8.3.1.6 Spread-Spectrum Mode

Systems with periodic switching signals may generate a large amount of switching noise in a set of narrowband frequencies (the switching frequency and its harmonics). The usual solution to decrease noise coupling is to add EMI filters and shields to the boards. The LP8756x-Q1 device has register-selectable spread-spectrum mode which minimizes the need for output filters, ferrite beads, or chokes. In spread-spectrum mode, the switching frequency varies around the center frequency, reducing the EMI emissions radiated by the converter and associated passive components and PCB traces (see Figure 12). This feature is available only when internal RC oscillator is used (PLL_MODE[1:0] = 00 in PLL_CTRL register), and it is enabled with the EN_SPREAD_SPEC bit (PIN_FUNCTION register), and it affects all the buck cores.

Where a fixed-frequency converter exhibits large amounts of spectral energy at the switching frequency, the spread-spectrum architecture of the LP8756x-Q1 spreads that energy over a large bandwidth.

Figure 12. Spread-Spectrum Modulation