TPS43336-Q1: External clock frequency

Tokumoto-san,
As Raymundo correctly pointed out, the part is not specified below 150kHz and above 600kHz external clock frequency, so applying a clock outside this range is entirely at customers own risk.
My ASSUMPTION (!) is:
The PLL will likely still be able to lock to a frequency slightly above or below the given range and thus operate at that clock
At such extreme values as 40kHz or 4MHz, it will most likely not be able to lock to such a frequency and interpret this as "loss of clock" and switch at the fall-back-frequency (note: there will be limited regulation-capabilities during the change of clocks). The default clock would be 400kHz typical. We do recommend to use a RT-resistor that sets a frequency close to the (desired!) external clock.
To your questions:
　　①Is there a possibility that the operation clock of the TPS43336Q switches between internal and external,
even though a same frequency(for example,40kHz or 4MHz) external clock is input to the SYNC pin?
--> unlikely that it switches between internal and such extreme external frequencies. However, it the SYNC-pin is toggling, it will likely attempt to synchronize to "something" and during the "transition" customer has to expect limited regulation capabilities (variance on Vout, in particular in case of line- or load-transients)
　　②Is there a possibility that the output voltage of the TPS43336Q goes to above and below from the range
that the customer designed?
--> yes, per above.
　　③Is there a tendency of behavior for low frequency(40kHz) and high frequency(4MHz)?
--> the part will not be able to switch at such frequencies.
Bottom line: the customer should design the system in a way that either the correct clock (between 150kHz and 600kHz) is applied, or NO clock is applied.

Best regards,
Frank