The device provides a digital spread spectrum which reduces the EMI of the power supply over a wide frequency range. This function is dynamically selectable during operation. The internal modulator dithers the internal clock when the DITHOFF pin is less than 1.0 V or the pin is grounded, and it stops clock dithering when the DITHOFF pin is greater than 2.0 V or the pin is connected to VCC. When an external synchronization clock is applied to the SYNC pin, the internal spread spectrum is disabled. DRSS (a) combines a low frequency triangular modulation profile (b) with a high frequency cycle-by-cycle random modulation profile (c). The low frequency triangular modulation improves performance in lower radio frequency bands (for example. AM band), while the high frequency random modulation improves performance in higher radio frequency bands (for example, FM band). In addition, the frequency of the triangular modulation is further modulated randomly to reduce the likelihood of any audible tones. To minimize output voltage ripple caused by spread spectrum, duty cycle is modified on a cycle-by-cycle basis to maintain a nearly constant duty cycle when dithering is enabled (see Figure 9-10).

Figure 9-10 Dual Random Spread Spectrum