When MODE/SYNC is set low, the device operates in PWM or PFM mode depending on the output current. The MODE/SYNC pin forces PWM mode when set high. The pin also allows the user 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 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. When there is no resistor from COMP/FSET to GND but the pin is pulled high or low, external synchronization is not possible.