When MODE/SYNC is set low, the device operates in PWM or PFM mode, depending on the output current. The MODE/SYNC pin allows the user to force PWM mode when set high. The pin also allows the user to apply an external clock in a frequency range from 2 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. The external synchronization frequency applied on the MODE/SYNC pin must be 10% to 40% higher than the nominal internal switching frequency set by R_CF (calculated with Equation 1 to Equation 3). Ensuring this makes sure that, if the external clock fails, the converter can continue normal operation with the internal switching frequency in a range where 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. If the device is externally synchronized, both converters are forced to run on that clock frequency preserving 180° phase relation. The internally generated spread spectrum clocking is turned off while running on an external clock.