The LM5176 provides an optional frequency dithering function that is enabled by connecting a capacitor from DITH to AGND. Figure 7-4 illustrates the dithering circuit. A triangular waveform centered at 1.22 V is generated across the C_DITH capacitor. This triangular waveform modulates the oscillator frequency by 10% of the nominal frequency set by the R_T resistor. The C_DITH capacitance value sets the rate of the low frequency modulation. A lower C_DITH capacitance will modulate the oscillator frequency at a faster rate than a higher capacitance. For the dithering circuit to effectively reduce peak EMI, the modulation rate must be much less than the oscillator frequency (F_sw). Equation 6 calculates the DITH pin capacitance required to set the modulation frequency, F_MOD. Connecting the DITH pin directly to AGND disables frequency dithering, and the internal oscillator operates at a fixed frequency set by the RT resistor. Dither is disabled when external SYNC is used.

Equation 6.

Figure 7-4 Dither Operation