Design Feature: May 9, 1996

IC vendors are increasingly providing system design engineers with much more than just packaged silicon or GaAs. They're supplementing their product offerings with complete details on a final design—schematic, layout, and bill of materials—plus data on final performance. In many cases, they are emphasizing the availability of this design package as much as the attributes of the IC itself.

In doing this, IC vendors become not just vendors, but designers of systems and subsystems, which solve increasingly large portions of the overall design challenge. Ironically, they are migrating to what they originally didn't want to be—vendors of anything more than ICs supported by printed data sheets. They now have application staffs, pc-board designers, magnetics specialists, and system-evaluation engineers to provide the reference design. Vendors do this because it helps them obtain design-ins—and because OEM designers demand this help.

For design engineers, these reference designs are mostly welcome news, even as they change the role of the designer and design team. They reduce risk and time to market and let you concentrate on those critical design areas in which, you hope, you and your company do your best and most competitive work (see box, "Reference designs have come a long way"). Using these designs means that designing and understanding each subsystem's circuitry takes less of your time, and system integration and production takes more.

Reference designs have come a long way

The idea behind a reference design is not new. IC vendors for many years have provided suggested circuit designs in their data sheets and application notes. Complete reference designs were relatively uncommon. However, Dave Fullagar, a vice president at Maxim, recalls that, when he was at Intersil (which RCA and GE and then Harris subsequently absorbed) in 1977, Intersil provided one of industry's first such designs. That design—for the ICL7106 LCD panel-meter IC—was so successful that customer orders reached 20,000 units for complete boards. In time, the suggestions and guidelines in application notes evolved into more complex demonstration/evaluation boards, which vendors provided so that designers could exercise the IC. These boards generally contained more than just the fewest required components to use the device; they also included switches, connectors, test points, and even prototyping areas, so that the designer could put the IC through its paces and check out its performance. They're still available and useful, especially for components such as A/D converters and other mixed-signal devices. They function as "teaching designs" rather than final designs for most user applications. Today's reference design, in contrast, comes with more support details—yet often contains fewer components—than a demo board, as a "design-ready," triple-output dc/dc converter from Linear Technology shows (figure). Such designs embody a complete, ready-to-use circuit, which you can use as a drop-in subsystem, rather than an exploratory vehicle, in the final design. It comes with detailed documentation on how it works; component values; sources and model numbers for all components in addition to the vendor's ICs; and an appropriate board layout, which comes as a photoplot or as an industry-standard CAD file. In short, it's like getting a ready-to-build kit, except that you buy the parts yourself using the bill of materials.

Despite their advantages, however, reference designs that are poorly chosen or improperly applied can put your product design in crisis and aggravate relations between you and your key vendors. You need to look at these offered designs carefully and understand the risks involved when you take on someone else's design to meet your needs.

The place to start

Although reference designs are available for wireless communication systems, PC multimedia boards, and even complete PCs, they are most common for power-supply and dc/dc-converter designs. This situation makes sense for several reasons. Every system has one or more power-supply subsections, even if they also have a larger, line-driven supply; battery-powered systems have additional power-supply constraints, such as efficiency, dropout, and even the need for supply rails that are both above and below the battery-voltage span. Also, most designers want to spend as little time as possible on the power supply, because they view it as a necessary evil and, usually, not as key when comparing their products to competitors' products. Finally, many designers lack expertise in power-supply topologies, such as buck, boost, hysteretic mode, and component selection and parameters, such as inductors, magnetics, capacitors, inrush currents, and equivalent series resistance (ESR). These designers may also lack both the time and the inclination to obtain such expertise.

Because it is designed for most applications, a reference design may not be the best solution to the unique needs of your application. More important, a reference design can optimally meet the needs of one or a few parameters of your design, but it may not with respect to all of them simultaneously. Trade-offs and compromises exist among various factors such as efficiency, cost, size, availability of components, or reliability. The reference design may be overdesigned in some ways, but it's unnecessary to optimize everything when your primary objective is to get a reasonably decent power-supply subsection designed in, so that you can go on to other areas of your project.

Some IC vendors sell limited quantities of the completed assembly to OEMs that don't want to manufacture the provided reference design. If your needs are greater than the vendor's production capability, they may work with you and an independent third party to produce what you need.

Carefully check references

Although using a power-circuit reference design helps get your product to market more quickly than designing your own would, do your homework. Carefully define your system needs and any flexibility you have, so that you can compare the available designs vs your requirements and broaden the number of vendors to consider. Keep requirements firm; otherwise, you'll never get past the design and redesign iteration that using an external design aims to minimize.

Be especially descriptive in defining transient, dynamic, fault, and start-up conditions, because this area is where most problems occur. Inrush currents, unanticipated changes or failures at the load, and thermal problems may cause a normally acceptable reference design to work improperly in your application. Ensure that the vendor understands your situation and verifies the design under corresponding conditions (see box, "Power-circuit surprises abound").

Some vendors offer a wide array of reference designs, covering most application needs. Maxim, for example, offers several dozen designs with documentation conveniently compiled into a several-hundred-page book. National Semiconductor uses a different ap-proach: Rather than many designs, the company offers the Simple-Switcher design and software package, which lets you specify the design input and output parameters and then chooses the appropriate IC and passive components (Figure 1).

Vendor-supplied documentation is critical, as both insurance and prudent design policy. This documentation should go beyond the schematic, layout, bill of materials, and performance data. Although you are using a tested and verified design, problems may later arise. For example, look for explanatory notes that show why the vendor selected a passive component, such as an inductor or a capacitor. Was the primary factor its size, ESR, dc resistance, or, simply, a relationship with the component's vendor? You need to know this for the inevitable substitution problem that occurs when a part is no longer available or when the vendor knowingly or inadvertently changes some specifications. Documentation that discusses the how and why of the design helps you understand its strengths and weaknesses.

Reference designs can also give you a false sense of security. Although you can use a reference design as a black-box, drop-in circuit, attempt to understand the basic principles of operation. Some reference-design suppliers provide their underlying design equations. Insist on these equations, so that you can understand both the operation and your options if your needs change, such as when you need 300-mA output instead of 250 mA. Design flexibility and circuit "hooks" may be mandatory if you expect your needs to change or if it's impractical to define them sufficiently early in your product-design cycle.

Minimize misery

Remember that the reference-design expertise resides with the IC vendor's application specialist, who designed, built, and tested the circuit. This person can leave, taking in-depth knowledge that you may need. Although key personnel departures also happen when you design the circuit in-house, at least you become immediately aware of it and can most likely work with that person to assemble a final documentation package together before he or she leaves. With an outside designer, you don't discover that the person has left until you call with a question, which may be months after the specialist is gone.

Credibility and confidence in the vendor are critical when you're using a reference design. Ask how, under what conditions, and how long the vendor tested the design. Look at test results, photos, and analysis, so that you're satisfied with the design's quality. Also, ask how many units the vendor built. It's one thing to get a single unit to work and meet specifications, but, as all experienced engineers know, it's difficult to get hundreds of production units to work when component tolerances, operating conditions, and other variations are factors. Also, look at what tests the vendor has done to verify satisfactory performance during those difficult start-up, marginal input, and other non-steady-state conditions.

Using a reference design intensifies the vendor/OEM relationship—for better or for worse. As an OEM, you count on more than just a well-designed and tested IC from the vendor. Problems in application, such as minor changes in layout or component placement, can lead to finger-pointing unless both sides understand what changes have occurred. Be alert for known component characteristics on which you rely; unknown component characteristics on which you rely; and manufacturability and tolerancing in a production environment.

As an added benefit, a well-designed and thoroughly tested reference design, even if you modify it, can make your task easier. You can substitute the reference design in your system, using it as a drop-in "known-good circuit" to see if the problem is with the rest of your circuit, your version of the reference design, or the reference design and IC as the vendor provides them. Using this information, the vendor can better help you with problems. Some vendors insist on this type of test using their reference designs before they will help you with your own design.

Table 1 shows some representative reference designs for power-supply circuits. Most vendors provide them free, some have a nominal charge, and some waive the cost for those OEMs buying ICs in large quantities.

The other end of the spectrum

Vendors of power-supply circuits are not alone in providing reference designs. Suppliers of communications ICs and chip sets are proving complete, or nearly complete, system designs that you can use as-is or adapt, such as the one for Maxim's quadrature digitizer (Figure 2). These circuits, although in some ways more complex than power circuits, are simpler in others. Certainly, power-supply circuits do not require µPs to operate, and they need not meet International Telecommunication Union or other interoperability standards, although they often must meet EMI/RFI standards. However, the simple appearance of power circuits often hides their operating and topology subtleties, and the circuits depend more on the characteristics of their passive components, layout, and input- and output-waveform parameters.

Communications reference designs offer you both opportunity and limitations. They allow an OEM to enter a market quickly, minimize the need for complex standard-compliance testing and certification, and provide even small start-up companies a platform on which to build, especially for embedded applications (Table 2). Some reference designs have licensing fees, so be sure to ask about them, as well as the legal situation and responsibilities if you modify the design.

Using a reference design lets you put more effort into product definition, system integration, and manufacturing issues and less into designing the hardware and software of each functional block. Instead, you can concentrate your design activities on those sections in which you add the greatest value and use reference designs for the other blocks. Also, using one of these designs may make it more difficult for you to determine the differentiating factors that make your product prominent vs similar products from competing sources.

You can reach Technical Editor Bill Schweber at (617) 558-4484; fax (617) 558-4470; email: bill.schweber@cahners.com

Acknowledgments

Thanks to John Pellegrino of AEG Schneider Automation, Brian Matthews of Harris Semiconductor Corp, Bob Dobkin and Ed Ritter of Linear Technology Corp, Dave Fullagar of Maxim Integrated Products, Daryl Sugasawara of Micro Linear Corp, Al Kelch and John Prendergast of National Semiconductor Corp, Gordon Cook of Siliconix (Temic), and Barry Sandefur of Vitesse Semiconductor Corp for their insights and comments.