【Introduction】Noise is an inherent and often unavoidable consideration in almost all system designs. Although some noise comes from outside and is not within the direct control of the circuit designer, it is also generated by the circuit itself. In many cases, designers must minimize noise sources, especially on power rails, that can affect sensitive analog and digital circuits.
Noise is an inherent and often unavoidable consideration in almost any system design. Although some noise comes from outside and is not within the direct control of the circuit designer, it is also generated by the circuit itself. In many cases, designers must minimize noise sources, especially on power rails, that can affect sensitive analog and digital circuits.
Less severe results can result in unstable circuit performance, reduced resolution and accuracy, and higher bit error rates (BER). But in the worst case, the result could be a complete system failure, or frequent or intermittent performance issues, both of which are difficult to fix.
Switching DC/DC regulators and their output rails have two main noise problems: ripple and radiated noise. To comply with electromagnetic compatibility (EMC) regulations, noise generated in circuits must be below specified levels in all frequency bands.
The challenge for designers is to understand internal noise and its sources, and to “clean it out of the design” or otherwise mitigate its effects. This article uses the DC/DC regulator from Monolithic Power Systems, Inc. as an example to discuss the choices that need to be made to minimize the problem of regulator noise.
First is the noise source and type
The easiest noise to observe, and the one that directly affects circuit performance, is ripple at the switching frequency. This ripple is typically in the range of 10 – 20 mV (Figure 1). While this ripple is not random in nature, it is still a manifestation of noise that can impact system performance. The millivolt level of this ripple is usually not a problem for high-Voltage digital ICs operating at 5V and above, but for low-voltage digital circuits operating below 3V A problem that cannot be ignored. Ripple on power rails is also a major problem for precision analog circuits and components, which is why power supply rejection ratio (PSRR) specifications for such devices are critical.
Figure 1: Ripple on the DC power rail is caused by the switching action of the regulator, which affects the basic performance or accuracy results of the circuit. (Image credit: Monolithic Power Systems, Inc.)
The switching action of DC/DC regulators also radiates radio frequency (RF) noise. Even if millivolt ripple on the DC power rail is tolerable, there is the problem of electromagnetic radiation weakening the EMC. This noise has a known fundamental frequency ranging from a few kilohertz to several megahertz (MHz), depending on the switching converter, and it also has many harmonics.
The most frequently cited EMC-related regulatory standards include CISPR 22 and CISPR 32, Information Technology Equipment – Radio Interference Characteristics – Limits and Methods of Measurement (CISPR is the acronym for International Panel on Radio Interference). In addition, there is the European standard EN 55022, mainly derived from the CISPR 22 product standard, which specifies tests under well-defined conditions.
CISPR 22 has been adopted by most members of the European Community. Although the US FCC Part 15 and CISPR 22 have achieved relative harmony, there are still some differences. CISPR 22/EN 55022 has been “absorbed” by CISPR 32/EN 55032, a new family of standards for Multimedia Equipment (MME) products. This standard is now in force as a harmonized standard in compliance with the EMC Directive.
Equipment intended primarily for use in a residential environment must meet Class B limits, and all other equipment must meet Class A standards (Figure 2). Products designed for the North American market must comply with the limits for unintentional radiators set forth in Federal Communications Commission (FCC) Part 15 Subpart B Section 15.109. Therefore, even if the electrical noise radiated by a DC regulator does not adversely affect the product itself, it may still be unacceptably high for meeting various regulatory requirements.
Figure 2: This is one of many charts provided by CISPR 32/EN 55032, which defines emission limits versus frequency for various consumer products. (Image credit: Academy of EMC, EMC Standards)
Dealing with EMC problems is a complex subject with no easy solutions. In addition, the measured and allowable limits for these emissions are a function of the circuit operating frequency, distance, power level, and application class. For these reasons, it makes sense to consult many technical resources, or even consult a consultant who can provide guidance and expertise.
That said, designers have three basic strategies to minimize noise and avoid circuit performance issues while meeting corresponding noise requirements:
· Use a low dropout regulator (LDO).
· Add an external filter to the switching regulator to reduce noise from the load on the DC rail.
· Select a switching regulator module. This module embeds components, such as inductors or capacitors, that would otherwise be external to the regulator IC. The resulting modules are designed to provide and guarantee a low noise rail and therefore require minimal or no external filtering.
Start with LDO
Because the LDO architecture has no clocks or switches, it inherently has low EMC noise and no output power rail ripple; hundreds of millions of LDOs are put into use every year. When used in a suitable design, this device will be an effective solution.
For example, Monolithic Power Systems’ MP20075 LDO is designed for active bus termination of double data rate (DDR) 2/3/3L/4 synchronous dynamic random access memory (SDRAM) (Figure 3). Housed in an 8-pin MSOP package, this LDO sinks and sources up to 3 A over a user-programmable voltage range of 1.05 – 3.6 V and features accurate VREF/2 tracking for precise termination .
Figure 3: The MP20075 LDO can source or sink up to 3 A and is optimized for the termination needs of various DDR SRAMs. (Image credit: Monolithic Power Systems)
The MP20075’s integrated voltage divider tracks the reference voltage (REF), ensuring accurate VTT and VTTREF output voltages, while Kelvin sensing helps achieve ±30 mV for VTT and ±18 mV for VTTREF. Also, like most LDOs, the purely analog closed-loop topology responds very quickly to output load transients, in the order of microseconds (Figure 4). This transient response is often critical in high-speed circuits, such as the design goal of this LDO: DDR SRAM termination.
Figure 4: The analog closed-loop design of the LDO helps it respond very quickly to load transient demands; this performance is necessary for applications such as DDR SRAM termination. (Image credit: Monolithic Power Systems)
Although LDOs are inherently low-noise and easy to use, they do have their limitations. First, it is far less efficient than a switching regulator, which presents two glaring problems: its self-heating increases the thermal load on the system, and the reduced efficiency affects the runtime of battery-powered portable devices. For this reason, LDOs are often used in applications with output currents up to about 1 A – 3 A (as shown by the MP20075), as the adverse effects of their efficiency tend to be more severe beyond this current range.
LDOs also have an inherent limitation: LDOs can only provide step-down regulation and cannot boost unregulated input DC power above its rated value. If a boost mode output is required, the LDO as a DC/DC regulator is automatically ignored.
Fine-tune the layout and add some filtering functions
When using a switching regulator, whether operating in boost or buck mode, the switching action is an inherent, unavoidable source of noise. When the regulator is operating at a fixed frequency, it is easier to add output filtering. Let’s take a look at the MP2145 device, a 5.5 V, 6 A synchronous buck switching regulator in a 12-lead, 2×3 mm QFN package with full 20 mΩ and 12 mΩ MOSFETs (Figure 5).
Figure 5: The MP2145 is a 5.5 V, 6 A synchronous buck switching regulator with complete 20 mΩ and 12 mΩ MOSFETs in its 2×3 mm QFN package. (Image credit: Monolithic Power Systems)
A synchronous buck converter like the MP2145 consists of an input capacitor CIN, two switches (S1 and S2 with body diodes), a storage power Inductor (L) and output capacitors (COUT). An output capacitor (COUT) is placed at the output to smooth the output voltage in steady state. These devices form a first-stage filter and reduce output voltage ripple by providing a low-impedance path back to ground for high-frequency voltage components. Typically, such a parallel output capacitor can effectively reduce the output voltage ripple to 1 mV.
To further reduce output voltage ripple, a secondary output filter is required with an inductor-capacitor (LC) filter in series with the primary output capacitor (Figure 6). The filter inductor (Lf) is resistive in the expected high frequency range, dissipating noise energy as heat. This inductor is combined with the added parallel capacitor to form a low-pass LC filter network.
Figure 6: Adding a two-stage LC filter to the output of a switching regulator such as the MP2145 reduces output ripple. (Image credit: Monolithic Power Systems)
The supplier’s data sheet and application note provide formulas and guidelines for sizing the filter’s inductive, capacitive, and damping resistive components. They also specified key secondary parameters such as maximum inductor DC resistance (DCR) and saturation current, and maximum capacitor equivalent series resistance (ESR). Typical inductance values are 0.22 μH to 1 μH.
The placement of these components is also critical to achieve the highest possible performance. Poor layout consideration can lead to poor line or load regulation, increased ripple, and other stability issues. The input capacitor (Cin) of the MP2145 should be placed as close as possible to the IC pins (Figure 7).
Figure 7: The MP2145’s input capacitors (Cin here, see bottom right; C1 in the schematic in Figure 5) should be placed as close as possible to pin 8 (power input) and pins 10/11/12 (Power GND pin). (Image credit: Monolithic Power Systems)
Modules guarantee performance
Modules enable the implementation of DC/DC regulators to a higher level of system integration. In this way, these modules minimize or eliminate concerns related to the selection and placement of external components and provide guaranteed specifications. Modules add components, mainly traditional external inductors that are somewhat troublesome. As a result, modules reduce the challenges associated with size, location and orientation of passive components, all of which affect EMC and ripple-related performance.
For example, the MPM3833C is a step-down module with built-in power MOSFETs and an inductor capable of delivering up to 3 A continuous output current from 2.75 V to 6 V input voltage with excellent load and line regulation (Figure 8) . The design is complete with only feedback resistors, input capacitors, and output capacitors. Inductors are often the most difficult external components to specify and place, but they are internal to a module, so consideration of proper placement in order to minimize electromagnetic interference (EMI) and ripple is no longer an issue. problem.
Figure 8: The MPM3833C DC/DC module includes potentially troublesome inductors in its design and performance specifications. (Image credit: Monolithic Power Systems)
The module is available in an ultra-small QFN-18 (2.5 mm × 3.5 mm × 1.6 × mm) package with a ripple voltage of 5 mV (typ). Its low-level radiated emissions (EMI) are EN55022 Class B compliant, as shown in Figure 9, with VIN= 5 V, VOUT= 1.2 V, IOUT=3 A, CO=22 pF, and a temperature of 25°C.
Figure 9: The datasheet for the MPM3833C DC/DC module shows that the device easily meets the EN55022 Class B radiated emissions requirements. (Image credit: Monolithic Power Systems)
With modern micropackaging technology, the overall size of the module is only slightly larger or higher than the internal chip; low profile is becoming an increasingly important parameter. Consider the MPM3650 device, a fully integrated, 1.2MHz, synchronous rectified step-down power module with built-in inductors (Figure 10). The device delivers up to 6 A continuous output current for 0.6 V to 1.8 V outputs and up to 5 A for outputs above 1.8 V with excellent load and line regulation over a wide input range of 2.75 V to 17 V . With its internal MOSFETS and embedded inductors, the QFN-24 package measures only 4 mm × 6 mm × 1.6 mm.
Figure 10: The MPM3650 module features an integrated inductor and can output up to 6 A at voltages up to 1.8 V and 5 A at above 1.8 V in a 4 mm × 6 mm × 1.6 mm package. (Image credit: Monolithic Power Systems)
Another advantage of the modular approach is that the ripple noise is well controlled, around 20 mV at no load and around 5 mV at 6 A at full load (Figure 11). This means that in many cases there is no need to add external filters, which simplifies the design, reduces the footprint and reduces the bill of materials (BOM).
Figure 11: The specified ripple noise for the MPM3650 module is approximately 20 mV at zero load and approximately 5 mV at full load. (Image credit: Monolithic Power Systems)
It is often useful to do some hands-on work with a DC/DC regulator module to evaluate whether its static and dynamic performance meets the system requirements or even exceeds the data sheet requirements. To speed up the process, Monolithic Power Systems offers the EVM3650-QW-00A device, a 63.5 mm × 63.5 mm × 1.6 mm four-layer MPM3650 evaluation board (Figure 12).
Figure 12: Using the EVM3650-QW-00A evaluation board, intended users of the MPM3650 DC/DC module can quickly evaluate its performance in their application. (Image credit: Monolithic Power Systems)
Evaluation boards and their datasheets serve a variety of purposes. First, users can easily evaluate the MPS3650’s many performance properties over a wide range of operating conditions, some of which are not obvious or easy to find in the datasheet. Second, the evaluation board’s datasheet includes a complete schematic, BOM, and detailed board layout, so users of the MPS3650 can use these devices in their designs to reduce risk and minimize uncertainty (Fig. 13).
Figure 13: The EVM3650-QW-00A evaluation board package includes a complete schematic, BOM and detailed board layout to reduce risk and uncertainty. (Image credit: Monolithic Power Systems)
Evaluation boards provide designers with an opportunity to gain a better understanding of module performance, increasing designer confidence and reducing time to market.
There is another type of noise
When designers talk about “noise,” they almost always refer to some manifestation of Electronic noise in a circuit, such as ripple or EMI. With switching regulators, however, there is another type of potential noise: acoustic noise. For voltage regulators whose noise exceeds the range of human hearing, the noise is not a problem, and this frequency is generally considered to be 20 kHz. However, some switching regulators do operate in the audio range, while others operating at higher frequencies drop into the audio range during no-load or standby to minimize power consumption.
This audible noise is caused by one or both of two well-known physical phenomena: the piezoelectric effect and the magnetostrictive effect. In the case of the piezoelectric effect, the clock-driven electrical oscillation of the circuit causes components such as ceramic capacitors to vibrate in sync with the switching clock due to the conversion of electrical energy into mechanical motion through the crystalline material of the capacitor. In the case of the magnetostrictive effect, which is somewhat the same as the piezoelectric effect, magnetic materials such as inductors or transformer cores change their shape and size during clock-driven magnetization cycles. The affected capacitor or inductor/transformer then acts as a mechanical “driver” that causes the entire board to resonate, amplifying and transmitting audible vibration noise.
Due to one or both of the above effects, people with good hearing often complain of a constant low-volume humming sound when they are in close proximity to electronic devices. Note that this noise is sometimes also produced by components of low frequency power circuits at 50/60 Hz, so even people without good high frequency hearing may hear humming.
Dealing with acoustic noise requires different methods and techniques than electronic noise attenuation.
LDOs provide a noise-free or low-noise solution to DC rail ripple and EMI issues, but are generally not a viable regulator option at currents over a few amps. A switching regulator with proper filtering or one designed for low noise performance is an alternative.
A complete DC/DC regulator module integrating components such as an inductor in its tiny package is another set of solutions. This regulator reduces design uncertainty in layout and component selection while providing well-tested and quantified subsystem performance.
(Source: China Power Grid, Author: Bill Schweber)