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Use Integrated Step-Down DC/DC Modules for High Density, Efficient Power Conversion with Low EMI

By Jeff Shepard

Contributed By Digi-Key's North American Editors

As the level of integration and proliferation of electronic devices increases, designers are under constant pressure to improve efficiency while lowering cost, size, and electromagnetic interference (EMI). While power supplies have improved in power density and efficiency, designers now are also faced with the challenge of developing multi-rail power solutions for heterogeneous processing architectures that might include a mix of ASICs, DSPs, FPGAs and microcontrollers.

Step-down DC/DC converters are traditionally used to power such architectures, but with a growing number of power rails, the use of traditional discrete step-down DC/DC converters with a control IC and internal or external power MOSFETs — plus external inductors and capacitors — can be complex and time-consuming. Instead, designers can use self-contained step-down DC/DC converter modules with multiple rails and programmable sequencing that better control EMI, have less generated heat, and smaller footprints.

This article will review the power system needs of embedded designs and discuss various approaches and what designers need to consider, before introducing the concept of self-contained step-down DC/DC modules. It will then use a sample device from Monolithic Power Systems to briefly review the design and layout considerations designers need to keep in mind in order to maximize the performance benefits of these modules.

Why embedded systems need many power rails

Embedded designs such as 5G base stations are intended to support ever-increasing data volume requirements from smartphones and smart connected devices in applications such as home and industrial automation, autonomous vehicles, healthcare, and smart wearables. Such base stations typically use a 48 volt input supply that is stepped down by DC/DC converters to 24 volts or 12 volts, then further stepped down to the many sub-rails ranging from 3.3 volts to less than 1 volt to power ASICs, FPGAs, DSPs and other devices in the baseband processing stages. Often, the power rails need sequencing for start-up and shutdown, further adding to power system complexities for designers.

In the example of 5G base stations, the traditional CPU by itself can no longer meet the processing requirements. However, there are advantages to using an accelerator card with an FPGA for system reconfigurability, flexibility, short development cycle, highly parallel computing, and low latency. But the space available for the FPGA power supply is shrinking, and the power-rail performance requirements are complicated (Figure 1):

  • Output voltage offset: The output voltage deviation of the voltage rail must be less than ±3%, and sufficient margin should be left in the design. By optimizing the control loop to increase the bandwidth and ensure its stability, the decoupling capacitor should be applied and designed carefully.
  • Monotonic start: The start of all voltage rails must rise monotonically, and the design should prevent the output voltage from returning to its starting value.
  • Output voltage ripple: In steady state operation, the output voltage ripple of all voltage rails (except the analog voltage rail) must be at most 10 millivolts (mV).
  • Timing: FPGAs must meet specific timing requirements during start-up and shutdown.

Graph of size of the processor on accelerator cardsFigure 1: Due to increasing computing requirements, the size of the processor on accelerator cards has grown, leaving little room for the power supply. (Image source: Monolithic Power Systems)

Processors require more current and power as the data processing bandwidth requirements become more demanding. The calculation density and floating-point speed requirements for accelerator cards is also becoming more difficult for the industry to meet. The accelerator card slot is usually PCIe standardized, so the size of the board is fixed. Due to increasing computing requirements, the size of the processor has grown, leaving little room for the power supply.

Power system design alternatives

The use of traditional discrete step-down DC/DC converters with a control IC and internal or external power MOSFETs, plus external inductors and capacitors, is one approach to embedded system powering. As discussed above, it is a complex and time-consuming process for designers when multi-rail power solutions are needed. In addition to considerations of efficiency maximization and solution size minimization, designers must be careful with filter component layout and placement to minimize conducted and radiated EMI caused by switching currents in the converter and inductor circuits (Figure 2).

Diagram of discrete step-down DC/DC convertersFigure 2: Discrete step-down DC/DC converters have multiple EMI sources that designers have to manage. (Image source: Monolithic Power Systems)

DC/DC converters typically generate conducted EMI via magnetic fields from the current loop path formed between the output power MOSFET switching node to ground, and the input capacitor to ground. They also generate radiated electric field EMI from the MOSFET switching node to the inductor connection, which has a high dV/dt since it is switching from the high input voltage level to ground continuously, and from the electromagnetic fields generated within the inductor. Failure to get the design right often results in time-consuming EMI lab retests and multiple design iterations.

A four-rail solution for powering an ASIC or FPGA using discrete step-down DC/DC converters can occupy 1220 square millimeters (mm2) (Figure 3). That can be reduced to about 350 mm2 using a power management IC (PMIC)-based solution. As an alternative, designers can use a self-contained quad-output DC/DC converter module to reduce the solution size to only 121 mm2, while also simplifying the design process and speeding time-to-market. Advances in semiconductor process technology and package construction mean that the latest generations of DC/DC modules achieve very high power density, high efficiency, and good EMI performance in a small form factor.

Diagram of integrated DC/DC module solution (click to enlarge)Figure 3: Use of an integrated DC/DC module solution can save up to 90% of the board space compared with a discrete solution. (Image source: Monolithic Power Systems)

New construction techniques, such as in-package flip-chip and “mesh-connect” lead frame technology mean that the IC, inductor, and passives can be mounted directly onto the lead frame without wire bonding or an additional internal pc board (Figure 4). Compared to older construction styles that use an internal pc board substrate or wire bonding, connection trace lengths can be minimized, and direct connection to passive components keeps the inductance low to minimize EMI.

Diagram of construction utilizing the lead frame for interconnectionsFigure 4: A new form of construction utilizing the lead frame for interconnections has a number of advantages: EMI is better controlled, heat dissipation is improved, and footprint size is reduced. (Image source: Monolithic Power Systems)

The use of a land grid array (LGA) package format that surface mounts directly to the target pc board offers a lower EMI profile than alternative single-in-line (SIL) or SIL package (SIP) style converters with leads that can radiate EMI.

Quad-output programmable integrated DC/DC modules

To meet the multi-rail, high power density needs of embedded systems, designers can turn to the MPM54304 from Monolithic Power Systems (Figure 5). The MPM54304 is a complete power management module that integrates four high-efficiency, step-down DC/DC converters, inductors, and a flexible logic interface. Operating over an input voltage range of 4 volts to 16 volts, the MPM54304 can support an output voltage range of 0.55 volts to 7 volts. The four output rails can support currents of up to 3 amperes (A), 3 A, 2 A and 2 A. The two 3 A rails and two 2 A rails can be paralleled to provide 6 A and 4 A, respectively. Designers should note that the maximum output current in parallel mode is also limited by the total power dissipation. This provides the flexibility to generate several output configurations (subject to the total power dissipation limitations):

  • 3 A, 3 A, 2 A, 2 A
  • 3 a, 3 A, 4 A
  • 6 A, 2 A, 2 A
  • 6 A, 4 A

Diagram of Monolithic Power Systems MPM54304 complete step-down power management moduleFigure 5: The MPM54304 is a complete 4 volt to 16 volt input quad-output step-down power management module. (Image source: Monolithic Power Systems)

The MPM54304 also provides internal sequencing for start-up and shutdown. The rail configurations and sequencing can be pre-programmed by the multiple-time programmable (MTP) e-fuse or through the I2C bus.

This fixed-frequency constant-on-time (COT) control DC/DC converter provides fast transient response. Its default 1.5 megahertz (MHz) switching frequency greatly reduces external capacitor size. The switching clock is locked and phase-shifted from buck 1 to buck 4 during continuous current mode (CCM) operation. The output voltage is adjustable through the I2C bus or preset by the MTP e-fuse.

Full protection features include undervoltage lock out (UVLO), over current protection (OCP), and thermal shutdown. The MPM54304 requires a minimal number of external components and is available in a space-saving LGA (7 mm x 7 mm x 2 mm) package (Figure 6). The low profile of the LGA makes it suitable for back-of-board placement or under a heatsink.

Image of Monolithic Power Systems MPM54304 power management moduleFigure 6: The MPM54304’s LGA package provides a compact and low-profile solution with low EMI (Image source: Monolithic Power Systems)

Design and layout considerations

The MPM54304 has a simple pinout along the edge, making layout and pc board design easier. Needing only five external components the total solution is small and compact. The LGA package allows a solid ground plane to cover most of the area beneath the module, which helps close eddy-current loops and further reduce EMI.

This step-down converter has a discontinuous input current and requires a capacitor to supply AC current to the converter while maintaining the DC input voltage. Designers should use low-equivalent series resistance (ESR) capacitors for the best performance. Ceramic capacitors with X5R or X7R dielectrics are recommended because of their low ESR and small temperature coefficients. For most applications, 22 microfarad (µF) capacitors are sufficient.

Efficient pc board layout is critical for stable operation of the MPM54304. A four-layer pc board is recommended to achieve better thermal performance (Figure 7). For the best results designers should follow these guidelines:

  • Keep the power loop as small as possible
  • Use a large ground plane to connect directly to PGND. If the bottom layer is a ground plane, add vias near PGND.
  • Ensure the high-current paths at GND and VIN have short, direct, and wide traces
  • Place the ceramic input capacitor as close to the device as possible
  • Keep the input capacitor and IN as short and wide as possible
  • Place the VCC capacitor as close to the VCC and GND pins as possible
  • Connect VIN, VOUT, and GND to a large copper area to improve thermal performance and long-term reliability
  • Separate the input GND area from other GND areas on the top layer and connect them together on the internal layers and bottom layer through multiple vias
  • Ensure there is an integrated GND area on the internal layer or bottom layer
  • Use multiple vias to connect the power planes to internal layers

Diagram of four-layer pc board layoutFigure 7: A four-layer pc board layout is recommended when using the MPM54304 quad-output power module. (Image source: Monolithic Power Systems)

Conclusion

As processing architectures evolve to address highly demanding data applications, designers are faced with the challenge of developing multi-rail power solutions that can support increased processing power and electronics in form factors that are either static or shrinking. Step-down DC/DC converters are critical components in designing power solutions for these systems but can be complex to implement.

As shown, designers can turn to self-contained step-down DC/DC converter modules with multiple power rails and programmable sequencing, simplifying the design process and speeding the time-to-market. Also, the new construction techniques that enable these self-contained modules have a number of performance advantages: EMI is better controlled, heat dissipation is improved, and footprint size is reduced.

Recommended reading

  1. Use Programmable Power Modules to Accelerate DC/DC Regulator Design

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Jeff Shepard

Jeff has been writing about power electronics, electronic components, and other technology topics for over 30 years. He started writing about power electronics as a Senior Editor at EETimes. He subsequently founded Powertechniques, a power electronics design magazine, and later founded Darnell Group, a global power electronics research and publishing firm. Among its activities, Darnell Group published PowerPulse.net, which provided daily news for the global power electronics engineering community. He is the author of a switch-mode power supply text book, titled “Power Supplies,” published by the Reston division of Prentice Hall.

Jeff also co-founded Jeta Power Systems, a maker of high-wattage switching power supplies, which was acquired by Computer Products. Jeff is also an inventor, having his name is on 17 U.S. patents in the fields of thermal energy harvesting and optical metamaterials and is an industry source and frequent speaker on global trends in power electronics. He has a Masters Degree in Quantitative Methods and Mathematics from the University of California.

About this publisher

Digi-Key's North American Editors