Rather than a Schmitt Trigger, a proper comparator will not have such excessive current usage. Take the MCP654x set from Microchip: http://ww1.microchip.com/downloads/en/DeviceDoc/21696H.pdf Figures 2-15 and 2-18 on page 8 show the supply current plotted against the input voltage, and it stays under 600 nA across the full range.

A few passives (4 resistors and a capacitor) should be able to turn it into an oscillator. I went through the trouble of working it out myself, and found out that I had reinvented this circuit: 

Sufficiently high values for the passives can keep your supply current down, but it also becomes more sensitive to leakages and external noise. I don't know how big of a deal that would be, since this isn't a precision circuit.

Yes, that's a good point.  There seem to be a number of nano-power comparators on the market which might work as the oscillator stage. 

Now, I need something better for the pulse shaper to create relatively stable pulses of 1us or less in order to be able to use smaller inductors.  Unfortunately, the nano-power comparators seem relatively slow - I can find 2us delays, but not much less.  Th Schmitt trigger inverters are much faster, and aren't power hogs in that stage, but the poorly specified hysteresis would allow for a large tolerance on the pulse width, and hence the LED current.

For the pulse shaper, how about a (non-Schmitt) buffer with a feedback resistor to create the hysteresis? Might have better tolerance.

And if that works, it might end up cheaper to use a 2+-input part (and, or, xor, xnor) and tie the unused inputs to the relevant rails.

@AlliedEnvy That's a possibility.  I'm also wondering how closely matched the thresholds of multi-gate packages are - at least they will be at the same temperature.  I think you could use a pair of gates to partially compensate for the threshold tolerance, as long as they're relatively matched.  For instance, use two RC's, one with a longer time constant, to drive gates, then combine the resulting pulse with a 2-input gate.  The resulting pulse has less dependence on the exact threshold.

@Ted Yapo It sounds like you're trying to reinvent a rising edge detector.

Instead of an inverter chain, you could use a single Schmitt Trigger with an RC filter on the input to tune the delay and adjust the pulse width.

@MarkRD yes, exactly - I've used rising edge detectors like that, except with an RC delay in there, the delay you get is not only a function of RC but also of the threshold. I can get 1% R's and C's, but the threshold on logic gates varies widely. You can substitute a comparitor for the gate after the RC for more precision, but fast nano-power comparitors don't seem to exist.

The thought was to use two legs with RC's so that you could compensate for the wide tolerance on the logic threshold - assuming that the threshold is similar for inverters on the same die.

This spec is in the datasheet. It's in the Electrical Characteristics table and labeled ΔIcc. You're measuring significant;y higher than the quoted spec though, maybe you've got some extra leakages in there that are hosing your current usage.

For your reading pleasure, I found an application note from TI called Understanding Schmitt Triggers: http://www.ti.com/lit/an/scea046/scea046.pdf

Given that this ΔIcc thing appears to be a common feature of Schmitt Triggers, I don't think there's much promise in this.

Thanks, I didn't know what the ΔIcc parameter was!

I see it in the datasheet specified as 40uA max (at 25C), but the conditions are with a 3.3V supply and the input at Vcc - 0.6 = 2.7V.  I think I'm seeing that the current is higher with the input closer to half the supply voltage - which would still meet the datasheet spec. if I weren't a few hours past bedtime, I'd measure it now.  Maybe in the morning - well, later morning.

But, with a reduced supply, this current can be made very low.  These parts are spec'd to run down to 0.8V - at around that voltage, I see sub-uA currents for the oscillator.  It actually seems pretty good.

I'm rather surprised that this issue is so obscure. I didn't know what that parameter was either until I found the TI app note. It's quite a major weakness using Schmitt Triggers in low power applications.

The major downside to reducing the voltage to the Schmitt Trigger is that it weakens the output drive capability. Which MOSFET are you using? Driving the gate with such low voltage might have too much RDS, wasting some of the power that should have gone into the inductor.

A thought I had is to clock a PIC timer with a 32 kHz watch crystal and use a PWM peripheral with very low duty cycle to generate gate pulse. That way the microcontroller can stay totally asleep except during initialization. I know the ATmega328P uses about 1 uA typical in that mode, but its timer is only 8-bit. Not enough for the small duty cycles you're shooting for. I'm having trouble finding a small PIC with a comparable feature.

Checkout page 14, figure 14

https://assets.nexperia.com/documents/data-sheet/74AUP1G14.pdf
Unhelpfully truncated, but I came across this while ordering some of these to test in some other circuits.

@Michael Stoiko yeah, that datasheet has the right info!  Nice to see suspicions validated.

@MarkRD The major offender in this circuit is the oscillator - the pulse shaper doesn't draw that much current even at 3V, because it mostly stays out of the linear region.  So, the idea would be to run the oscillator at a lower supply, then the pulse shaper at the battery voltage.  There's an example of this in the Art of Electronics V3, where they made a 32kHz watch crystal oscillator with these same gates.  The oscillator gate has 10k resistors in the power and ground connections to "float" it around the center of the supply voltage, while reducing the supply it sees.  A second inverter stage can boost the output back to the rails.

I could imagine doing the same thing here.  The Rds isn't too much of an issue with the circuit as it is - the inductor has 5 ohms of resistance anyway, which dwarfs decent MOSFETs (even weakly driven ones).

Newer PICs have ultra-low power 32kHz oscillators built-in.  You can't keep time with them, but that's OK in this case.  I looked around for a PWM solution, too, but didn't find one.

I'm still liking a separate oscillator/pulse shaper architecture.  The oscillator could just be a PIC, something made from CMOS gates or discretes, or one of the nano-power oscillator ICs.  The pulse shaper needs a little more thought, maybe a nano-power comparator, although I think they're all too slow.

IANAEE, just a well-read software guy, so take this with a grain of salt.

I was looking into low-power oscillator designs awhile ago, and thought about this same circuit with this same AUP Schmitt inverter part. I did think the shoot-through current was going to be a problem.

I was considering a comparator wired up in a Schmitt inverting configuration, but I decided power wasn't all that important in my case and ended on a CMOS 555. Still haven't finished the design for that project.

Maybe even a quartz oscillator with a frequency divider would work. You probably don't need the accuracy or stability, but if they use them in wristwatches that last years on a battery, I wouldn't expect them to use much power.

Hmm. Flipping through chapter 7 (oscillators and timers) of AoE, 3rd ed; fig 7.40 on page 448 looks relevant. They're using the same AUP logic family, too, but for a Pierce oscillator for a wristwatch.

I don't fully understand how it works, so I can't describe it accurately; it looks like their trick is running the oscillator from a low voltage, with 10k resistors on the inverter power rails (to reduce current?), and then boosting the low-voltage signal back to useful levels with an additional stage. They report 2.4 uA from 1.0 and 1.8 V supplies. If you don't have a copy, I'll be glad to send a pic of the page in question.

Would the final stage would even be necessary here, if you have a low-enough gate threshold on the drive transistor?

Oh, and if you're going to be using more than one inverter, they make em with 2 or 3 in a single package.

Thanks for the pointer to AoEv3.  I have a copy, and will check that out - I skimmed that section a while ago for something else.

And, thanks for the clue about the supply voltage!  I'm not sure how this will work over temperature, but the initial tests look promising. I just tried reducing the supply voltage - either with a variable supply or a series resistor in the power lead.  At 1.8V, the circuit only draws around 42 uA - so 1/4 of what it was.  It will actually continue to oscillate (crudely) down to 0.25V, and looks "normal" at around 0.4V. Neither one registers any current on my meter - I have to raise the supply to 0.76V before I get a consistent 0.1uA reading - and the current stays below 1 uA until a supply voltage of 0.95V.  This is very interesting, especially since the output stage works, too - yes, it could probably just drive a low-threshold MOSFET as-is.

Two issues remain - first, the varying supply voltage changes the hysteresis levels on the input, so the frequency changes with voltage.  At 0.95V, the frequency has increased to 93Hz.  Maybe you could use this to maintain brightness (by issuing more pulses) as the battery voltage drains.

The other issue is efficiently deriving a stable supply somewhere between 0.75 and 0.95 from the battery voltage of between 2 and 3V (or 3.6 for LiFeS2 cells).  You'd need some kind of nA-quiescent-current LDO regulator.  I don't think a resistor is going to work well over that input voltage range.

Still, this is a very interesting direction.

I've seen the multi-gate packages - that would be my preference with finished designs - but for prototypes like this, single-gates are awesome mounted on individual SMD adapters.  There's plenty of room to use through-hole passives, which are easy to work with and I have way too many of :)

Hmm. For the supply, how about using a forward-biased diode, sort of backwards to using a Zener, as a voltage regulator.

A forward-biased diode might work, but you'd typically run a shunt regulator at several times (at least) the load current for stability, which would be expensive in this case.  Also, the forward voltage of common diodes is pretty low at tiny currents - see the 1N4148 here:

at 1uA, it only develops 300mV.  So to get a 0.9V regulator that only consumes 1uA, you need 3 diodes in series.  So, now you have a 0.9V regulator that eats 1 uA quiescent, and may be good for a few hundred nA load current. Seems expensive for a regulator, but the whole thing may still be under the power budget.

I'm also not sure what the temperature coefficient does at these low currents.

It's a cheap solution, though, and maybe you could get it to work.

I mean, there are other, even more exotic chips to try. LTC's nanowatt comparators seem promising, but list specs similar to the 74xx14 you used. 

http://www.linear.com/product/LTC1540
This little guy looks intriguing, too.

https://www.sitime.com/products/datasheets/sit1534/SiT1534-datasheet.pdf

Thanks for the links!

I looked at LTC's comparators for this before - I can't remember if I ended up buying a few - I'll have to search through the boxes.  I think I decided against them for some reason, which I should have written down...

I hadn't seen the SiT1534 before - that looks really interesting. Digikey stocks versions programmed for 32 and 64 Hz, perfect.  The downside is that they are in crazy tiny packages and cost more than the PIC.

I'm measuring the PIC overhead at around 3uA in some cases, the SiT1534 might cut that in half.  On the other hand, you need maybe 10 uA minimum to drive the LED to decent brightness, so the percent savings you get is maybe 10%.

Worth playing with, anyway.

I've still had zero workshop time to play with this, but I'm wondering if a relaxation oscillator based around one of these old solarengine circuits could work. I say that only because the "tiny photopopper" circuit that launched me on this whole project was claimed to be operating off a panel that was outputting 1 ua.

https://www.fetchmodus.org/projects/beam/photopopper/

That one. I haven't been able to find an ISL88001 in the range this person used, but I have a couple in various voltage ranges. It would have the problem of varying frequency with voltage pretty heavily as the battery discharges, but if the voltage detector is only consuming nanoamps, and driving the MOSFET with it's miniscule current, you could theoretically have a 3 component drive circuit (heck, if you included the photodiode components, inhibit firing in the presence of bright light and saving energy).

I mean, controlling the frequency with the size of the accumulator cap is pretty dirty, but with really good tuning a compact, elegant circuit might be possible.

Thanks Ted. This isn't an area I have any expertise in but it's good to see fails as valid learning.

That's a bummer. Using the 74xx14's as an oscillator was previously billed as an extremely efficient way to flash LED and extend power supplies in BEAM pummers, you beat me to trying it out. Evidently not efficient enough.

Maybe it's not bad if the frequency is lower, or use a different circuit.  I'll have to play with them a bit more - it could be that something else is going on here.  Or, maybe the 74AUP isn't as good as the 74HC or even 4000-series parts in this role?

See the update - the oscillator current drops off dramatically with supply voltage, and runs down to 0.25V.  It looks very promising below 1V.

excellent!