Quieting Noisy Applications with ADI Silent Switcher μModule® Regulators

There's no such thing as silence in a sensitive electronic application—literally, it never happens. That's because eliminating all electromagnetic interference (EMI) noise from power supplies is virtually impossible. Different design approaches to mitigate this problem often involve tradeoffs that can create greater complexity.

Engineers jump through hoops to try and reduce EMI in noise-sensitive applications such as radio frequency power supplies (amplifiers), high-speed data converters, sensitive instrumentation, and medical imaging and diagnostics systems. That usually means bulking up on added components, shielding, and filters, all of which can increase complexity, cost, size, and weight.

Switched-mode power supplies (SMPS) and electronic-based converters are a leading cause of EMI, complicating the design of many applications in automotive systems, consumer electronics, industrial automation, and telecommunications.

Rapid switching minimizes energy loss for DC-to-DC converters, as well as AC-to-DC rectifiers, DC-to-AC inverters, and AC/AC converters. However, it comes with the cost of generating high-frequency energy and transients that can cause EMI conduction and radiation.

EMI can degrade system performance, interfere with radio frequencies, cause malfunction of components, and impede the operation of critical devices such as pacemakers and automotive safety systems. A major cause of EMI in such systems is common-mode current flowing in the same direction across two or more conductors, which induces magnetic fields.

Many—if not most—electronic applications in the U.S. must comply with Federal Communications Commission Part 15 regulations designed to prevent harmful interference, including from non-RF devices. International industrial and communications applications must comply with the CISPR 22 Class B and automotive applications with the CISPR 25 international standards. Other geographies have similar compliance certifications.

EMI testing often takes place late during the design cycle, so problems and corrective actions can lead to costly product delays. Worse, if EMI issues are discovered in the field, they may be more difficult to pinpoint and require costly remediation efforts.

Multiple types of components can be utilized to counter EMI. Low-dropout (LDO) linear regulators are a conventional, low-cost approach to protecting downstream loads from voltage transients and power supply noise. However, they can result in bulky solutions and often lack the necessary protection features.

More advanced LDOs with high-power supply rejection ratio (PSRR) improve noise suppression, but do not directly improve efficiency or thermal performance. Used in conjunction with switching regulators, they can combine high efficiency with low noise.

Designers can also focus efforts on PCB layout to minimize loop areas that propagate EMI and separate noisy and sensitive circuits. Another often complementary approach is to isolate or enclose components with EMI-shielding materials such as metals and metal alloys. Low-noise amplifiers can also be utilized.

Each of these EMI reduction techniques, often used in tandem, adds to the complexity of design, leaving developers searching for simplification.

Simplifying EMI design issues

The growth in applications reliant on SMPS designs is outpacing the number of designers skilled in meeting stringent EMI requirements. Many digital designers are being asked to fill the skill gaps caused by a shortage of analog power supply designers. That trend, combined with the growing complexity of SMPS design, signals the need for greater integration of SMPS components to simplify processes.

Analog Devices, Inc. (ADI) moved to simplify EMI design challenges with the introduction of its Silent Switcher^® technology in 2015. It aimed to optimize switching techniques while simplifying printed circuit board (PCB) design. First-generation Silent Switcher devices like the LT8640 reduced parasitic resistance by using copper pillar flip-chip packaging instead of bonded wire to connect dies to the substrate. They also incorporated a powertrain designed to improve high-frequency efficiency.

Those first-generation devices also split single high-current "hot loops" into dual loops with opposing flows that canceled out the EMI propagated. A single large hoot loop has high parasitic elements and strong magnetic fields that can contribute to EMI in the form of radiation. Silent Switcher devices also incorporated internal switch drivers to minimize switching power loss.

In 2017, ADI introduced a low-EMI monolithic synchronous buck converter based on a Silent Switcher 2 architecture. In this generation, devices such as the LT8640S-2 reduced reliance on external components by integrating capacitors, hot loops, and a ground plane inside a new LQFN package. This enabled smaller solution sizes and eliminated PCB layout sensitivity for better EMI performance. In addition, Silent Switcher 2 devices include more copper pillars and large exposed pads, increasing thermal performance and efficiency.

In 2021, ADI introduced an updated Silent Switcher 3 architecture with the LT8627SP synchronous step-down regulator, featuring ultra-low low-frequency noise performance, ultra-fast transient response, and high efficiency at high switching frequencies while maintaining ultra-low EMI. It also provides an exposed die top for optionally attaching a heat sink to accommodate high ambient temperature applications.

Silent Switcher 3 µModule regulators

Silent Switcher 3 technology is now available in ADI's µModule® highly integrated component on package (COP) power solutions. This packaging provides better thermal performance and saves even further on total solution size, enabling small, efficient, and reliable power solutions.

Other key benefits of µModule regulators include the time savings and reduced effort needed for designing, testing, and qualifying DC/DC regulators. ADI integrates the controller, power MOSFETs, inductor, and other supporting components into a single, compact package. They can be utilized as a power solution for a wide range of telecom, networking, and industrial equipment applications, RF power supplies, low-noise instrumentation, and high-speed and high-precision data converters.

The LTM4702 (Figure 1) is a complete 8 A step-down μModule regulator in an ultra-compact 6.25 mm × 6.25 mm × 5.07 mm BGA package, with incorporating silent switcher-based regulator IC for low EMI and high efficiency. It operates over an input voltage range of 3 V to 16 V and supports an output voltage of 0.3 V to 5.7 V.

Figure 1: ADI's LTM4702 μModule integrates a controller, power MOSFETs, inductor, and other supporting components for buck converter in an enhanced compact package. It alleviates the need for the post LDOs in noise sensitive applications. (Image source: Analog Devices, Inc.)

Multiple LTM4702s can be operated in parallel to produce higher output currents. A maximum of 12 phases can be paralleled to run simultaneously out-of-phase by programming the PHMODE pin of each LTM4702 to different voltage levels.

In addition, the LTM4702 synchronous switching regulator features exceptional low-frequency output noise (10 Hz to 100 kHz). It is highly suited for high-current and noise-sensitive applications. The device uses a constant-frequency PWM architecture that can be programmed to switch from 300 kHz to 3 MHz by using a resistor tied from the RT pin to ground.

A single resistor sets the LTM4702’s output voltage to provide unity gain on output voltage feedback and virtually constant output noise independent of the output voltage. For most noise sensitive applications, the LTM4702 removes the need for post regulation LDOs, and LC filters and only input and output capacitors are needed to complete a design.

The EVAL-LTM4702-AZ evaluation board (Figure 2) is available for setting up and evaluating the performance of the LTM4702.

Figure 2: ADI's EVAL-LTM4702-AZ evaluation board provides designers with a step-down DC/DC switching converter for evaluating the LTM4702 performance. (Image source: Analog Devices, Inc.)

The LTM8080 (Figure 3) is a 40 V_IN, dual 500 mA or single 1 A device that integrates dual ultra-high PSRR LDO regulators with a silent switcher DC/DC regulator separated by an integrated EMI shield in a thermally enhanced, 9 mm × 6.25 mm × 3.32 mm, over-molded BGA package. It supports a switching frequency range of 200 kHz to 2.2 MHz and an output voltage range of 0 V to 8 V.

Figure 3: ADI's LTM8080 μModule integrates dual LDOs, along with a silent switcher DC/DC regulator with an EMI shield between them in compact package. (Image source: Analog Devices, Inc.)

The front-end switching regulator is a non-isolated step-down switching DC/DC power supply that can deliver up to 1.5 A continuous current. The back-end LDO linear regulators utilize ADI's ultralow noise (2 nV/√Hz at 10 kHz) and ultrahigh PSRR (76 dB at 1 MHz) architecture. The LDO outputs can be paralleled to  increase output current

Designers can use the DC3071A (Figure 4) demonstration circuit, which has a wide operating range of 4 V to 40 V to evaluate the LTM8080.

Figure 4: The DC3071A demonstration circuit includes an LTM8080 µModule with two outputs, each of which is an adjustable 3.3 V/0.5 A. (Image source: Analog Devices, Inc.)

Conclusion

ADI's Silent Switcher μModule regulators provide a robust solution for the challenges of EMI in noise-sensitive electronic applications. By integrating advanced Silent Switcher 3 technology into highly compact and efficient system-in-package designs, these μModule regulators simplify design, improve thermal performance, and eliminate the need for LDO post regulators in most scenarios.

From high-speed data converters and RF systems to medical imaging and industrial equipment, these μModule regulators enable engineers to achieve ultra-low noise and high efficiency without the added complexity of traditional EMI reduction methods. With products like the LTM4702 and LTM8080, Analog Devices continues to lead in offering innovative solutions that meet the stringent demands of modern electronics, ensuring reliable performance in even the most noise-critical applications