SNVSBW1C December 2021 – August 2024 LM63440-Q1 , LM63460-Q1
PRODUCTION DATA
Use the EN/SYNC pin of the LM634X0-Q1 to synchronize the internal oscillator to an external clock signal ranging from 200kHz to 2.2MHz. The internal oscillator can be synchronized by AC coupling a positive clock edge to EN/SYNC, as shown in Figure 7-1.
TI recommends to keep the parallel combination value of RENT and RENB in the 100kΩ range. RENT is required for synchronization, but RENB can be left unmounted. The external clock must be off before start-up to allow proper start-up sequencing.
Referring to Figure 7-2, the AC-coupled voltage edge at EN/SYNC must exceed the SYNC amplitude threshold, VEN_SYNC, to trip the internal synchronization pulse detector. In addition, the minimum EN/SYNC rising pulse and falling pulse durations must be longer than tSYNC_EDGE and shorter than the blanking time, tB. A 3.3V or higher amplitude pulse signal coupled through a 1nF capacitor, CSYNC, is suggested.
After a valid synchronization signal is applied for 2048 cycles, the clock frequency quickly changes to that of the applied signal. Synchronization overrides spread spectrum, turning it off.