2 Low Ripple Noise

Several Ethernet controllers on the market integrate SERDES, PLLs and I/O ports on-chip that require low voltage ripple noise levels. The traditional method for achieving low output voltage ripple noise is is to use a DC/DC converter followed by a low-dropout regulator (LDO) such as the TPS7A52, TPS7A53 or TPS7A54. This family of LDOs supports output currents up to 4-A with a low dropout voltage as low as 65 mV. As the output current increases, a linear regulator becomes less feasible due to added cost, board space, and power loss. Besides a linear regulator, another method to achieve low noise along with a switching DC/DC converter is to implement a second-stage L-C filter following the DC/DC converter. Every DC/DC converter generates an output voltage ripple at its switching frequency. Ethernet controllers with integrated noise-sensitive circuitry need low voltage ripple to minimize frequency spurs in the spectrum, which typically varies with switching frequency, inductor value, output capacitance, and equivalent series inductance and resistance. Low-ripple buck converters such as the 2-A TPS62912 and 3-A TPS62913 leverage an external ferrite-bead filter by integrating the compensation to accommodate the ferrite-bead. Using the inductance of the ferrite-bead along with an additional output capacitor removes the high frequency components of the output voltage ripple and achieves less than 10 uV_rms ripple. Figure 2-1 shows the output voltage ripple with an input voltage of 12 V, an output voltage of 1.2 V, and output current of 1-A.

Figure 2-1 TPS62913 Output Voltage Ripple After the Second-stage L-C Filter