However, a new generation of noise-optimized switching regulators for mobile phones is making a difference in practice by minimizing interference with nearby low-noise and power amplifiers. They enable high-speed data converters to be powered directly by DC/DC converters without significantly degrading AC performance. This design can immediately improve power efficiency by 20% to 50%.
Compared to previous generations, modern high-speed converters consume approximately 50% less power, partly due to reducing the supply voltage from 3.3V to 1.8V. In designs based on low-dropout regulators, as the power rails decrease, the regulator's dropout voltage and the available power rails become more critical for power efficiency. In the digital section of a board, many voltage rails typically serve the various core and I/O voltages of FPGAs and processors. However, in the analog section, perhaps only a few "clean" options are available, such as 3.3V and 5V.
For high-speed data converters, you can use a linear regulator to generate a 3.3V supply from a general-purpose 5V rail. The 1.7V dropout in a low-dropout regulator translates to approximately 35% power loss. When using a low-dropout regulator to supply 1.8V to an ADC (such as the ADS4149) from a 3.3V bus, the power loss of the linear regulator increases to approximately 45%, meaning almost half the power is consumed by the low-dropout regulator. This example illustrates how easily an inefficient power supply design can lose 50% of its power. Switching regulators, on the other hand, are completely independent of the input power rail, thus offering significant power savings. With careful design, you can minimize the impact on AC performance.
Power supply filtering
The key component for isolating switching noise from the ADC is the power supply filter, which consists of a ferrite bead and a bypass capacitor. Several key characteristics should be considered when selecting a ferrite bead. First, the ferrite bead must provide sufficient rated current to the data converter, and it must have a low DCR (DC resistance) to minimize the voltage drop across the bead itself. For example, a 200 mA supply current through a bead with a DCR of 1 Ω will result in a 200 mV drop in supply voltage. When you consider standard supply voltage variations, this voltage drop can push the ADC voltage near the edge or even below recommended operating conditions.
Secondly, the ferrite beads must have high impedance at the switching frequency and harmonics of the DC/DC converter to suppress switching noise and spurious signals. Most available ferrite beads have an impedance of 100 MHz, while modern DC/DC converters typically have switching frequencies from 500 kHz to 6 MHz. In our example, the ADS4149 evaluation module uses a TPS625290 switching regulator with a switching frequency of 2.25 MHz. Since the DC/DC regulator has a square wave output, you must also consider higher harmonics. Murata's NFM31PC276B0J3 EMI filter provides high impedance and low DCR in this frequency range.
The insertion loss of a conventional ferrite bead with a resistance of 68Ω was compared with that of a Murata EMI filter at 100 MHz. The power supply circuit has low impedance, and the insertion loss was measured in a 50Ω environment. Therefore, the insertion loss amplitude of the power supply filter may differ slightly, although the resonant frequency remains unchanged.
Other components of the power filter are bypass capacitors. You should choose the values of these capacitors so that they create a low-impedance ground path with a resonant frequency close to the switching frequency. This shorts the switching noise through the ferrite bead to ground. Insertion loss comparisons of power filters show that appropriate bypass capacitor values produce a resonance close to the switching frequency, even when used in conjunction with conventional ferrite beads such as the EXCML32A680. However, at low frequencies, it makes little difference if replaced with a 0Ω resistor. On the other hand, Murata EMI filters provide approximately 20dB of additional attenuation near the switching frequency. The power filter uses a 33μF tantalum capacitor for wideband decoupling, while the 10μF, 2.2μF, and 0.1μF ceramic capacitors have narrower resonant frequencies.
AC performance
According to the PSRR of the data converter, a certain amount of noise on the power rail will still enter the ADC and degrade its AC performance. SNR and SFDR (spurious-free dynamic range) scans were compared with reference power supplies such as 1.8V, clean laboratory power, low dropout regulators, and the ADS4149 evaluation module of DC/DC converters with different power filter options.
Test results show that the SNR performance decreases by approximately 0.3 dB when powered by a switching regulator compared to a low-noise, low-dropout regulator at a 300 MHz IF. The SFDR performance is also nearly identical across the settings. A close examination of the normalized FFT plot, which plots noise versus offset frequency starting from the input signal, shows a slight increase in the Nyquist noise floor when using suboptimal EXC ferrite beads, but no evidence of switching frequency feedthrough.
Power efficiency
The main advantage of using a DC/DC converter instead of a linear regulator is energy saving. In all experiments with the ADS4149 evaluation module, an external 3.3V supply (one common analog power rail) powered both the low-dropout and switching regulators. The power efficiency and their respective quiescent currents were measured. This comparison shows that the low-dropout regulator consumes almost as much power as the ADC. The switching regulator consumes only 32 mW more power than the ideal approach, enabling a highly efficient power supply design. You can further improve the efficiency of the low-dropout regulator by reducing the input voltage (starting from 3.3V to, for example, 2.5V or 2.2V), but at the cost of increased system cost and size.
Despite having more external components than low-dropout designs, the overall footprint of a DC/DC converter design can be smaller because newer DC/DC converters have higher switching frequencies, which greatly reduces the size of the inductor; for example, for 2.25 μH, it is about 2.2 μH MHz instead of 33 μH at 500 kHz.
Conversely, linear regulators may require less power supply filtering, but they also have size limitations because they typically consume more power. From a cost perspective, switching regulators can be slightly more expensive due to the greater number of components. Nevertheless, improving efficiency can save on costs related to thermal design and system power budget.
As system designers seek more energy-efficient components, changing the power architecture in high-speed data converter designs to switching regulators can significantly reduce power consumption. You can power low-power, high-speed data converters directly from switching regulators without significantly degrading their AC performance.
