When MODE/SYNC is set low, the device operates in PWM or PFM mode, depending on the output current. The MODE/SYNC pin allows you to force PWM mode when set high. The pin also allows you to apply an external clock in a frequency range from 1.8 MHz to 4 MHz for external synchronization. The specifications for the minimum on-time and minimum off-time has to be observed when setting the external frequency. For use with external synchronization on the MODE/SYNC pin, the internal switching frequency must be set by R_CF to a similar value than the externally applied clock. This ensures that, if the external clock fails, the switching frequency stays in the same range and the compensation settings are still valid.