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FLT Pin for LT3796 PWM Input

Hello ADI:


When PWM pin  is driven high (EN pin high), FLT pin output low and is not in switching state. One of many devices will not transition to switching. I checked the waveforms and when PWM pin is driven high, the VC pin is swinging.

Wave form :

Circuit :

Please tell me why the FLT pin output low. Is there a maximum time that the PWM pin goes low while the EN pin is high? If the PWM pin is low for less than 10 seconds, FLT pin stays high, but if the PWM pin is low for 30 seconds, FLT pin goes low.