When MODE/SYNC is set low, the device operates in PWM or PFM mode, depending on the output current. The MODE/SYNC pin allows 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. Similar to COMP/FSET, the specifications for the minimum on-time and minimum off-time have to be taken into account 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 a fast settling to the external clock and, if the external clock fails, the switching frequency stays in the same range and the compensation settings are still valid. When there is no resistor from COMP/FSET to GND but the pin is pulled high or low, external synchronization is not possible.