Yesterday I was researching some old computers, machines that required substations' worth of power. Today I'm thinking about systems that run for years off a single coin cell. How much things have changed!
Ultra-low-power embedded systems can run off a CR2032 for 10 years if they spend most of that time in a deep sleep, waking only occasionally and for brief periods to do something useful. But it's not easy to squeeze the last coulomb from these small cells.
We know that a processor's power consumption is proportional to the voltage squared times the operating frequency. But we're not really interested in power, since battery capacity is rated in amp-hours. Current is the issue, and current in a resistive network is linear with voltage.
Is it linear with an active (very active) component like a microprocessor? Here's what the datasheet says about Microchip's PIC18LF46K22, a typical very low-power part:
TI's MSP430F2013 also exhibits the same behavior, though it's unclear if this is the pretty-much-meaningless “typical” data or worst case.
This linear Vdd vs. current data is a bit puzzling. Depending on clock rate, running at a lower Vdd results in substantial energy reductions. A number of vendors are locked in a battle for dominance in the ultra-low-power space. One would think they'd put a regulator on board to operate the chip at the lowest possible Vdd, optimizing their current numbers. Obviously, there may be some pin issues, though those could be powered pre-regulator.
The processor will, in these applications, mostly be sleeping. But we really don't care much about the relationship between Vdd and current when it's in a snooze mode. For a 10-year life from a CR2032, the average current draw over that decade cannot exceed 2.5 uA. With sleep currents of very-low-power MCUs in the tens to hundreds of nanoamps, sleep is practically irrelevant to these issues.
To reduce the processor's needs, why not add an external LDO (low-dropout) regulator?
Unfortunately, during the processor's long periods of sleep, the typical LDO will require tens or more of microamps. I can't find one that is frugal enough with coulombs.
Touchstone Semiconductor has a boost converter that looks like a perfect solution. Its TS3310 will do all sorts of cool things to crank low-voltage sources up to MCU levels. But it will also regulate a battery down to lower levels. When the processor is sleeping, it only needs 180 nA from the battery, which is just 7% of the 2.5 uA budget. Regulate from 3 to 1.8 V, and you'll gain, even accounting for the regulator's needs, around a third more energy. Using the regulator will boost the amount of current a system can consume by around a third for the same battery lifetime.
The 2mm square IC itself is $0.99 in 2,000-unit quantities. It does require a small inductor, but those are about seven cents in volume. A couple of capacitors are needed, as well, which would also be the case with an LDO.
It may be tempting to assume one can use this trick to build a system that will run for more than 10 years off a coin cell. Alas, that's not to be. The vendors specifically claim a 10-year life on their batteries. It was just last summer that Duracell changed its ratings from seven to 10. If a part supplier says, “Don't use this part in this mode” (in this case, for more than a decade), it's poor engineering practice to ignore the warning. Sure, the system might work, but it's a crapshoot.
Of course, a cynic might note that he will probably not be employed at the same place after all those years go by, so he can't be held accountable…
(Note: Touchstone has fallen on hard times. Silicon Labs recently acquired its IP and will hopefully continue selling this part.)
This article was originally published on EBN's sister publication Embedded.