Want to minimize noise and ripple? Choose a low-noise buck converter!

When engineers design power supplies for noise-sensitive systems such as clocks, data converters, or amplifiers for test, measurement, and radio applications, one common question they face is how to minimize noise. Since different people understand the term "noise" in different ways, I would like to state that the noise discussed in this article refers to the low-frequency thermal noise generated by resistors and transistors in the circuit. You can usually think of the noise in the 100Hz to 100kHz bandwidth of the noise spectral density curve (in microvolts per square root hertz ) as the integrated output noise (in millivolts rms) . Power supply noise can degrade the performance of analog-to-digital converters and cause clock jitter.

In the past, clocks, data converters, or amplifiers were powered by a DC/DC converter, followed by a low dropout regulator (LDO) (such as the TPS7A52, TPS7A53, or TPS7A54), and then a ferrite bead filter, as shown in Figure 1. This design approach minimizes power supply noise and ripple, and maintains good performance when the load current is less than about 2A. However, as the load increases, the power loss in the LDO causes efficiency and thermal management issues . For example, a post-regulator LDO will add 1.5W of power loss in a typical analog front-end application. Is a low-noise, efficient design impossible? Not necessarily.

Figure 1: Using a DC/DC converter, LDO

Typical low noise architecture of ferrite bead filters

One way to prevent power loss is to minimize the voltage drop across the LDO. However, this approach can negatively impact noise performance. In addition, higher current LDOs are typically larger, which increases design size and cost. A more effective way to ensure low noise without increasing power loss is to use a low noise DC/DC buck converter instead of the LDO in the design, as shown in Figure 2.

I know what you are asking: How can you still provide a low-noise power supply by removing the main device for noise reduction? In fact, many LDOs have a low-pass filter at the bandgap reference to minimize the noise entering the error amplifier. The low-noise buck converters of the TPS62912 and TPS62913 families use a capacitor connected to the noise reduction/soft-start pin and form a low-pass resistor/capacitor filter with the integrated Rf and externally connected CNR /SS , as shown in Figure 3. In essence, this structure simulates the performance of the bandgap low-pass filter in the LDO.

All DC/DC converters generate output voltage ripple at their switching frequency. In precision systems, noise-sensitive analog power rails require ultra-low supply voltage ripple to minimize frequency spurs in the spectrum. Supply voltage ripple is usually determined by the DC/DC converter's switching frequency, inductor value, output capacitance, equivalent series resistance, and equivalent series inductance. To reduce the ripple generated by these components, engineers typically use LDOs and/or small ferrite beads and capacitors to form a π-type filter to minimize load ripple. Low-ripple buck converters such as the TPS62912 and TPS62913 take full advantage of ferrite bead filters by integrating ferrite bead compensation and remote sensing feedback. By utilizing the inductance of the ferrite bead and the additional output capacitor, the high-frequency component in the output voltage ripple is eliminated and the ripple is reduced by about 30dB, as shown in Figure 4.

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

Output voltage ripple after using ferrite bead filter (b)

By integrating features that reduce system noise and ripple, low-noise buck converters help engineers achieve low-noise power solutions without LDOs. Of course, different applications require different noise levels, and different output voltages require different performance. So, you can only choose the right low-noise architecture for your design. If you want to simplify noise-sensitive analog power design, reduce power losses, and reduce the overall design size, consider using a low-noise buck converter.

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