The LM61440 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. It is recommended to keep 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. A 3.3-V or higher amplitude pulse signal coupled through a 1-nF capacitor, C_SYNC, is suggested.

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.