The device can be synchronized to an external clock by applying a square wave clock signal to the SYNC/FSEL pin with a duty cycle from 20% to 80%. The clock can either be applied before the device starts up or during operation. If the clock is to be applied before the device starts, a resistor between SYNC/FSEL and AGND is not needed. If the clock is to be applied after the device starts, then the clock frequency must be within ±20% of the frequency set by the SYNC/FSEL resistor. When the clock is applied after the device starts, the device begins synchronizing to this clock after counting four consecutive switching cycles with a clock pulse present. This is shown in Figure 6-4.