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LT8391 Flashing

Hi All, 

As power is increased to about 42 watts (42V @ 1amp) LED output starts flashing.  Noticed SS pin is dropping low quickly.  From the data sheet page 13 I see 5 conditions that would cause a rapid discharge of the SS pin.  They are CTRL 1 or 2 below 0.2V, UVLO low, Visp-Visn > 0.75V,  and FB > 1.05V.  I dont see any of these conditions being broken.  The SS cap is being quickly discharge do switch to ground from Inhibit Control looks like its being activated.  What could be going on?

  • Hi SunStone,

    If you say flashing, do you mean that the LED outputs are blinking on and off? The LT8391 has a fault protection and detection feature that provides information whether there is an open or short LED. The complete cycle of fault detection and shutdown can be seen on page 17 of the data sheet. After the start-up sequence has been initialized, the circuit will try to run and detect if there is fault (open or short LED). the dropping of SS pin is a result of this fault protection where it is pulled low in and then the switch is disabled after the soft-start is pulled low. The IC will try to check if there is still fault and will repeat again until the fault is corrected. Since the switch is enabled and disabled in cycles, this may be the result why the LED outputs are blinking (turning on and off).

    For a more detailed information and discussion about the fault protection and its cycle, check page 17 of the data sheet.