When optimizing spread spectrum, the most important factor is the modulation index. The larger the modulation index, the more distributed the spectrum becomes; however, there are limits when spreading in a switching-regulator application. Modulation index is a factor of the energy spread and the resulting fundamental/harmonics. Note the switching frequency of the converter is the fundamental frequency of the voltage ripple. Therefore, as the switching frequency deviates from the fundamental frequency it can increase the output ripple, create large variations in the inductor current ripple and can start spreading energy into bands where it is not desired.