The controller can be used in external synchronization mode by applying a clock to the SYNC pin. The external clock that is applied must be set to the desired switching frequency. The external clock must be in the range of 100kHz to 1MHz with a duty cycle of 50%. It is recommended to use an external clock with 50% duty cycle. The controller will switch in phase with the clock signal that is applied at the SYNC pin during this mode. Note that the SYNC pin only operates as an input for the controller.

When operating in external synchronization mode, the RT pin must be populated with a resistor to AGND. The switching frequency set by the RT resistor should be within ±10% of the external clock frequency. The switching frequency is 1:1 with the external clock frequency. In the event that the controller is operating in internal oscillator mode before the application of the clock on the SYNC pin, it will take approximately 2-5 cycles after the clock is applied before the detection and switchover is made. If operating in external synchronization mode and the clock is lost, it takes approximately 2-5 cycles for the detection to be made, after which the switch to the internal oscillator will occur.