The LM708x0-Q1 provides a Dual Random Spread Spectrum (DRSS) function which reduces the EMI of the power supply over a wide frequency range. The DRSS function 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 the lower radio frequency bands, while the high frequency random modulation improves performance in the higher radio frequency bands.

Spread spectrum works by converting a narrowband signal into a wideband signal which spreads the energy over multiple frequencies. Industry standards require different spectrum analyzer resolution bandwidth (RBW) settings for different frequency bands. The RBW has an impact on the spread spectrum performance. For example, the CISPR-25 requires 9kHz RBW for the 150kHz to 30MHz frequency band. For frequencies greater than 30 MHz, the required RBW is 120kHz. DRSS is able to simultaneously improve the EMI performance in the high and low RBWs with the low-frequency triangular modulation and high-frequency cycle-by-cycle random modulation as shown in Figure 7-3. In the low-frequency band (150kHz -30MHz), the DRSS function can reduce the conducted emissions by as much as 15dBμV, and in the high-frequency band (30MHz - 108MHz) by as much as 5dBμV.

The DRSS function can be enabled by connecting either a 41.2kΩ or 71.5kΩ resistor from the CONFIG pin to AGND. The DRSS function is disabled when an external clock is applied to the PFM / SYNCIN pin.