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My microcontroller operates at 3.3 V, but all the CR2032 bateries are rated at 3 V. Is it safe to drive my microcontroller using this 3 V? Will it operate correctly?

I don't have to draw too much current, so the current is not an issue. If not, do I have to use something like NCP1450 to actually step up the voltage? Given that I have to reduce size, what is the cheapest way to achieve this?

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What MCU is it? –  geometrikal Jan 18 at 6:12
    
If you don't absolutly need to use the CR2032, the rechargeable Li-ion batteries like the Ultra-Fire 18650 are 3.7V. They are sold all over eBay with chargers for low costs and I am finding them to be very nice, though I have been using them for only 6 months or so. They are probably over-rated in amp-hours but have been working well. –  C. Towne Springer Jan 18 at 7:51
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@C. Towne Springer They most certainly are not 3.7 V in the sense in which non-rechargeable batteries are 3.0 V. The nominal voltage of a single rechargeable lithium cell is 3.7 V, but a charged cell will go as high as 4.2 V, which may be dangerously close to absolute maximum rating of a 3.3 V MCU and it may be outside of recommended operational voltage. On the other hand, a 3.0 V coin cell's voltage will remain very close to its nominal voltage until it's almost depleted. –  AndrejaKo Jan 18 at 13:11
    
I changed the question. Nowhere in the question is efficiency discussed (only price is) so having 'most efficient' in the question title makes no sense. –  Adam Lawrence Jan 18 at 13:31

2 Answers 2

up vote 13 down vote accepted

While the question does not specify the microcontroller under consideration, the popular 3.3V rated microcontrollers from Atmel, Texas Instruments and Microchip all tolerate supply rails down to 3 Volts, 2.7 Volts or in some cases even lower.

Note also that a CR2032, or for that matter pretty much any battery, will drop in voltage as it depletes. Thus the constraint to be considered is not the nominal voltage of the battery, but the lowest voltage the battery can drop to and still be considered operational.

The microcontroller datasheet will mention what the absolute lower limit is.

The datasheet will also reveal whether some parameters of the microcontroller will behave differently under lower voltage.

Many microcontroller families specify lower clock speed limits for lower supply voltages: Thus a microcontroller which is rated to operate on a 20 MHz clock at, say, 5 Volts, would be limited to no higher than perhaps 12 MHz at 3.3 Volts, and 10 MHz at 3 Volts.

Another potential point of difference would be any analog reference voltage output available from the microcontroller: Some microcontrollers offer a reference pin with output valid only as long as the supply rail voltage is sufficiently higher than the reference voltage.


In order to be able to operate a microcontroller even as the battery depletes, a simple 3-terminal boost regulator, such as the Holtek HT7733, is an option to consider:

Schematic

This particular boost works down to 0.7 Volt battery voltage, and is very inexpensive (20 cents each in my part of the world). The circuit is simple enough to add to an existing design. It also does not require an external MOSFET, an advantage over the NCP1450.

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That helped. Thanks a lot –  Developer Android Jan 18 at 7:42
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Nice chip, but HT7733 does not give a single hit on www.findchips.com :( –  Wouter van Ooijen Jan 18 at 9:20
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@WoutervanOoijen Yeah, even Octopart lists the product but doesn't find any distributors for it. Shops in Mumbai stock both the 7733 and the 7750 as they are quite popular. Maybe I could start a drop-ship business from here ;-) –  Anindo Ghosh Jan 18 at 10:12
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@WoutervanOoijen I found an inexpensive module on eBay that uses the HT7750: "0.9V-5V to 5V DC-DC Booster Module" for under $1 shipped. –  Anindo Ghosh Jan 22 at 4:17

The cheapest way is to use the battery directly. Referring to a typical CR2032 datasheet, look at the discharge curves, and temperature dependence. If your entire circuit draws 190uA or less, you can depend on 800+ hours of life at an end-of-life voltage of 2.75V or so. enter image description here

Look at the data sheet for your CPU and see if it is GUARANTEED to operate at 2.75V at the clock frequency you are using. Don't use "typical" figures. For example, here is a figure from the Microchip PIC12F1501 datasheet. figure This particular part is good down to 2.5V for any clock frequency up to 20MHz, and down to 2.3V for 16MHz or less, over the entire temperature range. It will also easily accommodate the highest voltage from a new CR2032 (a bit over 3.0V at high temperatures).

Then check carefully through the MCU datasheet to make sure that all peripherals will operate over that voltage range. In the case of this particular chip we can select the brownout voltage to be 2.7V nominal or 2.4V nominal. In the first case, we can use any clock up to 20MHz, but we might only get half the battery life (for the same load current) due to the higher brownout voltage. In the second case, we're restricted to 16MHz maximum clock, but we will get 1000 hours battery life. enter image description here

(Or we could pick the PIC12LF1501, which is guaranteed to operate down to 1.8V (and can accept up to 3.6V) at up to 16MHz, and go through the analysis again).

If you do find you need a boost converter, the NCP1450 is a cheap controller, but it requires an external transistor and Schottky diode. If your circuit happens to draw in the hundreds of uA, you can optimize for efficiency by picking a smaller transistor. For example, below you can see efficiency of 70% at 200uA out, so the drain from the battery should be a bit more than 300uA. enter image description here

If your current requirement is in the mA, you might want to consider a complete regulator with synchronous rectification such as the AAT1217ICA-3.3-T1, which requires a smaller inductor. It's about 35 cents in quantity. If your factory is in Asia, there are even less expensive choices. The same analysis as for the MCU applies to the voltage range that the regulator is guaranteed to operate at (and can accept without damage).

The efficiency of these things is not great at low currents, however, as you can see here: enter image description here

At 200uA, it's only about 35% efficient (about 660uA in for 200uA out!), so a lot depends on the actual operating current of your circuit.

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