Table 1-1 highlights the latest point-of-load DC/DC converters with integrated MOSFETs, linear regulators and power modules applicable to powering NICs; however, they are designed to accommodate the requirements for a wide range of markets. These devices are designed to achieve fast transient response, low output voltage ripple, high efficiency, good thermal performance, and high output voltage accuracy. Notice that different control architectures are suggested in the table below.

Fixed-frequency control architectures provide a predictable switching frequency and can be synchronized to an external clock. Current mode or voltage mode control is desirable in noise-sensitive applications that use data converters and high-speed analog circuits. On the other hand, devices implementing constant on-time control deliver a faster transient response than voltage or current mode control to quickly changing load profiles, since there is no internal clock to control the switching frequency. Several devices feature PMBus or I²C with adaptive voltage scaling and margining. Devices integrating PMBus or I²C with telemetry report voltage, current, and temperature information to a host.

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

As the semiconductor process technology advances, processors require tighter voltage accuracy and lower operating voltages. The processor data sheet specifies the voltage tolerance as either a percentage or as a value in mV, which includes DC, AC and ripple variations over the entire operating temperature range. Designers also consider the tolerance of the resistor divider used by the DC/DC converter, the routing and trace losses of the circuit board, and the variations of the application, like the input voltage variations, temperature swings, and fast changes in the load.

Check the feedback voltage accuracy of the DC/DC converter in the data sheet rather than the front page. Table 3-1 shows the regulated feedback voltage, or the internal voltage reference specification of the TPS548A29, which is a 2.7 V to 16 V, 15-A converter, and shows that the reference accuracy is ±6 mV or ±1% over the full temperature range. The total output voltage accuracy is improved by choosing tighter tolerance resistors. If more headroom is needed, designers can choose 0.1% or 0.5% resistors, even though they may cost a little bit more. The additional headroom allows a total ±3% or ±5% output voltage variation to be met with less bulk and bypass capacitance. ⁽¹⁾

Layout constraints, connectors, and board density requirements affect the total output voltage accuracy. A remote sense feature of a DC/DC converter compensates for voltage drops from long trace lines to accommodate processors needing high accuracy output voltage. This feature is especially useful when routing higher currents since the voltage drop can be a large portion of the overall DC error.

Figure 3-1 shows the TPS543B20 using the remote sense feature with voltage feedback resistors used to program the output voltage. Figure 3-2 shows the TPS543B20 using the remote sense feature without voltage feedback resistors when the VSEL pin selects the reference voltage. The RSP and RSN pins are extremely high-impedance input terminals of a true differential remote sense amplifier.

Figure 3-1 TPS543B20 Remote Sense Without Feedback Resistors

Figure 3-2 TPS543B20 Remote Sense With Feedback Resistors

Figure 3-3 TPS548A29 Differential Remote Sense Implementation