The device provides an optional programmable spread spectrum (clock dithering) function that is activated by connecting a capacitor between DITHER and AGND. A triangular waveform centered at 1.0 V is generated across the dither capacitor. This triangular waveform modulates the oscillator frequency by –6% to +5% of the frequency set by the RT resistor. The dither capacitance value sets the rate of the low frequency modulation.

Figure 8-10 Switching Frequency Dithering

For the dithering circuit to effectively reduce peak EMI, the modulation frequency must be much less than the RT switching frequency. The dither capacitance which is required for a given modulation frequency (f_MOD), can be calculated from Equation 9. Setting the f_MOD to 9 kHz or 10 kHz is a good starting point.

Connecting DITHER to AGND deactivates clock dithering, and the internal oscillator operates at a fixed frequency set by the RT resistor. Clock dithering is also disabled when an external synchronization pulse is applied.

Figure 8-11 Dynamic Dither On/Off Example