Other Parts Discussed in Thread: LM3409HV, LM3409
We have an LED Light Bar design using the LM3409HV driver powered by +48Vdc to run a string of 12 Cree XPL LEDs. The PWM switching frequency of the driver is set at 400KHz (Roff 24K3, Coff 470pf). We use the EN input to strobe the LEDs to synchronize with an external camera, sort of like a flash but running up to 1KHz with a 100us ON duration (10% Duty Cycle). Everything works as expected until we drop the EN strobe frequency to around 60-80Hz (still 100us ON). Below that frequency, there is a large delay before the LEDs turn on (70-80us compared to 2us normally). I can see switching noise on the LED power through the inductor that corresponds to the PWM switching frequency, but no increase in LED voltage until 70-80us later. I ran a test on an LM3409HV Evaluation Board we have and found that a similar issue happens with EN input around 80-90Hz.
Is this the expected operation?
What is the minimum EN switching frequency that can be used?
What changes do I need to make to ensure the LEDs turn on fast when EN is driven by a single pulse?