From this article: Power Tips: Power converter topology... they say that:
Moreover, buck-converter duty cycles become extremely small in high-voltage AC/DC conversion. For a 440VAC to 15VDC conversion, the duty cycle could be smaller than 2% in a DCM buck converter. When operating the converter at a high switching frequency, it is possible to hit the controller minimum on-time limitation; for example, the UCC28911 requires a 420nS minimum on time. In other words, if you use the UCC28911 in a 440VAC to 15VDC conversion as a buck-converter controller, the converter must operate at a switching frequency lower than 45kHz.
What I understand is this. Let's say the duty cycle is 2%. If the switching frequency is 60 kHz, then the minimum time would be 0.02*(1/60k) = 333 ns. And the UCC28911 wouldn't start because their minimum time is 420 ns according to the datasheet. Then the article says that the converter must operate at a switching frequency lower than 45kHz. However, the datasheet says the minimum switching frequency is 420 Hz. Would the controller adjust the frequency itself if this situation with the minimum time happen?