Easily resist noise and make a quiet and simple Silent Switcher device~

First, because there aren’t enough analog power designers, more and more digital designers are being asked to design switch-mode power supplies! While most digital designers know how to use a simple linear regulator, not all designs require step-down (buck mode). In fact, many are step-up (boost) or even buck-boost topologies (buck and boost modes combined).

Obviously, many electronic system manufacturers face a problem: How to implement all the switch-mode power supply circuits required for the system?

In this article, I will introduce some basic principles of buck regulator operation, including how high di/dt and parasitic inductance in the switching regulator hot loop can cause electromagnetic noise and switch ringing. We will then look at how to reduce high frequency noise. I will also introduce ADI's Power by Linear TM Silent Switcher ® technology, including how it is constructed, and demonstrate how it can help solve EMI problems without sacrificing performance. It also includes how the SilentSwitcher device works.

I will also provide an overview of Silent Switcher packaging and layout and discuss how these can improve the overall performance of a buck converter. In addition, I will demonstrate how this technology can be incorporated into our μModule® regulators, increasing the level of integration of SilentSwitcher devices. These simple and easy-to-use solutions can be useful for designers who are new to switch-mode power supply design techniques.

One of the most basic power supply topologies is the buck regulator, as shown in Figure 1. EMI starts with high di/dt loops. The supply and load lines should not have high AC current components. Therefore, the input capacitor C2 should transfer all the AC components of the relevant current to the output capacitor C1, where all the current AC components end.

Referring to Figure 1, during the on-cycle when M1 is off and M2 is on, the AC current flows in the solid blue loop. During the off-cycle, when M1 is on and M2 is off, the AC current flows in the dashed green loop. Most people have a hard time understanding that the loop that produces the highest EMI is neither the solid blue loop nor the dashed green loop. It is the fully switched AC current flowing in the dashed red loop, switching from zero to I peak and back to zero. The dashed red loop is often referred to as the hot loop because it has the highest AC current and EMI energy.

What causes electromagnetic noise and switch ringing is the high di/dt and parasitic inductance in the hot loop of the switching regulator. To reduce EMI and improve functionality, the radiation effect of the dotted red loop needs to be minimized. If we can reduce the PC board area of ​​the dotted red loop to zero and can buy an ideal capacitor with zero impedance, this problem can be solved. However, in the real world, all design engineers can do is find a best compromise!

So, where does this high-frequency noise come from? In electronic circuits, high-frequency harmonics are generated during the switching process through parasitic resistance, inductance and capacitance coupling. Knowing where the noise is generated, how to reduce high-frequency switching noise? The traditional way to reduce noise is to slow down the MOSFET switching edge. This can be achieved by slowing down the internal switch driver or adding a buffer externally.

However, this reduces the efficiency of the converter due to increased switching losses – especially when the switching regulator is operated at a high switching frequency such as 2MHz. Speaking of which, why would we want to operate at 2MHz?

Filters and shielding can also be used to reduce radiation, but this requires more external components and board area. Spread spectrum frequency modulation (SSFM) can also be used, but this will cause the system clock to jitter within a known range. SSFM helps meet EMI standards. The EMI energy is scattered and distributed in the frequency domain. Although the switching frequency selected for ordinary switching power supplies is usually outside the AM band (530 kHz to 1.8 MHz), the unmodulated switching harmonics within the AM band may still not meet the strict automotive EMI requirements. Adding SSFM can significantly reduce EMI in the AM band and other areas. Or just use ADI's Silent Switcher technology, which can meet all of the above requirements:

Silent Switcher devices eliminate the need to slow down the switching edge rate, solving the trade-off between EMI and efficiency. So how can this be achieved? Consider the LT8610, shown on the left side of Figure 2. This is a monolithic (with internal FETs) synchronous buck converter that supports 42 V input and can provide up to 2.5 A of output current. Note that it has an input pin (VIN) in the upper left corner.

However, comparing the LT8610 to the LT8614 (a monolithic synchronous step-down converter that supports 42V input and can deliver up to 4A of output current), we can see that the LT8614 has two VIN pins and two ground pins on the other side of the package. This is important because it is part of the implementation of ultra-low noise switching!

How is this achieved? Placing two input capacitors between the VIN and ground pins on the other side of the chip cancels the magnetic field. This is highlighted in the slide with red arrows pointing to the capacitors’ locations on both the schematic and the demo board, as shown in Figure 3.

The LT8614 includes a Silent Switcher feature. With this feature, we are able to reduce parasitic inductance by using a copper pillar flip chip package. In addition, there are reverse VIN, ground, and input capacitors that cancel the magnetic field (right-hand rule applies) to reduce EMI radiation.

Since the long bond wires required by wire bonding assembly technology are not required, no large parasitic resistance and inductance are generated, thereby reducing the package parasitic inductance. The opposite magnetic fields generated by the two symmetrically distributed input hot loops cancel each other, and there is no net magnetic field in the electric loop.

We compared the LT8614 Silent Switcher regulator to the current state-of-the-art switching regulator, the LT8610. Standard demo boards for both devices were tested in a GTEM chamber using the same load, the same input voltage, and the same inductor. We found that while the LT8610 had very good EMI performance, there was a 20dB improvement when using the LT8614, especially in areas where it is more difficult to manage higher frequencies. In the overall design, the LT8614 switcher requires less filtering and shorter distances than other sensitive systems, allowing for a simpler and more compact design. Additionally, in the time domain, the LT8614 performs well at the switch node edge.

Despite the excellent performance of the LT8614, we did not stop at the pace of improvement. As a result, the LT8640 step-down regulator uses a Silent Switcher architecture designed to minimize EMI/EMC radiation while providing high efficiency at frequencies up to 3 MHz. It uses a 3 mm × 4 mm QFN package with an integrated power monolithic structure while providing all the necessary circuit functions, together forming a solution with the smallest PCB space. Transient response performance is still excellent, with an output voltage ripple of less than 10 mV pp at any load (from zero current to full current). The LT8640 allows high VIN to low VOUT conversion at high frequency with a minimum switch on time of 30 ns.

To improve EMI/EMC, the LT8640 can operate in spread spectrum mode. This function adjusts the clock with a 20% triangular frequency modulation. When the LT8640 is in spread spectrum modulation mode, the switching frequency is adjusted between the RT setting and approximately 20% above that value using the triangular frequency modulation function. The modulation frequency is approximately 3 kHz. For example, when the LT8640 is set to 2 MHz, the frequency at a 3kHz rate will vary from 2MHz to 2.4MHz. When the spread spectrum mode of operation is selected, Burst Mode® operation is disabled and the device will operate in pulse skipping mode or forced continuous mode.

However, despite all our instructions in the Silent Switcher data sheet, such as providing schematics and layout suggestions, and placing the input capacitors as close as possible to both sides of the IC - some customers still make mistakes. In addition, our in-house engineers also spend too much time to solve customers' PCB layout problems. Therefore, our designers came up with the best solution to this problem - the Silent Switcher 2 architecture.

With Silent Switcher 2 technology, we only need to integrate capacitors inside the new LQFN package: VIN capacitor, IntVCC and boost capacitor - placed as close to the pins as possible. The advantage is that all hot loops and ground planes are included, which reduces EMI. Fewer external components means a smaller solution size. In addition, we have eliminated PCB layout sensitivity.

As shown in Figure 5, you can see how the schematics for the LT8640 and LT8640S differ. The marketing breakthrough was to give the new, more integrated version with internal capacitors an “S” suffix. Because it is “quieter” than the first generation!

Silent Switcher 2 technology improves thermal performance. Multiple large ground exposed pads on the LQFN flip chip package help dissipate heat from the package and PCB. Since we have eliminated high resistance bond wires, conversion efficiency is also improved. The EMI performance of the LT8640S easily meets the radiated EMI performance CISPR 25 Class 5 peak limit requirements with a large margin.

Silent Switcher technology is so attractive that we chose to incorporate it into our μModule regulator product line. All components are integrated in a small package, providing users with a simple, reliable, high-performance and high-power density solution. LTM8053 and LTM8073 are micromodule regulators that integrate almost all components, with only a few capacitors and resistors connected externally.

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