Question about an external PMW dimming for LT3922

Hi,

I have some questions about duty signalof external PWM signal to turn LEDs off?

Please take a look at the attachment.

When my external PWM duty cycle is set to 1020/1024 (D=99.6%), my LEDs are not turned off completely.

When my external PWM duty cycle is set to 1023/1024 (D=99.9%), my LEDs are turned off.

Question

1. What is the possible reason about the difference in those results? (D=99.6% and =99.9% seem to be almost same.)

2. Toff (PWM duty signal OFF time) and Ts (switching period) are related to turning LEDs off?

     Considering the toff of each duty cycle, D=99.6% has 3.9us, which comprise app 7.8 times as long as Ts=0.5us.

     On the other hand, D=99.9% has 0.98us comprise app 1.95 times, which is much smaller than 7.8times

3. Any other reason LEDs are not turned off completely inspite of D=99.6%?

Thanks,

Kazu

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Changed subject from LT3992 to LT3922
[edited by: Kazu at 1:30 AM (GMT -4) on 7 Sep 2020]