The device provides a digital spread spectrum, which reduces the EMI of the power supply over a wide frequency range. This function is enabled by a single resistor (37.4 kΩ or 100 kΩ) between the MODE pin and the AGND pin or by programming the MODE pin voltage (370 mV or greater than 1.0 V) during initial power up. When spread spectrum is enabled, the internal modulator dithers the internal clock. 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, the AM band), while the high frequency random modulation improves performance in higher radio frequency bands (for example, the FM band). In addition, the frequency of the triangular modulation is further modulated randomly to reduce the likelihood of any audible tones. In order 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 9-9).

Figure 9-9 Dual Random Spread Spectrum