8.3.8 Spread Spectrum

For the LMR36506-Q1, spread spectrum is a factory option. To find which parts have spread spectrum enabled, see the .

The purpose of spread spectrum is to eliminate peak emissions at specific frequencies by spreading these peaks across a wider range of frequencies than a part with fixed-frequency operation. In most systems containing the LMR36506-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 and inductor. The LMR36506-Q1 uses a ±3% spread of frequencies which can spread energy smoothly across the FM and TV bands, but is small enough to limit subharmonic emissions below the switching frequency of the part. Peak emissions at the switching frequency of the part are only reduced slightly, by less than 1 dB, while peaks in the FM band are typically reduced by more than 6 dB.

The LMR36506-Q1 uses a cycle-to-cycle frequency hopping method based on a linear feedback shift register (LFSR). This intelligent pseudo-random generator limits cycle-to-cycle frequency changes to limit output ripple. The pseudo-random pattern repeats at less than 1.5 Hz, which is below the audio band.

The spread spectrum is only available while the clock of the LMR36506-Q1 device is free running at its natural frequency. Any of the following conditions overrides spread spectrum, turning it off:

• The clock is slowed due to operation at low-input voltage – this is operation in dropout.
• The clock is slowed under light load in auto mode. Note that if you are operating in FPWM mode, spread spectrum can be active, even if there is no load.
• The clock is slowed due to high input to output voltage ratio. This mode of operation is expected if on-time reaches minimum on-time. See the Timing Characteristics.
• The clock is synchronized with an external clock.