8.4.5 Spread Spectrum Operation

The spread spectrum is a factory option in the select variants.

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 LMR36015-Q1 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 LMR36015-Q1 devices with a triangular spread spectrum use typically a ±4% spreading rate with the modulation rate set at 16 kHz (typical). The spread spectrum is only available while the internal clock is free running at its natural frequency. Any of the following conditions overrides spread spectrum, turning it off:

• At high input voltages/low output voltage ratio when the device operates at minimum on time the internal clock is slowed disabling spread spectrum.
• The clock is slowed during dropout.
• The internal clock is slowed at light load in the PFM variant (LMR36015SC3QRNXTQ1, LMR36015SC3QRNXRQ1). In FPWM mode, spread spectrum is active even if there is no load.