TPS53315: About tON(min) and tOFF(min)

The specifications are determined by the application and are determined by the input and output voltages. 

A low duty cycle (high input, low output) higher switching frequency application would want a small on time.

A high duty cycle application (low input, high output) application would prefer a 0 ns off time. 

The parameter will depend on the type of control or protection techniques that are employed which will determine the on time and off time contraint.

As you mentioned, one of the consideration for off time is the boot strap and recharging the boot capacitor.

But in some devices such as the TPS53315, it is sensing the current  in the low side fet for current limiting.

Some converter have a pmos on the high side will not have on time or off time limitation.

If there is a high side current limit or if using peak current mode control, the on time will be

constrained by the minimum time to sense current accurately in the high side fet 

On the TPS53315, the on time will be determined by linear ramp generation.