circuits around it. I have already began a PCB design but really need to prove this out on the bench before getting it manufactured. Thanks in advance!

Kris