The 32kHz internal PWM oscillator has a timeslot that is divided into 15 master intensity timeslots. Theoretically the fastest output you can achieve is 32kHz / 15 which is roughly equal to about 2kHz using some combination (say 8/16) of the individual timeslots per master slot. The oscilloscope shots on page 5 and Figure 11 can help to visualize the output based on different individual and master register settings.