EDN hands-on project: Online design tools: 7 tips for safe passage

At EDN, we have been listening to design engineers talk about the nature of their work, how it's been changing, and the pressures driving those changes. We meet at conferences and trade shows, listen to comments during focus groups, touch base at meetings of professional organizations, and read research reports that summarize the experience of more than 1000 practitioners (Reference 1). The clear and consistent impression from all quarters is of a discipline under increasing pressure to develop more products in less time with smaller staffs than just a few years ago. Engineers aren't necessarily working on more projects during a given interval. But the substantial thinning of the engineering fleet during the past few years leaves many OEM designers sailing out of the familiar waters of their specialties and looking for methods and tools that let them work faster and extend their technical reach.

Meanwhile, the longer term trend in mixed-signal-IC technology has led to ever-higher levels of integration and a greater entanglement between IC and system designs through ASSPs (application-specific standard products). Some highly integrated mixed-signal devices offer simple interfaces that exhibit few parametric interactions with off-chip circuitry. Other ASSPs, however, depend on surrounding components to determine parametric behaviors. Neither of these device types yields often to characterization by a few simple equations: You can't regard most complex ICs using simple linear expressions as if they were op amps with tricked-up transfer functions. For the same reason, it's unlikely that your Spice engine or other design automata with which you work will readily support many of these devices.

With OEM designers needing to do more with less, semiconductor vendors producing ever more complex chips, and key function-block ICs fitting less well into OEM designers' design-automation suites, customary support documentation, such as data sheets, selector guides, and application notes, no longer suffice.

The same factors add significantly to the work the IC makers' traditional second lines of customer support—their application-engineering staffs—face. Just as the data sheets, application notes, and other paper-based customer support information have moved to the Web, the natural evolution has been toward Web-based support tools to help customers with, at minimum, the most commonly experienced problems and most frequently asked questions.

The type and scope of online support tools vary from vendor to vendor and, within a given vendor's site, from topic to topic. For most IC manufacturers, the Web-based tools are evolving rapidly, but only a few vendors have developed particularly sophisticated notions of what the medium can do. With finite development resources, a tug of war always takes place over which subjects to address, which product lines to support, and how best to cover the most territory. But looking across many vendors and many product types, a range of perspectives also exists on what types of tools best serve customer needs: Sometimes a simple, narrowly focused online calculator can more quickly provide the information you need than can a glitzier looking multipurpose tool that takes time to set up.

IC vendors are not serving a homogeneous clientele. Visitors range from relatively inexperienced engineers to seasoned experts. They vary from designers well-steeped in a given topic seeking specific information to survivors of the great downturn who are trying to come up to speed at best—and keep from drowning at worst—in technologies and application areas with which they have little or no experience.

Most published descriptions of Web-based design tools are glowing, and many of these tools deserve the pearly adjectives. Vendors put significant effort into building accessible, successful design aids. Some contract with commercial software authors; others write their own. In most cases, there is a mix: The vendor's staff develops the simpler calculators and database-query front ends, and the contractors develop or adapt the more complex and general analysis software. The results are mostly helpful; some are excellent.

That said, important gaps apply to many of these programs and the portals through which designers gain access. Having cranked through some simple design tasks on several vendors' sites, I'd like to offer seven suggestions on how you can make the most of your online-design time.

Most online-design tools reside behind a registration wall. Don't wait until you need an answer before registering on your vendor's site. Familiarize yourself with how the vendor arranges the design tools. Every vendor will point out how easily navigable its site is. However, they fail to point out that no two vendors use the same arrangement. Their staffs may have to concern themselves only with their sites. But you need to be able to move around and switch between vendors' sites, adapting to each in a reasonable amount of time. That task requires familiarity that comes from more than a five-minute tour.

The same notion holds for the individual tools. Although "intuitive interfaces" abound, you'll find that some align themselves well with your intuition and that others seem unfathomable at first exposure. Take a look at a number of tools and familiarize yourself with the way your key vendors approach their construction.

Although most of my experiences were reasonably trouble-free, a few incidences reminded me that, if I had been in a hurry and new to a site, I might have been in for some difficulty. One vendor's registration process determined my password for me and sent it to me by e-mail. I copied and pasted the utterly nonmnemonic character string into the login form on my next visit, only to find out that it was invalid. An e-mail to the vendor's support group yielded an automated reply informing me that my message had been received and that a reply would be forthcoming within 48 hours. As it happened, the problem was resolved the next morning—perhaps owing to the good fortune of light traffic in August—but if I had planned on getting a significant amount of work done that day, I would have set myself up for a disappointment.

It's easy to spend a lot of time on a Web site, find several useful tools tucked away, and then forget where you saw the one you need weeks or months later when you need it. Similarly, you can collect a stack of bookmarks with marginally mnemonic names and then waste time later trying to sort through them to find the one you need. Just as there is no one right way to organize a CD collection or books on shelves, there is no single best organizing principle to manage the information you find during your periodic Web-site visits.

I ended up using a combination of browser bookmarks and a small spreadsheet with headings for the vendor, the tool name, brief descriptions of the tool type and area of coverage, the URL, and a check column if I bookmarked the page in my browser. In an effort to minimize bookmark clutter, I didn't bookmark every item I found worth logging in the spreadsheet, instead limiting those bookmarks to tools I thought I would want to use somewhat regularly. I still haven't decided whether I prefer all the bookmarks in one "design-tools" folder or broken out by vendor or by topic. The organizational principle to which you subscribe may be less important in the end than the fact that you've chosen one.

Keep in mind that organizing your design-tool links doesn't prevent the vendor from later torpedoing you. There's no guarantee that your links to your favorite tools will work when you need them if the vendor's site has undergone a redesign in the interim. This is particularly true when a redesign introduces "Web-delivery technologies." Often, a site design leaves old links intact. But if a redesign breaks your favorite link, you many need to allow extra time to find its new URL, locate its replacement, and fire off that e-mail of appreciation to the vendor.

OK, so you've dealt with the registration walls and learned your way around. Now, the fun begins. If you're planning to sit down with online tools and get instant results, you may be disappointed. But in the spirit of giving credit where credit is due, your expectation, not the software, may be at fault. Recognize that you often need to spend as much time learning the software as you would a commercial program or a piece of test hardware. Many relatively simple tools feature shallow learning curves, but several of the more ambitious programs take time to master.

Unlike commercial products, vendors' Web-based software rarely provides more than simple instructions and little or nothing to guide you when you run into errors. One area in which online tools consistently fail is in the level of documentation and runtime support.

For example, one high-speed-backplane design tool I tried provided a simple set of drop-down menus that allow you to select from various parameters, such as the signaling technology, termination impedance and voltage, driver and receiver chips, clock rate, connector characteristics, the number of connectors on the bus, and stub lengths, most preloaded with default values. I selected a transceiver, reviewed its data sheet, and chose appropriate values—or so I thought—from the tool's menus. The program's response was an unhelpful pop-up screen that informed me that "A ERROR has occurred Code = 103" (sic). Spelling and grammar issues notwithstanding, the program offers no guidance on what error code 103 means other than to say "NOTE errors maybe caused by inappropriate parameter values" (sic). This message translates to: "Two or more of the choices you have made are mutually incompatible, and, although we have determined that fact, we're not going to give you a hint about which one or combination caused the problem. You're on your own."

You can chew up a great deal of time trying to figure out what you have done to offend the software's sensibilities, and, until you do, you won't know whether your inputs are faulty or the software is broken. Indeed, such cryptic error messages indicate less about a user's faulty inputs than about how poorly some developers adhere to industry-standard programming practices. These practices include input error checking with appropriate feedback and error messages that require no special knowledge of the program's internal workings—concepts whose age you can measure in AFCs (appreciable fractions of a century).

Help is not a click away due to a dearth of support documentation. It isn't even an IT call away. Help is at the far end of an e-mail to the vendor's support organization, the response time to which you can count in hours at best and usually a couple of days at worst—certainly not minutes. Schedule your work accordingly.

Many vendors have contracted outside developers to create some of their online-design tools. When I started looking at the tools available for analog- and mixed-signal-design support, I assumed that the third-party tools would be better crafted and documented than the programs that IC makers' staffs write. Overall, this situation was not what I experienced. The software developed outside the IC manufacturers' organizations tended to be more sophisticated in scope and to feature more elegant user interfaces. But, overall, they were neither better documented, despite the increased need, nor easier to learn. With few exceptions, their more finished appearances were for the most part just that: appearances.

Several third-party tools do provide helpful features, such as symbolic circuit representations with clickable nodes, graphical data representations, and highly refined vendor-part-database navigation including hot links and on-screen sort capabilities. But with few notable exceptions, the third-party software as solver engines appeared on average no better than the support software. So, although you might be able to work with more complex circuit blocks using third- party tools, and access a vendor's component documentation directly from a third-party online tool, you are likely going to spend much more time working with and at software than you would with the more simple and narrowly focused programs written by the IC maker's staff.

Some of the online tools that felt best in my hand were those that appeared to have been developed with the least formal presentation methods. Perhaps outside the large EDA companies, people who can capture and codify a semiconductor vendor's expertise are more rare than those who can write slick GUIs. Because hardware engineers' concerns reside with the circuit issues and component-application problems, you may find that the tools that get you closer to a vendor's expertise serve you better than those that look best at first glance.

It would be a gross mischaracterization to claim that speed is what best distinguishes a Spice engine from a No. 2 pencil. Electronic tools codify and provide you access to an enormous body of knowledge. But for anybody who has ever been caught in a Spice loop, the lesson is clear: You've got to do your design homework before you open the toolbox. Tools have greatly changed the nature and scope of that homework since the dark days when the closest thing most designers had to a simulator was a cheat sheet of transforms and a pad of graph paper. The need remains constant for designers to understand their craft and continually hone their skills. Just as owning a power saw does not make you a furniture maker, tools can significantly aid—but not replace—a knowledgeable designer.

Many who market and publicize design tools claim that they enable you to work with parts and in technologies you don't well understand. I take issue with that claim and recommend that you do, too. It is true that, once you master the tools, they can help you extend the depth of your understanding and assist you in performing the diligence that a production-ready design requires. Again, I find appropriate the comparison with test-and-analysis hardware. However, as with the test hardware, the software tools neither do the design work for you, nor teach you how to work within a given technology, explain cryptic error messages, or explain where you went wrong in your design.

Ironically perhaps, some of the best IC-specific and applicationwide tutorial materials are the traditional paper-based materials, now also largely available online. White papers, application notes, data sheets, and published articles may still be your best source of part-specific design information. They can get you started with unfamiliar technologies and deepen your understanding of areas with which you have experience.

Several vendors arrange links to these materials in pages depicting design flows, which I found to be a great time saver. But IC manufacturers also report a typical 6-sec/pageview, causing one to wonder whether visitors are getting the most benefit from the most readily accessible—if least glitzy—parts of the Web sites.

As online tools move from helpful accessory to critical facility, your assessment of competing ICs may need to include an evaluation of their associated design automata if an IC is bound to support software. It's easy to underappreciate the significance of these bindings: Few of us are accustomed to thinking about analog functions as being associated with software at all. Yet, if you design with modern power ICs, just to name one example, you'll likely be unable to break the association between the IC you choose and its vendor's design aids.

So, you choose your IC and its support software in a single decision. That correspondence is not difficult to imagine. But the implications go one step deeper: IC vendors have come to understand these links and how they can affect an IC's market acceptance, which is hastening the evolution of online tools and the adoption of the foundation technologies on which they run. But most design tools run as server applications with a Web-client interface. You don't possess the software, so you don't get to choose whether or when you move to the next version. You're not just buying an IC and a design aid. When you choose an IC vendor, you choose an evolutionary track for the software you use.

If you design in a Spice environment, you'll discover that no bridges exist between vendor-supplied design software, including Spicelike simulators, and your in-house tool suite. This issue is potentially more problematic than it may appear at first glance. Using power ICs again as examples, when you go to the vendor's site, you might select, say, a power-converter-circuit topology and operating parameters. You may fill in the topology by selecting a converter and its surrounding components, such as power-MOSFET switches and filter components, from context-generated lists. Then, depending on the vendor's tool complement, you can simulate the results and assess whether the design meets your electronic needs. In some cases, you can run a thermal profile, as well. You don't, however, get to simulate the interactions between, in this example, the power subcircuit and your application. You may be able to specify a current sink or a resistive load, but if your application makes dynamic demands on the power-converter circuit, particularly at high frequency, it's unlikely you can study their interactions on the vendor's simulator. A notable exception is Linear Technology's LTspice/SwitcherCADIII software, which allows you to simulate mixed-signal ICs—switch-mode regulators—with application circuits that you define.

If you wish to simulate mixed-signal ICs in your own design environment along with your application subcircuits, you'll find yourself on the opposite side of the gulf. With rare exceptions, behavioral models are not available for highly integrated functions. When available at all, these models tend to be generic and may not characterize the performance issues most important to your application.

IC manufacturers acknowledge the gulf between the simulation environments. They candidly admit that the evolutionary course their online design tools are following will not span the gap in the foreseeable future. They recommend two things you can do to help bridge the two software environments.

First, consider ways of segmenting your design that minimize the complexity at the intersegment interface. Consider both the number of lines that cross the gulf and their sensitivity to events and conditions on opposite sides. Note the circuit parameters that affect the performance at the subcircuit interface. For example, your vendor's software environment may directly report an IC's nominal output voltage noise under simulated operating conditions. However, your evaluation of the subcircuit interface may also benefit from a calculation of the dynamic output impedance. Even if you can derive this information from simulations in the online environment, it is unlikely that the software will just hand you the number. You need to determine for yourself that this information is meaningful and then determine whether and how you can tease it out of the simulation environment.

Second, learn to write simple behavioral models that capture information you glean from the vendor's online software. This area is one in which many members of a design team can share and hone information about ICs your team uses often. The purpose of the model is to account for the behaviors and characteristics you can observe in the IC vendor's design environment and allow you to transport that information to your in-house design suite.

Models are fine, but confirmation is better. If you are designing with a mixed-signal IC that you've used before, accessing production-test data from an earlier design could offer some insights and allow you to improve both your understanding of the circuit behavior and the model you use to represent it. Your investment in your models can pay dividends each time you reach for the same or even a similar part for another design.

Most of the large analog and mixed-signal IC vendors provide tools. Most have staffs to develop tools that respond to the needs of the OEM-design community. A tool-by-tool software review holds little value unless the reviewer's needs and criteria closely match yours—essentially an impossibility with the size and range of interests that EDN's readership represents. So, instead of attempting the impossible, I note (in alphabetical order) a few of the sites that are developing online-software tools as integral parts of their overall product-line strategies.

Like many of the other semiconductor manufacturers active in providing design aids, Analog Devices offers a variety of support automata. Its "virtual-design center" is a good place to start to perform fast parametric-based component searches of amplifiers, DACs, and ADCs. You can also access online versions of the more traditional static-selector guides for the company's full product line, check cross references, and order samples—capabilities that most vendors' sites offer.

Analog is also an example of the several companies that provide application-specific signal-chain maps—block diagrams with hot links to information about appropriate components. In the Analog Devices case, you can access signal-chain maps for applications in the audio, communications and networking, computer, video and imaging, industrial, instrumentation, medical, and military segments. A map depicts function blocks that the company offers in IC form with clickable icons that typically link to a context-generated selection table. Signal-chain maps are a quick and easy way to locate an IC maker's most appropriate components for your application.

Analog Devices also has a number of design "wizards," "assistants," and interactive tutorials, many written as Java applets (Figure 1). Typical of most manufacturers, compatibility issues can arise with Web browsers other than Microsoft's Internet Explorer—a potential source of difficulty for those in the OEM community who use Unix, Linux, or a non-Microsoft browser within a Windows environment. Analog's wizards, assistants, and tutorials cover a range of interesting topics, such as antialias filtering, photodiode signal conditioning, harmonic images in data converters, and op-amp stability. The site also provides gain, common-mode range, and error-budget calculators for the company's family of instrumentation amplifiers and configuration aids for devices ranging from direct digital synthesizers to accelerometers.

It is not entirely clear whether the company organizes this information by application or by device type, so a bit of both is currently available. Despite the large number of tools available, you can scan through the list and fairly quickly locate the resources of greatest interest to you, issues of organization or presentation notwithstanding. It is also unclear which features or capabilities distinguish wizards, assistants, and interactive tutorials from each other. Although I expect dialectal differences, particularly in naming conventions, among vendors, extra nomenclature within one vendor is harder to justify, potentially confusing, and likely annoying.

The company offers development tools and code examples for its line of DSPs, Saber and Spice models, a frequently asked questions list, and extensive packaging information. Lastly, you can access the company's traditional paper-based information resources, including application guides and notes, manuals, and technical articles.

Fairchild Semiconductor has this year been focusing on the power sector and is supporting its growing line of power ICs and discretes with online software and information resources. Application-specific selectors cover automotive, communications, computing, consumer, industrial, and "ultraportable" segments and point to analog and mixed-signal ICs, interface and logic devices, and optoelectronics in addition to devices from the power-product line.

Fairchild's design automata include calculators that compute MOSFET power-switching losses. The FPS Designer software is a clientside offline switch-mode power-supply design tool you can download and run under Windows (Figure 2). The program's documentation is noteworthy for its coverage, including a walkthrough of the calculations that the transformer design requires.

Perhaps best known among Fairchild's online software is the FETBench simulation tool, a Transim product tied to Fairchild's component database. FETBench includes a simulated curve tracer that characterizes static and dynamic device behaviors. Alternatively, you can choose standard power-MOSFET application topologies, such as synchronous-rectified-buck and boost converters, or unidirectional and bidirectional load switch applications. FETBench also provides a thermal simulator, though the software's unwillingness to recognize devices from its own database thwarted my attempts to use it.

International Rectifier offers single-phase and multiphase synchronous-buck-converter tools based on the company's iP1001 and iP2001 converters, respectively (Figure 3). The online simulator offers steady-state, ac, step-input, and step-load outputs. IR offers thermal-profile information and application notes explaining how you can manage the thermal issues in your layout. You can download sample Gerber files in PDF for designs with one through four phases. An interactive power-MOSFET selector helps you locate the optimum control and synchronous-rectifier devices for a synchronous-buck topology.

Linear Technology's tool set is unusual in that it exclusively comprises clientside software that the company makes available for you to download. The software available from the company's Web site includes LTspice/SwitcherCADIII, which is a fully functional Spice III simulator with enhancements geared toward simulating switching regulators (Figure 4). Unique among the design tools I examined, LTspice/SwitcherCADIII includes macro models for 80% of the company's switching regulators. It also includes models for 200 of the company's op amps, various discrete MOSFETs and bipolar transistors, and passives.

The QuickEval System software includes drivers and control software for a family of evaluation boards that use a USB-control link. The system supports device-specific modules for a number of ADCs, DACs, LED drivers, and the company's new powered-Ethernet controllers (Reference 2).

The company also provides FilterCAD, which helps you implement lowpass, highpass, bandpass, and notch filters with your choice of response, including Butterworth, Bessel, Chebyshev, or elliptic. The program also allows you to input a response definition of your own devising.

National Semiconductor's Webench launched some years ago on the same Transim technology that drives Fairchild Semiconductor's FETBench software. Since then, Webench has evolved into a sophisticated tool suite, primarily for power and wireless applications. A set of 32 application-specific signal-chain maps include interactive block diagrams for consumer, industrial, medical, display, automotive, wireless, and broadband segments (Figure 5).

The scope of National's signal-chain maps is among the broadest in the industry. Within a map, national's selector front end allows you to sort components by nearly any spec-sheet parameter you can name plus package and price. The selectors are tied to a BOM (bill-of-materials) generator that includes inventory information from several popular distributors.

The selector front end allows you to specify the components for the circuit topology given by your chosen map. You can then simulate the circuit and order samples, often with shipping within 24 hours.

Texas Instruments offers FilterPro, a design program and associated application note that helps you implement multiple-feedback and Sallen-Key filter topologies with Bessell, Butterworth, or Chebyshev responses. TI also provides a family of calculators for simple single-ended op-amp and fully differential-amplifier circuits, mostly written as Java scripts. TI's site offers a host of small aids such as a decibel calculator, an op-amp-noise-to-effective-number-of-bits converter, and a common-mode-range calculator for instrumentation amplifiers.

Swift Designer is clientside software that TI makes available for free download. It supports TI's Swift family of power converters that feature integrated control FETs. The program presents a schematic representation of the converter circuit and allows you to set the operating parameters, including input voltage range, output voltage and current, ripple, and phase and gain margin. The program calculates values for the IC's external components and provides access to component data.

Although not an example of IC-vendor-supplied design tools, B2Spice from Beige Bag Software deserves mentioning. B2Spice provides schematic capture and analysis for analog- and mixed-signal designs in either a Windows or a McIntosh environment. Even if you have a large-scale EDA suite available in your office, B2Spice makes an attractively portable and affordable system for your laptop for use while traveling, for home projects, or for trying out circuit ideas when you're off the clock. Version A/D V4 Pro's facilities include schematic entry, 17 simulations with highly flexible graphing capabilities, pc-board export, netlist generation, 25,000 digital and analog parts, and parameterized subcircuits. If you're new to manipulating simulation models, this $300 (one) design tool can serve as an excellent platform on which to learn, and it's powerful enough to use for most projects that you would consider running on a PC platform.

Author Information

You can reach Technical Editor Joshua Israelsohn at 1-617-558-4427, fax 1-617-558-4470, e-mail jisraelsohn@edn.com.