LM5146: LM5146 Low Dropout Conversion Issue

Hi Letao, 
From the datasheet, the Typical max duty cycle for Fsw = 400kHz is 94%. With your output at 30V, that would give an approximate Vin = 31.9V. To maintain a stable regulation you can slightly increase it by 1.2V. So, it could be around 33V or slightly more than. 

I also encourage you to utilize the quick start calculator if you haven't.0211.LM5146_quickstart_calculator_revB1.xlsm

Here is the quickstart tool folder https://www.ti.com/tool/LM5146DESIGN-CALC

PS: use the toff(min) spec in the EC table to derive the max duty cycle for a particular Fsw. The LM5146 does not have frequency foldback when max duty cycle is reached (contact Fsw is maintained).

Regards,

Tim