The LM25149 provides a digital spread spectrum, which reduces the EMI of the power supply over a wide frequency range. DRSS combines a low-frequency triangular modulation profile with a high frequency cycle-by-cycle random modulation profile. The low-frequency triangular modulation improves performance in lower radio-frequency bands, while the high-frequency random modulation improves performance in higher radio frequency bands.

Spread spectrum works by converting a narrowband signal into a wideband signal that spreads the energy over multiple frequencies. Since industry standards require different EMI receiver resolution bandwidth (RBW) settings for different frequency bands, the RBW has an impact on the spread spectrum performance. DRSS is able to simultaneously improve the EMI performance in the low and high RBWs with its low-frequency triangular modulation profile and high frequency cycle-by-cycle random modulation, respectively. DRSS can reduce conducted emissions by 15 dBμV in the CISPR 25 low-frequency band (150 kHz to 30 MHz) and 5 dBμV in the high-frequency band (30 MHz to 108 MHz).

To enable DRSS, connect either a 41.2-kΩ or 71.5-kΩ resistor from CNFG to AGND. DRSS is disabled when an external clock is applied to the PFM/SYNC pin.

Figure 8-3 Dual Random Spread Spectrum Implementation