The LM656x0-Q1 provides a Dual-Random Spread Spectrum (DRSS) function, which reduces EMI of the power supply over a wide frequency range. As shown in Figure 7-6, DRSS combines a low-frequency triangular modulation profile with a high-frequency cycle-by-cycle pseudo-random modulation profile. The low-frequency triangular modulation improves performance in the lower radio-frequency bands, while the high-frequency random modulation improves performance in the higher radio-frequency bands.

Spread spectrum functions by converting a narrowband signal into a wideband signal that spreads the energy over multiple frequencies. Because industry standards require different EMI receiver resolution bandwidth (RBW) settings for different frequency bands, the RBW has an impact on the spread spectrum performance. For example, the CISPR 25 spectrum analyzer RBW in the frequency band from 150kHz to 30MHz is 9kHz. For frequencies greater than 30MHz, the RBW is 120kHz. DRSS is able to simultaneously improve the EMI performance with low and high RBWs through the low-frequency triangular and high-frequency cycle-by-cycle random modulation profiles, respectively. DRSS can reduce conducted emissions by up to 10dBμV in the low-frequency band (150kHz to 30MHz) and 5dBμV in the high-frequency band (30MHz to 108MHz) for CISPR 25. Applying an external clock signal to MODE/SYNC disables DRSS.

Figure 7-6 Dual Random Spread Spectrum Implementation

The device provides a wide low-frequency modulation profile that spreads the switching frequency by ±10% with a 6kHz average modulation frequency. As shown in Table 7-4, the LM656x0-Q1 also provides a switch-node waveform shaping feature that, when enabled, adjusts the switch-node voltage rising transition for reduced ringing and overshoot.