Hello,

There is no way to disable light load burst mode.  When the converter's voltage amplifier demands a duty cycle less that is less than the minimum on time programed by Rtmin.  The controller will skip duty cycle pulses until the voltage amplifier demands duty cycle.

You are correct that this can be seen during startup as described in page 10 of the UCC28951 data sheet and below.

https://www.ti.com/lit/ds/symlink/ucc28951.pdf

During soft start while charging the output capacitors you should only pulse skip for a few cycles as the duty cycle is increasing.

This generally does not cause a noticeable delay during the startup time.

The following link will bring you to the UCC28950 600 W Evaluation module. Section 8.8.5 shows the startup waveforms of the Out A..D drivers going into burst mode during soft start; as well as, the output voltage.  From these waveforms the output gradually charges up with no disruption from burst mode. 

https://www.ti.com/lit/ug/sluub02a/sluub02a.pdf

Regards,