The LMQ61460 EN/SYNC pin can be used to synchronize the internal oscillator to an external clock. The internal oscillator can be synchronized by AC coupling a positive clock edge into the EN pin, as shown in Figure 8-2. TI recommends keeping the parallel combination value of R_ENT and R_ENB in the 100-kΩ range. R_ENT is required for synchronization, but R_ENB can be left unmounted. Switching action can be synchronized to an external clock ranging from 200 kHz to 2.2 MHz. The external clock must be off before start-up to allow proper start-up sequencing.

Figure 8-2 Typical Implementation Allowing Synchronization Using the EN Pin

Referring to Figure 8-3, the AC-coupled voltage edge at the EN pin must exceed the SYNC amplitude threshold, V_{EN_SYNC_MIN}, to trip the internal synchronization pulse detector. In addition, the minimum EN/SYNC rising pulse and falling pulse durations must be longer than t_{SYNC_EDGE(MIN)} and shorter than the blanking time t_B. TI recommends to use a 3.3-V or higher amplitude pulse signal coupled through a 1-nF capacitor, C_SYNC.

Figure 8-3 Typical SYNC/EN Waveform

After a valid synchronization signal is applied for 2048 cycles, the clock frequency abruptly changes to that of the applied signal. Also, if the device in use has the spread spectrum feature, the valid synchronization signal overrides spread spectrum, turning it off, and the clock switches to the applied clock frequency.