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LT8391 Flickering LEDs are certain supply voltages

Good afternoon,

I have a custom PCB that just arrived and am testing the LED driver based on the LT8391A. The design is mostly sourced from the DC2345A reference design, however, a number of components were swapped due to the voltage ratings and needs for the project.

The LED string is designed to receive 36V and 5.5A from the driver. The board is also setup for external PWM dimming, and at 100% duty cycle it seems to always work great. However, the PWM dimming feature only seems to work between 45-50V on the buck side and below 18V on the boost side. When it is not working, the LEDs are still illuminated but are rapidly flickering. Scoping the fault pin shows that the fault pin is being triggered and shutting off the output. It is then resetting and tripping again very rapidly resulting in flickering lights.

I'm looking for suggestions on what might be wrong and how to debug this. I did place a 100k resistor on the SS pin to disable hiccup mode but it doesn't seem to be the short/open LED voltage causing the fault.

Thank you!

Kris