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 you to apply an external clock in a frequency range from 1.8 MHz to 4 MHz for external synchronization. When an external clock is applied, the device only operates in PWM mode. As with the switching frequency selection, the specification for the minimum on-time has to be observed when applying the external clock signal. When using external synchronization, TI recommends to set the switching frequency (as set by R_CF) to a similar value as the externally applied clock. This action makes sure that, if the external clock fails, the switching frequency stays in the same range and the settling time to the internal clock is reduced. When there is no resistor from COMP/FSET to GND, but the pin is pulled high or low, external synchronization is not possible. An internal PLL allows you to change from an internal clock to external clock during operation. The synchronization to the external clock is done on the falling edge of the applied clock to the rising edge of the internal SW pin (see Synchronizing to an External Clock). The MODE/SYNC pin can be changed during operation.