7.3.5 Frequency Dithering (Spread Spectrum)

The LM5141-Q1 provides a frequency dithering option that is enabled by connecting a capacitor from the DITH pin to AGND. A triangular waveform centered at 1.2 V is generated across the C_DITH capacitor. Refer to Figure 22. The triangular waveform modulates the oscillator frequency by ±5% of the nominal frequency set by the OSC pin or by an RT 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 the peak EMI, the modulation rate must be less than the oscillator frequency (F_SW). Equation 3 calculates the DITH pin capacitance required to set the modulation frequency, F_MOD.

Equation 3.

If the DITH pin is connected to VDDA during power-up the dither feature is latch-off and cannot be changed unless VCC is allowed to drop below the VCC_(UVLO) threshold. If the DITH pin is connected to ground on power up, dither will be disabled, but it can be enabled by raising the DITH pin voltage above ground and connecting it to C_DITH. When the LM5141 is synchronized to an external clock, Dither is disabled.

Figure 22. Dither Operation