How-to-RTFM
Understanding data sheets is one of the most important skills an engineer needs to master. Data sheets are the cause of, and solution to, most of your problems. Elite hardware trolls from around the world have aggressively plastered RTFM to every proverbial forum wall. What rarely happens is someone explaining how you actually Read The F***ing Manual. It’s not always easy and it’s not always right.
Structure
Electronic data sheets have a general standard layout. You won’t see this 100% of the time but it will be close enough to make it useful to understand.
Marketing will have a general overview of product and best case specifications. This is the pitch section to get you to read further.
Sample Application will show you why or for what industry the part was designed.
Pin Out section will label all the pins and have a one line explanation of their function.
Technical Specs will be tables of max/min values for different metrics. It will be very dense and may hide gotcha issues.
Graphs will have test data the a quality engineer produced during validation. This is your easiest defense to see how much the marketing section exaggerated claims.
Dimensions will show you physical size and electrical pads. It should be enough to make a custom footprint in the PCB design tool of your choice.
Ordering Info will have part number and packaging information. This is where you find out how part 123A is different than part 123B.
Legal is jargon you can likely avoid.
Marketing
This is the most useful section when trying to find a new part. In the beginning of a search you want to look for reason not to use something. It will be much faster to find one thing that doesn’t work than ten things that do work. If you need 8 inputs and the part only has 4, move along. You don’t need to read any further because you know it won’t work. The other specs could be amazing but it still wouldn’t be the right part for your application.
You don’t need to read any further because you know it won’t work.
The three key sections I use to find faults are description, target market, and high level specs. The description is useful to confirm the part does what you think it is going to do. Occasionally a parametric search will lead you down a wrong path and this is a sanity check that it didn’t happen. The target market gives you an idea of the price, size, and operating conditions of the part. For example, if you see the word automotive, it will be more expensive than other similar parts because it will have a wide operating temperature. Anything with cellphone or wearable will only come in small sizes. The bullet point specs will be the best the part can do. If you need a 1A output and the bullet point says 1.5A you can read further. If it only says 500mA, you need to find a different part.
Sample Application
The description will tell you what the part does but the sample application will tell you what else you need to make that happen. Two parts may appear to do the same thing from the description, but the application circuit will look very different. This is because one device may integrate more of the circuit inside the silicon while the other relies on discrete supporting components. The example below shows two different LED current regulators. Both can regulate a string of LEDs to ~350mA, but they each do it differently.
The IC on the left requires 10+ passive components to work and the IC on the right doesn’t require any. If we were to just compare prices of the ICs we would not have a fair comparison of the total cost of the system. The IC on the left cannot operate independently, and we‘ d need to calculate the additional cost of the supporting components to find out which one is cheaper. By looking at the application circuit we quickly know what we are getting into when we select the part. Maybe I don’t want to source a bunch of different parts, so I use the IC on the right. Maybe I value the additional features and performance gains, so I use the IC on the left. Remember to keep looking for a reason the part won’t work.
Pin Out
This one is pretty self explanatory. There is going to be a picture of the IC package with the pins labeled, and a list of what they do. This is important to reference when making PCB layout schematic symbols and footprints. Microprocessors can be more complex because they often share functions on a single pin. Always double check where pin 1 is located and how the other numbers are allocated. This counter clockwise numbering method is common, but there is no standard. Even when there are standards it doesn’t mean everyone is going to follow them.
Technical Specs
This section gets deep into the weeds. I usually skip over this on the first pass. There is useful information and gotchas hidden inside, but it’s not the easiest way to find them. For the sake of making up a number, 95% of this information is not going to be useful. The 5% that you will need is only useful after you’ve nearly picked your part. This is the last place to check for anything that is going to break your design. Maybe you need to care about rise and fall time on a digit circuit, but it is likely you don’t.
Graphs
Before any device is sold it is thoroughly tested. The tests produce graphs, and those beautiful technical graphs are grouped into this section. This is the third or fourth section I look at when browsing. It is the place were I see how much the marketing section exaggerated claims. Let’s say the top of the data sheet says this part can handle 1.2W. Looking at the de-rating curve, that 1.2W is at 25C with a 50 square mm thermal pad. This isn’t as catchy and may not be appropriate for your application. If you need to put this outside in the desert(50C), then you can only use it at 0.9W. If it needs to be smaller, maybe you only get 0.75W. These qualifiers matter and you rightfully won’t find them in top level information.
People say a picture is worth a thousands word for a reason, and these graphs are no exception. Because you have curves and lines you can extract formulas to characterize the part in numerical format. There are a few online tools that make this easy. Web Plot Digitizer is the one I use most often. Once all the charts are turned into formulas, you can then construct a model to automate some of the design process. I will go into further detail on this subject in an later article.
Dimensions
This is the first section I use when making a new PCB footprint. It should have all the information to make a rough 3D model and electrical footprints. Everyone has a different style of dimensioning the drawing, so it may take a little bit of math to get the numbers you actually need. There are a few gotchas you can run into with this section. Sometimes you don’t have all the dimensions you need and you will need to measure an actual part. The units may not be obvious or you may mix them up. This example and most supplied use mm, but many will use both, and some may only use inches. Unfortunately, you will be switching back and forth a lot. The last one is drawing view. This LED does not have any views marked. By referencing the cathode mark I can figure it out but it would be nice it was listed. You can listen to Chris Gammell explain how this happened to him in his latest design here.
Ordering Info
I probably enjoy this section more than I should because I have become a sucker for good part numbers. Logistics is hard, and it can be easier when part numbers (P/N) make sense. This section is like a decoder ring to tell you how to actually get the thing you want. You can see that all the P/N below start with Si7006. Si is short for Silicon Labs, and 7006 is the top level part number. The rest of the information denotes variations of that top level part. A20 is the firmware version as described in the footnote. This is useful to document in production because any change is an opening for potential problems. Based on other products from the company, I know the I means industrial temperature range, but I am not sure what the M means. R gets us a full reel, and 1 comes with a teflon cover. These conventions will carry over to other products by the same company. Does a Si7021-A20-IM1 come with a protective cover? Because we have the decoder ring, we know the 1 at the end means it does.
Legal
Pursuant to sub section 5a of part 15 class II of Title 9, no-one reads this and every company reserves the right to be wrong. There will be mistakes in data sheets and only some of them will be corrected. This will be a get out of jail free card to say you can’t sue anyone. You can still complain LOUDLY, IN ALL CAPS, ON TWITTER. Make sure to @junesphd so I can commiserate in your time of need.
