The cookie is used to store the user consent for the cookies in the category "Other. This cookie is set by GDPR Cookie Consent plugin. The cookies is used to store the user consent for the cookies in the category "Necessary". The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional". The cookie is used to store the user consent for the cookies in the category "Analytics". These cookies ensure basic functionalities and security features of the website, anonymously. Necessary cookies are absolutely essential for the website to function properly. However, there are other ways to stretch battery life, which I’ll cover in coming weeks. Summing up, in the use case I’ve described (ten years of system life from a coin cell) it’s generally impractical to get a pulse boost of Vdd from a capacitor across the battery. Be sure to use a low-leakage part, and factor that into your battery capacity calculations. To add another wrinkle, don’t forget that every MCU vendor requires one or more decoupling capacitors, even when running from a battery. That’s impractical since even at room temperature it’ll eat just about the entire 2.5 uA current budget. Alas, the lowest-leakage supercap I can find is from Cap-xx, and they quote 2 uA at 23C, doubling at 70C. What about a supercapacitor? These are astonishing devices that offer farad levels of capacity. You can’t buy a bunch of smaller parts and parallel them, since the leakage will go up. And, the capacitor doesn’t exist the biggest such part on the market is 220 uF. The leakage will eat about half the battery capacity. Using the graph above we pick off a 400 uF part for a 10 mA 20 ms load (remember, this is before all of the derating that must be done). Maybe this is a government project where costs don’t matter. Digi-Key does have a 50 V version available today, but that will set you back $36.54.Ī complete Raspberry Pi with 700 MHz ARM processor and half a gig of RAM costs less than that capacitor. They’re $14.50 each! Neither Digi-Key nor Mouser has them in stock, and quote a 25 week lead time. Let’s ignore all of the unpleasantness and assume that the 100 uF part fits the bill, and that we’re using a low-leakage AVX part. So we’ve done the math, and have figured out what size capacitor to buy. X7Rs are indeed very temperature stable, but not as a function of leakage! Here’s a typical graph of normalized leakage versus temperature: And, one must be careful which X7R is selected as some leak as badly as Y5Vs, though exhibit better temperature stability, which has always been the primary reason to use X7Rs. It’s clear Y5V dielectrics can’t be used. How about a better part? The best low-leakage capacitors with the C values needed are MLCC. Who would have thought that a cap could leak more than Edward Snowden? Just the capacitor’s leakage will suck the battery dry in a fraction of a decade. A CR2032 offers about 220 mAh of capacity, which means the average current draw over 10 years cannot exceed 2.5 uA. I started this series showing that MCU vendors who claim decades of potential operation off a CR2032 are completely off-base, and demonstrated that the longest life one could hope for is a decade (though few will ever achieve that).
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