Starting from several practical circuits, we will explain several application scenarios for achieving low electromagnetic interference in complex electronic systems.

As an integral part of any electronic system design, power supply performance has a crucial impact on system performance. Electromagnetic interference (EMI) characteristics are one of the most critical performances. This interference affects the circuit through electromagnetic induction, electrostatic coupling or conduction. Any requirements for power supply performance (increased power density, higher switching frequency and higher current) will amplify the impact of EMI. Therefore, if it is not considered in the early stages of design, it will seriously affect product performance and time to market. In response to this situation, ADI has launched the Silent Switcher series architecture regulator solution designed specifically for low EMI scenarios.

The most commonly used EMI regulation standard is CISPR 25 Class 5, which details acceptable limits at frequencies between 150 kHz and 1 GHz. Meeting this requirement typically involves a complex design and test program that includes trade-offs in solution size, overall efficiency, reliability, and complexity. Traditional approaches to controlling EMI have slowed down switching edges or reduced switching frequency, which results in reduced efficiency, increased minimum switching times, and larger solution size. Alternative mitigation solutions include bulky and complex EMI filters, snubbers, or metal shielding, which can significantly increase board space, component and assembly costs, and complicate thermal management and testing.

Silent Switcher buck regulators are designed to provide high efficiency and ultra-low EMI radiation at high switching frequencies (>2 MHz), enabling very compact and low-noise buck solutions. This series uses special design and packaging techniques: in design, the hot loop is divided into two loops with opposite polarity to form a local magnetic field that can cancel each other; in packaging, the distance from the internal FET to the package pins and input capacitors is shortened by flipping the silicon chip and adding copper pillars to reduce the range of the hot loop. Using the above technology, Silent Switcher can achieve >92% efficiency at 2 MHz, while easily meeting CISPR 25 Class 5 peak EMI limits.

Magnetic Field Cancellation in Silent Switcher Regulators

The internal structure of the new generation Silent Switcher 2 technology uses copper pillars instead of bonding wires, increases internal bypass capacitors, and integrates substrate ground planes to further improve EMI, making it insensitive to PCB layout, thereby simplifying design and reducing performance risks.

Typical Silent Switcher application schematic and how it looks on a PCB

Automotive applications require systems that do not generate electromagnetic noise that could interfere with the normal operation of other automotive systems. For example, switching power supplies are highly efficient power converters, but they generate unwelcome high-frequency signals that can affect other systems. Switching regulator noise occurs at the switching frequency and its harmonics. Sensors and other instruments affected by this type of noise may operate improperly, causing audible noise or serious system failures.

The figure below shows a low IQ (quiescent current), low noise solution that supports high current applications for automotive I/O and peripherals. The LT8672 on the front end protects the circuit from reverse battery faults and high-frequency AC ripple with a forward voltage drop of only tens of mV. The LT8650S has a switching frequency of 400 kHz, an input range of 3 V to 40 V, and an output capability of 8 A when two channels are operated in parallel. Two decoupling capacitors are placed close to the input pins of the LT8650S. Due to the Silent Switcher 2 technology, high-frequency EMI performance is excellent even without an EMI filter installed. The system meets the CISPR 25 Class 5 peak and average limits with a large margin.

The LT8672 and LT8650S are configured for high output

The figure below shows the radiated EMI average test results for vertical polarization from 30 MHz to 1 GHz. The complete solution has the characteristics of simple schematic, very low total component count, compact size, and the EMI performance is not affected by changes in board layout.

LT8672 and LT8650S EMI Performance: 30 MHz to 1 GHz

Many applications for LED lighting require wide dimming ratios. In automotive, for example, LED backlights used in head-up displays, infotainment systems, and instrument panel lighting must be bright enough to compete with the direct sunlight that pours into the car during the day, but also able to reduce the brightness by several orders of magnitude to avoid momentary blinding the driver at night. Such extreme LED dimming requirements can make it difficult to meet CISPR EMI standards without adding expensive noise reduction components and complexity.

The LT3932 enables both high dimming ratios and low EMI by incorporating many built-in features designed to minimize EMI:

2MHz Automotive LED Driver Features Low EMI with Internally Generated PWM Dimming and 90% Peak Efficiency Over Input Range (~91% Efficiency Without EMI Filter)

The LT3932 synchronous step-down LED driver with integrated 36V, 2A switch houses its high efficiency integrated power switch in a small 4mm x 5mm QFN package and can operate at up to 2MHz switching frequency for compact, high bandwidth designs. With built-in fault protection for handling open and shorted LEDs, and spread spectrum frequency modulation to help reduce EMI, the LT3932 meets the demanding requirements of automotive and industrial LED lighting applications.

LT3932 circuit passes CISPR 25 Class 5 radiated average EMI test

To ensure high accuracy, precision test and measurement systems require power solutions with low ripple and radiated noise so as not to degrade the performance of the high-resolution converter signal chain. In these test and measurement applications, generating bipolar and/or isolated system power supplies presents challenges to system designers in terms of board area, switching ripple, EMI, and efficiency.

Many precision test and measurement instruments, such as source meters or power supplies, require multi-quadrant operation to acquire and measure positive and negative signals. This requires efficient generation of positive and negative supplies from a single positive supply input with low noise. Using Silent Switcher, μModule regulator LTM8074 to improve high efficiency solutions down to lower voltages is shown in the figure below.

Power solutions that step down voltage to lower voltage rails with low EMI

The LTM8074 is a Silent Switcher, µModule step-down regulator in a small 4 mm × 4 mm BGA package capable of delivering up to 1.2 A with low radiated noise. The high efficiency and very low radiated noise of this µModule device make it an excellent choice for powering noise-sensitive precision signal chains. Depending on the PSRR of the components connected to the power supply, such as amplifiers, DACs, or ADCs, it may be possible to power them directly from the Silent Switcher output without the need for an LDO regulator to further filter the supply ripple, as is required with traditional switchers. The high output current of 1.2A also means that it can be used to power digital hardware in systems such as FPGAs if needed. The small size and high level of integration of the LTM8074 make it ideal for space-constrained applications while simplifying and speeding up the design and layout of switching regulator power supplies.

Electronic systems are developing and becoming more popular now and in the future. It is foreseeable that the requirements for low EMI will become more and more stringent. Based on this, ADI will continue to develop the Silent Switcher architecture and provide more and more solutions to help system designers more calmly deal with various challenges now and in the future.

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