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 than a part with fixed frequency operation. In most systems containing the LM53625-Q1 and LM53635-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 LM53625-Q1 and LM53635-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 part’s switching frequency are only reduced by slightly less than 1 dB, while peaks in the FM band are typically reduced by more than 6 dB.

The LM53625-Q1 and LM53635-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 8 Hz which is below the audio band.

The spread spectrum is only available while the clock of the LM53625-Q1 and LM53635-Q1 devices is free running at its natural frequency. Any of the following conditions overrides spread spectrum, turning it off:

1. An external clock is applied to the SYNC/MODE terminal.
2. The clock is slowed due to operation low input voltage – this is operation in dropout.
3. The clock is slowed due to high input voltage – input voltage above approximately 21 V disables spread spectrum.
4. The clock is slowed under light load in Auto mode – this is normally not seen above 200 mA of load. In FPWM mode, spread spectrum is active even if there is no load.