The MAX5974DETE+T is a current-mode PWM controller for flyback and forward converter topologies in a 16-TQFN (3x3 mm) package.
Switching frequency range and duty cycle — design margin
The controller's switching frequency is programmable from 100kHz to 600kHz. At the high end you shrink the transformer core and output capacitor, but you trade off switching losses and snubber dissipation. The 80% maximum duty cycle is typical for flyback designs — it gives enough on-time for step-up ratios without starving the transformer reset. If your design needs a duty cycle above 80%, you are looking at a different topology or a controller with a wider pulse-width window.