The spread spectrum is a factory option. In order to find which parts have spread spectrum enabled, see Section 5.
The purpose of the spread spectrum is to eliminate peak emissions at specific frequencies by spreading emissions across a wider range of frequencies. In most systems containing the LMS36x5-Q1 devices, 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 LMS36x5-Q1 devices use a ±3% spread of frequencies which spread energy smoothly across the FM band but is small enough to limit sub-harmonic emissions below its switching frequency. Peak emissions at the switching frequency of the part are only reduced by slightly less than 1 dB, while peaks in the FM band are typically reduced by more than 6 dB.
The LMS36x5-Q1 devices use a cycle-to-cycle frequency hopping method based on a linear feedback shift register (LFSR). Intelligent pseudo random generator limits cycle to cycle frequency changes to limit output ripple. Pseudo random pattern repeats by approximately 1.2 Hz which is below the audio band.
The spread spectrum is only available while the clock of the LMS36x5-Q1 devices is free running at its natural frequency. Any of the following conditions overrides spread spectrum, turning it off: