For designers who choose high-speed data converters, power consumption is the most important system design parameter. Whether it's a portable design that requires a long battery life or a small product that consumes less heat, power consumption is critical. System designers used to use low-noise linear regulators to power data converters, such as low-dropout regulators, rather than switching regulators, because they were concerned that switching noise would enter the converter's output spectrum, greatly reducing AC performance.

However, a new generation of noise-optimized switching regulators (for cell phones) minimizes interference with adjacent low noise and power amplifiers, causing a shift in applications. They can power high-speed data converters directly from a DC/DC converter without significantly degrading AC performance. This design immediately increases power efficiency by 20% to 25%.

Modern high-speed converters can reduce power consumption by about 50% compared to the previous generation, in part because the supply voltage is reduced from 3.3V to 1.8V. In a design with a low-dropout regulator, the power rail is reduced. The pressure differential of the press and the available power rail are more important for power efficiency. The data portion of the board typically has a number of voltage rails that provide various core and I/O voltages for the FPGA and processor. In the analog part, there may be only a few "clean" voltages to choose from, such as 3.3V and 5V.

For a high speed data converter, a linear regulator can be used to obtain 3.3V from a common 5V rail. Thus, the low dropout regulator has a 1.7V drop, which is equivalent to about 35% power loss. When using a low dropout regulator (such as the ADS4149) to provide 1.8V from an ADC on a 3.3V bus (Reference 1), the power loss on the linear regulator increases to approximately 45%, which means low voltage The difference regulator consumes almost half of the power. This example shows that a low-efficiency power supply design can easily lose 50% of the power. The efficiency of the switching regulator has nothing to do with the size of the input rail, so it can save considerable power. By carefully designing, the impact on AC performance can be minimized.

Power supply filtering

One of the key components that isolate noise from the ADC is the power supply filter, which includes a ferrite bead and a bypass capacitor. Several key characteristics should be considered when selecting a ferrite bead. First, the ferrite bead must have sufficient current rating for the data converter, which must have a low DCR (DC impedance) to minimize the voltage drop of the magnetic ring itself. For example, when a 200 mA power supply passes through a magnetic ring with a DCR of 1 Ω, a voltage drop of 200 mV is produced. This voltage drop can push the voltage on the ADC to the edge. The ADC voltage may even be lower than the recommended operating voltage, taking into account the standard deviation of the supply voltage.

Second, the ferrite bead must have high impedance to the switching frequency and harmonics of the DC/DC converter to block switching noise and switching spurs. The impedance of most ferrite magnetic rings on the market is 100MHz, and the typical switching frequency of modern DC/DC converters is 500kHz~6MHz. In our example, the ADS4149 evaluation module uses a TPS625290 switching regulator. The switching frequency is 2.25 MHz (Reference 2). Since the DC/DC regulator is a square wave output, higher order harmonics must also be considered. Murata's NFM31PC276B0J3 EMI filter has high impedance and low DCR in this frequency range.

Figure 1 compares a conventional ferrite magnetic ring insertion loss using a Murata EMI filter with a resistance of 68 Ω at 100 MHz. The power supply circuit has a low impedance and the insertion loss is measured in a 50 Ω environment. Therefore, the insertion loss magnitude of the power supply filter may vary slightly, although the resonant frequency does not change.

Figure 1. Murata's NFM31PC276B0J3 EMI has high impedance and low DCR compared to a conventional ferrite bead with a resistance of 68Ω at 100 MHz.

Figure 1. Murata's NFM31PC276B0J3 EMI has high impedance and low DCR compared to a conventional ferrite bead with a resistance of 68Ω at 100 MHz.

The other components in the line filter are bypass capacitors. These capacitor values ​​should be chosen such that their resonant frequency (which produces a low impedance path to ground) is close to the switching frequency. Thus, the switching noise through the magnetic ring is shorted to ground. The power supply filter insertion loss comparison of Figure 2 shows that the correct bypass capacitor value produces a resonance close to the switching frequency, even for a conventional ferrite magnetic ring, such as the EXCML32A680. However, at low frequencies, if Putting it together with a 0Ω resistor is not that big difference. On the other hand, the Murata EMI filter provides approximately 20 dB of additional attenuation around the switching frequency. The power supply filter in Figure 3 uses a 33μF tantalum capacitor for wideband decoupling, while the 10μF, 2.2μF, and 0.1μF ceramic capacitors have a narrow resonant frequency.

Figure 2. The correct bypass capacitor value produces a resonance close to FS (switching frequency), even when combined with a conventional ferrite ring (such as the EXC-ML32A680).

Figure 2. The correct bypass capacitor value produces a resonance close to FS (switching frequency), even when combined with a conventional ferrite magnetic ring (such as the EXC-ML32A680).

Figure 3. This power supply filter uses a 33μF tantalum capacitor for wideband decoupling, while 10μF, 2.2μF, and 0.1μF ceramic capacitors have a narrow resonant frequency.

Figure 3. This power supply filter uses a 33μF tantalum capacitor for wideband decoupling, while the 10μF, 2.2μF, and 0.1μF ceramic capacitors have a narrow resonant frequency.

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