TPS54427 max duty cycle

TPS54427 has an adaptive on time / hysteretic off time control scheme.  There is an absolute minimum off time that determines the maximum duty cycle.  For a given output voltage the required duty cycle will depend on Vin. If you were to plot the output voltage vs duty cycle, ideally you would like to see a constant output voltage until the maximum duty cycle is reached where the output voltage would immediately fall to 0 V.  That ideal condition would represent perfect line regulation.  In real life, as duty cycle increases (Vin decreases) Vout will decrease by some fraction of a percent.  Then as the duty cycle approaches the maximum duty cycle, rather than a sharp break point, the output voltage starts to fall out of regulation around 80%.  So while the absolute maximum duty cycle may be 95%, at that point, the output voltage is well out of regulation.  So for duty cycle < about 80%, the output voltage has a normal line regulation characteristic.  For duty cycles between 80% and 95%, the line regulation is degraded.  Above 95 %v duty cycle the output voltage falls rapidly towards 0 V.

Additionally, the TPS54427 device is intended for applications which require good transient response to load step changes.  The adaptive on time control allows the duty cycle to change immediately to load changes without the lag associated with traditional feedback compensation.  Operating near the maximum duty cycle impairs that ability.  That is why the Webench design tool is limited to 65% duty cycle.

For most target application, the TPS54427 is used with a 12 V input voltage.  With a maximum output voltage of 7 V the required duty cycle is 58%, so that 65% duty cycle limit does not usually apply.

So in your case you can operate at 83% duty cycle as long as you can tolerate reduces regulation and transient performance.

Max duty cycle is a minimum of 87% for TPS5450, so that should not be an issue.  There are somethings you need to know about TPS5450.  It is internally compensated, so you will be limited in your selection of output capacitors.  It is possible to modify the compensation with external components, but that approach is limited and may not work well with every possible output capacitor.  Also TPS5450 is at the limits of thermal performance for that package.  The junction temperature rise can be quite high as the output current increases.  You will have to pay close attention to PCB design if your ambient temperature is above 50 C.