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Ending at the beginning
The very last thing we think about when we start a new project is the power supply. And that is not a bad way of doing things. After all, how can you even know how much power you will need until you have spec’ed out the project. Having said that, how you provide power to your prototype is very important. If you are designing a battery powered device, that contains an MCU and you provide a battery powered prototype to your firmware developer, you are sure to get a bag of coal for the holidays.
The most important tool you have
Most inventors (I define an inventor as someone with a great new idea and a credit card), don’t understand the role that the prototype plays in product development. Even some seasoned companies don’t realize the benefits of approaching a new design in steps rather than trying to design the final product out of the bag.
A prototype is like a new clean smooth piece of paper. Think of a design as an essay or a painting and you can see that until it starts to take shape the final solution is unknown. Even the great painters in history have layers or painting under the final work of art. That is their prototype. It is a place where we get to try things out, see if they match up with our needs and expectations.
The prototype is also your greatest tool. It is a platform that allows you to develop future parts of the product. What I mean by this is that you don’t, or shouldn’t try to make the final design right off the bat. If you are going to put a flash chip on your device and there is something special about the chip or the way you are using it, or maybe there is some doubt about how much you will need, why not plan to have two or three chips on the bus for the prototype. You could make three boards and populate a different footprint for each one. If you guess, I always thought guessing was lazy until taking the partial differential equations classes, correctly then it only took you a little time and no extra money to drop down the addental footprints. If the first chip is a failure, you have a ready platform to try other ideas and don’t have to waste time and money making another board.
Back to power supplies
If I am going to make a battery powered device, I also want a way to power it using a benchtop supply. I don’t want to have to fool with batteries until the device is done. But I don’t want to ignore the power supply early on, thinking I don’t have to worry about that until later, either. Double paths and contingencies are your friend. You may want to drop down a 2-pin header on 0.1” pitch so you can literally attach a variable voltage bench top power supply to your circuit. Why? Well, if you are going to have a battery pack, you don’t have a constant voltage source. You might want to test out the limits of your regulator before you get to beta testing. I know that manufactures make incredible claims about switching supplies, but there is a fine line between lying and giving a specification that has 12 criteria that must be met to get that 97% efficiency.
I also want to have a barrel connector with the nominal voltage expected available for developing firmware. It is such an easy thing to do. No fuss, just plug it in and do what you need to do to test out the circuit or firmware.
You also want to connect the actual target power supply early on, whether you use it much in development or not, you want to test it and make sure there are no surprises.
Linear or Switching
Everyone loves switching supplies because they will capture those last few joules out of your battery, they run cooler and are much more efficient. They are not, however, plug and play, no matter what the datasheet says. So, having a linear supply as an alternate means of regulating the incoming power is a safe bet if you have not used the particular switcher. Again, if it works out the first time, exactly as you planned, you don’t even have to populate the linear regulator. On the other hand, if it all hits the fan, you have a backup parachute to save your precious skin.
How much power
Something I got in the habit of doing many years ago is to put a small resistor in line with my various voltage regulators or power paths so that I know how much power I am really using. The first prototype has lots of unknowns. The final product should be tight. To get from one extreme to the another you need to have information and that is exactly what a series of prototypes can provide you. I say can, because you must plan the hardware to get the answers you need.
Knowing the current going to various parts of the circuit is very valuable and that information should get put into your design notes that follow the design into production. Some technician, or perhaps a future you, will thank you for providing detailed notes about how the circuit is preforming.
Another useful thing to do is to sprinkle the circuit with test points. This comes in really handy in early prototypes. You will want to remember to remove many of them as your design matures. The same goes with the sense resistors. No need to burn up power once you have the information you need.
Use your tools to their maximum and good designing.
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