From Low Noise to No Noise

In previous TechTalk columns, I’ve talked to Frederik Dostal about how Analog Devices’ Silent Switcher technology has been designed to minimize noise in voltage conversion, and how the company has continued to improve the devices to decrease those noise levels in later generations of products. Recently, the company has launched its latest Silent Switcher device. Frederik spent some time with PSD describing how the company has improved the technology even further to offer the ultimate in noise reduction.

Initially, Dostal recaps the earlier advances in the Silent Switcher technology, “Developed around a decade ago, the first generation of Silent Switcher devices reduced the radiated emissions by clever layout techniques. After that, the second generation offered further noise emission reductions by integrating an internal bypass capacitor into the package. This move also eliminated PCB layout sensitivity. The next generation of devices added an ultra-low-noise linear regulator into the package, reducing low frequency conducted emissions in frequencies between 0.1 Hz and 100 kHz. Then, the question after this is, how can we reduce noise even more?”

The answer to that question was to incorporate shielding. There are some applications that are particularly sensitive to noise. These include signal processing applications, particularly for medical, industrial and test and measurement applications, or for use in RF, avionics or ultrasound circuits. When designing solutions for this type of use, manufacturers usually cover the product in a metal can. It is an inelegant way to reduce the radiated noise, but it was the best solution available. It also has some drawbacks. It adds costs to the solution, both in terms of the additional item, and the fact that it adds manufacturing steps to the production process. It also makes the final design larger and heavier than it needs to be.

For the next stage in the evolution of Silent Switcher technology, Analog Devices took a different approach, and incorporated the shielding into the package itself. Not only does that negate the drawbacks from using an external can, it can offer better overall performance by reducing noise in both the E field and the H field.

Dostal expands, “Firstly, we integrated the Silent Switcher devices into our µModule. Then we put the µModule into a package designed with EMI shielding. It's still a plastic moulded package, but it also has a metal coating around it. The bottom of the package already has a substrate where the active components and the inductors are mounted, which is like a little PCB with metal planes, so it's shielded anyway. Now, with the shielding on the top side as well, the very little radiated EMI that still existed in the Silent Switcher devices is almost completely gone.”

The first device to be launched with the new shielded package is the LTM8060F module with 40V maximum input voltage, and four output channels capable of delivering a peak current of 4A each. The EMI shielding enables a Faraday cage to be applied directly to the LTM8060F package. The EMI shield is electrically connected to the GND pins, eliminating all electric field noise, and giving a 10dB reduction in magnetic field noise. The device integrates controllers, power switches, inductors, and supporting components.

Figure 1, a comparrison of the LTM8060F and unshielded LTM8060 shows the additional noise reduction provided by the new shielding technology

The company also has a LTM8060 variant without shielding, allowing a direct comparison of the devices to show the effectiveness of the new shielding. Dostal describes the experiment, “Figure 1 shows two plots - on the left side is the magnetic field and on the right side is the electric field. The plots are between 1MHz and 1GHz. Typically, we look at high switching frequencies for radiated emissions, and we can see that here in yellow, the noise improvement is good, especially at frequencies above 143MHz. The emissions have been reduced by around 10dB. Then on the electric field test it's reduced by almost 100%. With the shield, the noise disappears into the noise floor”.

www.analog.com