UCD3138: Related to Burst Mode

Part Number:UCD3138

Hello guys,

I set the minimum duty as 400 ns but when UCD3138 goes into burst mode, it generates smaller pulses like 150 ns. When I disable the burst mode and decrease the duty, it gets no smaller than 400 ns which is what I expect. Why does it generate smaller pulses than the minimum value when the burst mode is enabled? 

I would be appreciated if someone helps me.

Thanks in advanced,

Regards, 

Merih