Minimize noise and ripple using low-noise buck converters

Minimizing noise is a common challenge for engineers designing power supplies for noise- sensitive systems in test and measurement and radio applications such as clocks, data converters, or amplifiers . Although the word "noise" can mean different things to different people, for the purposes of this article, I will define noise as the low-frequency thermal noise produced by resistors and transistors in a circuit. You can identify the noise by its spectral noise density curve in microvolts per square root hertz, and output the noise as an integral in microvolts rms, typically within a specific range of 100 Hz to 100 kHz. Noise in the power supply can degrade analog-to-digital converter performance and introduce clock jitter.

The traditional setup for powering a clock, data converter, or amplifier is to use a DC/DC converter, then a low dropout regulator (LDO) such as the TPS7A52 , TPS7A53 , or TPS7A54 , and then a ferrite bead filter, as shown in Figure This is shown in Figure 1. This design approach minimizes noise and ripple on the power supply and is suitable for load currents below approximately 2 A. However, as the load increases, power losses in the LDO can cause efficiency and thermal management issues; for example, in a typical analog front-end application , a post-regulated LDO can add 1.5 W of power loss . Are those of you looking for low noise and low efficiency options in your designs? Not completely.

Figure 1: Typical low-noise architecture using DC/DC converters, LDOs, and ferrite bead filters

Use a low-noise buck converter instead of an LDO

One way to control power loss is to minimize the voltage drop through the LDO. However, this approach can have a negative impact on noise performance. Additionally, higher current LDOs are typically larger, which increases design footprint and cost. A more effective way to ensure low noise while controlling power losses is to eliminate the LDO from the design entirely and use a low-noise DC/DC buck converter, as shown in Figure 2.

Figure 2: Using a low-noise buck converter without an LDO

I know what you're thinking: How do you still provide low-noise power by removing the primary equipment that reduces noise? Many LDOs have a low-pass filter on the bandgap reference to minimize noise entering the error amplifier. The TPS62912 and TPS62913 family of low-noise buck converters implement a noise reduction/soft-start pin for connecting a capacitor, using integrated R and externally connected  C NR/SS to form a low-pass resistor -capacitor filter as shown Shown in Figure 3. This implementation essentially mimics the behavior of a bandgap low-pass filter in an LDO. 

Figure 3: Block diagram of low-noise buck with bandgap noise filtering

What about the output voltage ripple?

Every DC/DC converter generates output voltage ripple at its switching frequency. Noise-sensitive analog rails in precision systems require the lowest supply voltage ripple to minimize frequency spurs in the spectrum, which typically depends on the DC/DC converter switching frequency, inductor value, output capacitance, equivalent series resistance and equivalent series inductance. To mitigate ripple from these components, engineers often use LDOs and/or small ferrite beads and capacitors to create pi filters to minimize load ripple. Low ripple buck converters such as the TPS62912 and TPS62913 take advantage of this ferrite bead filter by integrating ferrite bead compensation and remote sensing feedback. Combining the inductance of the ferrite bead with additional output capacitors removes the high-frequency component of the output voltage ripple and reduces the ripple by approximately 30 dB, as shown in Figure 4. 

Figure 4: Output voltage ripple before ferrite bead filter (a); after ferrite bead filter (b)

in conclusion

By integrating features that mitigate system noise and ripple , low-noise buck converters can help engineers implement low-noise power solutions without the need for an LDO. Of course, the required noise levels will vary from application to application, as will the performance at different output voltages, so only you can determine the best low-noise architecture for your design. However, if you want to simplify the design of noise-sensitive analog power supplies , reduce power losses, and reduce the overall design footprint, consider using a low- noise buck converter.