SNVSBK9C November 2019 – September 2020 LM63635-Q1
The LM63635-Q1 is available with a spread spectrum clock dithering feature. This feature uses a pseudo-random pattern to dither the internal clock frequency. The pattern repeats at a 0.98-Hz rate while the depth of modulation is ±3%.
The purpose of the spread spectrum is to eliminate peak emissions at specific frequencies by spreading emissions across a wider range of frequencies than a part with fixed frequency operation. In most systems containing the LM63635-Q1 device, low frequency conducted emissions from the first few harmonics of the switching frequency can be easily filtered. A more difficult design criterion is reduction of emissions at higher harmonics which fall in the FM band. These harmonics often couple to the environment through electric fields around the switch node. The LM63635-Q1 device use a ±3% spread of frequencies which spreads energy smoothly across the FM band but is small enough to limit subharmonic emissions below its switching frequency.