App note: A microwatt charge pump boosts 1 V to 1.8 V at 90% efficiency, providing “Always On” standby power for microcontrollers

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Silicon Labs’ app note featuring low power boost for microcontrollers. Link here (PDF)

Boosting the output voltage of common alkaline button-cells to at least 1.8 V needed by microcontrollers provides an “always on” standby power source sufficient for low-power oscillator interrupt/sleep state operation. Two ultralow power op amps are used in a charge pump configuration to double an input voltage, creating an output voltage of approximately 2x the input voltage. Output currents up to 100 µA are available at 90% efficiency; even load currents as low as 10 µA achieve 80% efficiency, beating commercially available charge pump ICs and inductorbased boost regulators.

 

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4 Comments

  1. Fascinating. That would be a pretty pricey keep-alive supply, TS1001 is in the USD1 to 2 range. Now, leaving aside the fact that a standard lithium coin cell will be better, I’m thinking why not do intermittent boosting? Instead of a comparator, there are supervisory IC parts (which I found for a similar idea) from ROHM that work down to 0.7V and uses under 1uA, now that may help stretch the keep-alive duration a bit more…

  2. Power consumption of this circuit is in the ballpark of 5uA, given that the AN’s graph shows ~50% efficiency at 5uA output load. Say 5uA load and 10uA battery current, a 150mAh LR44 will last just under 2 years.

    In the real world, it is in fact easy to use a nanowatt-type MCU and sleep at ~1uA or run a crystal clock peripheral at ~2uA. Then a lithium coin cell with a Schottky will solve all your problems, and coin lithiums have a nice discharge curve too. Or even 2 x LR44 is not that far worse either.

    But it is still an interesting circuit in that most parts can’t start or operate <1V, but this design (or the TS1001) can. Using electrolytics for C1 22uF and C3 47uF won't have terribly good ESR, while multilayer ceramics at those values will be eye-wateringly expensive. So is it a good solution? Are there significant improvements that can be made?

  3. Hi KH, thanks for your interest in our application note. I apologize for the delayed follow-up, but I am with Silicon Labs and I would like to take the opportunity to address a few of your comments.

    By way of background, the circuit in this application note was conceived specifically for the designer that needs to use an alkaline button cell battery to power a design using an 8-bit MCU with sleep current in the 10 – 100 µA range.

    I agree that one alternative to implementing this circuit would be use an MCU with ~1 µA sleep current, but unfortunately this may not be an easy option for many designers. A designer may need to modify an existing hardware and software design that based on an 8-bit MCU (e.g. 6800, 8051, etc., with sleep currents in the 10 – 100 µA range). If a designer simply needs to introduce a new version of an existing product with longer battery life, or move to a smaller form factor enabled by a button cell, this circuit will enable the designer to get their derivative design to market as quickly as possible. (Changing the design to use a new 1 µA MCU and porting the software would take much more effort.)

    Similarly, switching to a 3V battery type such as a coin cell may not always be an option, alkaline button cells such as the LR44 are quite a bit smaller than the typical Lithium coin cell, e.g. CR2032.

    I checked pricing for the key components and see that the 1ku MSRP for the TS1001 is $0.75 (even less on Digi-Key). I agree that electrolytic capacitors are not suitable for this application, but I don’t find ceramic capacitors to be particularly expensive: 22 µF, ±20%, 6.3V parts are available on Digi-Key in modest quantities for less than $0.03, while similar 47 µF capacitors are available for just over $0.10.
    /www.silabs.com/products/analog/

    1. Thank you for your reply! It’s rare that a company rep responds in a non-corporate blog. I agree with your views, oftimes product constraints rule these things.

      This is more of the perspective of embedded product engineering versus the perspective of a hobbyist who is looking at such solutions. The issues of mass production is unfortunately often different from issues faced by hobbyists, for example, I often buy enough parts to get a discount, but I cannot buy thousands or hundreds of parts. Someone like me might also pay more for a few X5R caps rather than going for optimal cost. Or choose to buy from somewhere due to shipping costs. All this does not imply anything negative on the part of the app note, it’s just that there will be diverse views, moreso in a blog like this.

      Take car key fobs, they are mature designs, but as a consumer I have always felt like I am trapped under the thumb of battery manufacturers, uneasy about consuming so many A27 batteries over the years. For the sake of form factor, A27s have less capacity and more metal. So as an embedded hobbyist would I eagerly pick A27s? My choices would be quite different from designing for mass production.

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