August 1, 1997

Have you done your homework?

Good products result from conscientious efforts at all stages by the designer.

It's easy to get so caught up in adding the latest features to a design--whether it's memory, a faster CPU, multimedia capability, or Internet support--that you don't have the time or the inclination to rigorously check your fundamentals. A recent open letter, entitled "Alert!: PCs built to fail," from Bob Dobkin, vice president of engineering at Linear Technology Corp (Milpitas, CA), examines this topic. Dobkin's thesis is not the speculative ranting of a tabloid news show looking to boost ratings. It's exactly the opposite: Detailed technical documentation and scope photos support his assertions.

In his letter, Dobkin details how and why many of the latest PCs have marginal power-supply-system design, primarily due to vendors' cutting corners and using lower cost, higher ESR, less stable aluminum-electrolytic capacitors instead of the superior but more costly tantalum electrolytics. In some cases, the supplies didn't meet Intel's detailed specifications even under favorable conditions; in others, the supply worked properly but only at nominal temperatures or early in the inevitable aging cycle. The power-supply shortcomings appear to PC users as soft failures or random crashes.

Beyond the immediate warning, the open letter reminds us that good products result from conscientious efforts at all stages by the designer. The relatively high reliability of modern components and the inherent tolerance of digital systems can lull you into thinking that there is little to worry about once you accommodate timing margins and propagation delays. Although your product may be primarily digital, it still has a power supply--battery- or line-powered--and that's an analog function.

Unlike software and most aspects of digital circuitry, analog-component characteristics change with temperature and age with time. The increasingly stringent requirements of big CPUs make design margins ever tighter. Lower supply rails mean that a given ESR-related ripple, such as a mere 200 mV, is an increasingly large fraction of the supply's nominal value. Faster clocks mean that supply transients take up more CPU cycles. Because of these facts, designers can no longer treat power-supply functions as a last-minute, "hey-if-it-works-it's-OK" part of the design.

It's not enough for you to have one unit or even a hundred units that seem to work fine. Validate your design by going through detailed time- and temperature-tolerance modeling and -analysis. Build some supplies with parts that have non-nominal or aged part values. Stress the supply and carefully study its waveforms. If you use an outside supplier, make the effort to go through the supplier's analysis. Investigate other hardware-simulation and -test tools that are available to you. For example, when I spoke to Mr. Dobkin, he noted that you can get (or build) a power-consumption-simulation module that plugs into a CPU socket and replicates its static and dynamic load.

The rigor of doing your homework applies especially if you design non-PC products. Unlike PC vendors, you can't assume that users automatically think that any inexplicable, intermittent crashes are due to bugs in Windows 95 or in the application software. Users may realize that such errors are the result of the marginal design of your product!

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