Issue link: https://iconnect007.uberflip.com/i/1256432
JUNE 2020 I DESIGN007 MAGAZINE 51 I relate this story to drive home the point that PCB design should not always involve ig- noring app notes or manufacturer's recom- mendations. Good design may involve consid- ering the app notes first to understand the con- text. But an app note usually provides only one solution. I think we designers need to be able to come up with more ways to proceed. Ex- plaining to your engineering stakeholders that an app note is the only way to move forward may not end well. Shaughnessy: Did you end up trusting the app note or disregarding it? Dack: By applying previous design experience, I was able to apply the thermal dissipation re- quirements addressed in the app note and ap- ply its veracity in a re- quired, alternate form factor. I'd previous- ly worked with other manufacturers' LEDs who recommended the use of thermally conductive vias under- neath the part, which were to be intercon- nected with thermal planes on the second- ary side of the PCB. This technique dis- persed the required surface area for cool- ing to both sides of the PCB. To make room for the secondary side planes, I had to add internal routing layers. To provide solid return paths for the internal sig- nal layers, I had to add inner layer planes. By the time this design was completed, it looked nothing like the simple but expansive single layer solution illustrated in the app note. But by leveraging experience gained by us- ing techniques for dissipating heat from other types of components, I was able to merge the requirement with the given mechanical con- straints. Granted, this was not an example of good engineering, but it is an example of using design experience to quickly consider all of the design constraints and come up with a viable solution for an awkward hand that was dealt. Matties: My understanding is that a component may have an app note, and there's also a data- sheet. I'm trying to understand, and help our readers understand, the difference between an app note and a datasheet because they're al- most used synonymously in conversation. Dack: It can be confusing, but it shouldn't be. My take, as I alluded to earlier, is that suppliers publish datasheets for most components. But only complex parts or ICs with critical applica- tions require accompanying app notes. The app note describes how the electronic aspects of a part is supposed to perform as applied to a giv- en set of criteria. Our advanced PCB de- sign educators make a good point that in PCB design, we can- not always stick with the manufacturers' given layout crite- ria to achieve perfor- mance. We must of- ten work the compo- nent into a PCB en- vironment that does not exactly match the optimized two- layer, four-square- inch surface area that the manufactur- er had based all of their testing on. Without design experience or engineering ability to run complex analyses or time-consuming DOE and testing, new design- ers can get into a lot of trouble taking things for granted and applying the component to a design scenario that does not exactly match the app note. Datasheets for components, on the other hand, have been knocked as unreliable too. Datasheets are supposed to document the physical and mechanical attributes of an elec- tronic component, including package size and