High-efficiency 12V to 1.8V converter at very low (~100uA) load - Page 1
Forum section: electronics / general

author: eddie1
author_rank: Contributor
timestamp: December 21, 2019, 07:47:45 pm
content: Most switching converters, even those advertised as "nanopower," "ultra-low-load efficiency," or similar terms, seem to have very poor efficiency at such low loads. There are only two I've found that get to the ~80% area under these conditions, and one of those I'm unsure about:Linear/Analog LTC3388which claims efficiency in the 80% area at these voltage levels and loadTI WEBENCH claims the LMZ12001 (and its higher-current variants) can do 90.7% at a switching frequency of 22 Hz (yes, Hz). Thedata sheetis not so optimistic, but TI doesn't say what switching frequency is being used there, so maybe WEBENCH really is telling the truth.TI has some other high-efficiency parts except they are limited to much lower (~5-6V) Vin.Is anyone aware of any other parts? Or for that matter, has anyone tested the LMZ12001 using one of those optimistic WEBENCH-generated circuits to see if it can actually hit 90%? I'm going to do that and will post back with the results, but might need to order some parts first (and possibly a PCB, which WEBENCH provides a design for, but I think the eval board might be the same circuit/layout and I already have that).Thanks!

author: bob91343
author_rank: Super Contributor
timestamp: December 21, 2019, 07:52:43 pm
content: At such a low power level I might consider a battery. It depends on the specifications you have. If you can live with about 1.5V an ordinary flashlight cell can work. Otherwise a Lithium battery at 3.7 V with a series diode string or even a regulator should last for years.

author: james_s
author_rank: Super Contributor
timestamp: December 21, 2019, 08:28:38 pm
content: Switching converters excel at providing high efficiency to relatively high current loads but for such low currents you're almost certainly going to be better off with a linear regulator. There are numerous parts specifically designed for low quiescent current battery powered applications.

author: eddie1
author_rank: Contributor
timestamp: December 21, 2019, 09:48:11 pm
content: A linear regulator is definitely the "normal" solution here. Trying to optimize beyond that is more of an exercise in overengineering than anything, that I'm just doing for fun. Given that pretty much everyone would just use a linear regulator for this, it sounds like the LTC3388 might be the only switching solution (and if that didn't exist, safe to say linear would be both more efficient and simpler).As for the LMZ12001, I'm certain WEBENCH is not accurate. The data sheet says the non-switching input current is 1 mA typical (no min or max values specified). I don't see how it can possibly attain such high efficiency (even higher than the LTC3388 claims despite that part specifically being designed for extremely low loads like this) with that. I suspect the inputs to their efficiency calculator were incomplete.

author: eddie1
author_rank: Contributor
timestamp: December 21, 2019, 10:11:24 pm
content: Upon further analysis, theTI TPS62125looks like a potentially good option, even if difficult to hand-solder. WEBENCH claims it would be about 60% efficient and the data sheet supports that. It's also very cheap and TI has reasonably-priced ($25) eval boards, unlike Linear/Analog who wants an incredible $175.The LTC3388 is more efficient on paper, but the 1.8V output is a nominal value and it can dip slightly below that. The minimum voltage spec on the MCU I'm using is 1.8V without any extra tolerance. My guess is the MCU would work fine at, say, 1.75V, but out of spec is out of spec and never guaranteed, so I'd probably want to increase the voltage to compensate for the tolerance. That would mean increasing to 2.5V, since the LTC3388 only has fixed voltage options. The MCU will draw higher current at higher voltage. At that point the increased efficiency of the LTC3388 is probably gone.

author: langwadt
author_rank: Super Contributor
timestamp: December 21, 2019, 10:26:07 pm
content: (1.8V*0.0001A)/12V = 15uA so to get 80-90% efficiency the switcher including everything will have to run on a 1-2 uA that doesn't add up

author: eddie1
author_rank: Contributor
timestamp: December 21, 2019, 10:47:48 pm
content: Yes, another reason the LMZ12001 numbers are definitely off. The data sheet claims 92% best-case efficiency and looking at the efficiency curves, that would be at about 0.5A or maybe higher.The LTC3388 numbers, on the other hand, I can believe. That part seems to have been specifically designed for ultra-low-power applications (50mA max output current) and the data sheet has detailed efficiency graphs at microamp-level load currents (1uA, 10uA, 100uA, and so on). And Linear, from what I've picked up, had a reputation for doing some really cool things, so I'm sure their numbers are trustworthy. If only it supported external feedback resistors for adjusting the output voltage like TI's parts tend to do...

author: ahbushnell
author_rank: Frequent Contributor
timestamp: December 21, 2019, 11:27:06 pm
content: Quote from: james_s on December 21, 2019, 08:28:38 pmA linear regulator would be 1.8V/12V=15% efficient at best.

author: David Hess
author_rank: Super Contributor
timestamp: December 22, 2019, 12:31:42 am
content: Switching regulators optimized for high efficiency at low currents either use a burst mode type of circuit to gate the oscillator or a self oscillating hysteretic type of control.

author: james_s
author_rank: Super Contributor
timestamp: December 22, 2019, 02:30:39 am
content: Quote from: ahbushnell on December 21, 2019, 11:27:06 pmSure, but when your load is 100uA the total consumption is still very low. Ideally one would start at a lower input voltage.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 22, 2019, 09:00:37 am
content: There is no fundamental physical reason why a switching converter couldn't be scaled down to such low currents while maintaining efficiency. If you can convert 1A at 90% eff, you should be able to convert 100uA at 90% eff as well; at least in theory.The problem is, you need to scale down everything, you can't do it with an IC designed for currents 1000 times higher. Now, there seems to be little market for such parts, making the problem so difficult.Because all the switching ICs are designed for much higher currents, they will include quiescent losses, gate drive losses, losses driving parasitic capacitances which are higher in a larger design, etc. Practically, the dynamic range of the currents that an IC can provide at good efficiency is limited (say, 10x), and while special low-load modes increase this range (say, to 100x), they won't increase it by many orders of magnitude; efficiency is still poor if the current is say less than 1-2% of the nominal "design" current of such converter IC.As you are already accepting the fact you are overengineering, maybe look at DIYing the whole control thing. The key would be finding the smallest possible CMOS stage to drive the smallest possible inductor, while minimizing quiescent current in the feedback and oscillator circuitry. It could also be some hysteretic self-oscillating thing generating some 2.5V-ish with high ripple, which is then fed to LDO to generate 1.8V.

author: Marco
author_rank: Super Contributor
timestamp: December 22, 2019, 01:28:24 pm
content: If you want to do it with more discrete components I'll recycle my suggestion fromthisthread.So in this case, use an OV-7604-C7 with a MAX9645 to turn the output on/off?

author: ahbushnell
author_rank: Frequent Contributor
timestamp: December 22, 2019, 05:09:58 pm
content: Quote from: james_s on December 22, 2019, 02:30:39 amBut the spec if 12 volts in. The question was efficiency.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 22, 2019, 05:33:49 pm
content: Quote from: ahbushnell on December 22, 2019, 05:09:58 pmYeahbut... the theoretical maximum amount of loss a 100% efficient switcher could save here is a mere 1.02mW. If this is a battery-operated device that needs a long run time then I would instead be looking at a lithium coin cell so that the terminal voltage is a lot closer to the needed supply voltage; dropping 3V down to 1.8V is 60% max efficiency, after all.It's just going to be incredibly difficult to make a buck switcher that uses substantially less than 100uA in operating current. Heck, even a micropower CMOS error amplifier and bandgap reference are likely going to need 10-3uA each, and as the OP already found out, the op-amp will be so slow that the switching frequency ends up below 100Hz.

author: eddie1
author_rank: Contributor
timestamp: December 22, 2019, 07:34:15 pm
content: Thanks, everyone!The application is wall-powered, an MCU-based fan monitoring thing (oversimplification). The 12V supply is from a standard computer power supply (and no, I don't have easy access to the 5V or 3.3V PSU rails). There's no actual need to save nanowatts; it's just a challenge I've set to see how low I can get the circuit down to. (It's just for fun, which is how I can justify going to absurd lengths to try to save tiny amounts of power on a wall-powered application!)Quote from: Siwastaja on December 22, 2019, 09:00:37 amThat's definitely the case. The LTC3388 seems to be the only part on the market designed for this with such a wide input range and, despite being around since 2010, does not seem very popular. It's easy to see why nobody else has spent the R&D money on designing a similar part.Quote from: Marco on December 22, 2019, 01:28:24 pmThis sounds fun; I predict a UPS overnight shipment from Digi-Key showing up Tuesday.Extra benefit I just thought of: probably could share the oscillator output for the (PIC) MCU, which might (would have to test) lower the PIC power consumption a little compared to the integrated RC.edit: actually, the max input voltage of those isn't enough, but I'll see if there are similar parts with higher Vin.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 22, 2019, 09:16:43 pm
content: Okay, so it's a challenge thing... I'd probably start with the Roman Black 2-transistor buck converter:https://www.romanblack.com/smps/smps.htmOtherwise, a hysteretic converter based on a micropower comparator driving a PMOS switch would be worth considering, though as I mentioned before, the switching frequency is going to be really low (generally speaking, you trade operating current for speed in comparators and op-amps).

author: Marco
author_rank: Super Contributor
timestamp: December 22, 2019, 09:33:01 pm
content: Quote from: eddie1 on December 22, 2019, 07:34:15 pmPreferably you run almost everything off the low voltage rail.Would a 1.8 V oscillator capacitively coupled to a high side sub 1V Vgs P-MOSFET work? With a 100+Meg resistor in parallel with a small signal diode from the gate to the rail.

author: mzzj
author_rank: Super Contributor
timestamp: December 22, 2019, 10:09:19 pm
content: Trick is to use very slow switching frequency (hysteretic, burst or whatever control) and you can use reasonably sized(large) switching components with good efficiency.One 100us pulse per year would be really low power yet decent efficiency without the need to scale down every component to nano-size.Brewing the control part is still fun if you want to minimize the power consumption.

author: james_s
author_rank: Super Contributor
timestamp: December 22, 2019, 11:40:42 pm
content: If it's strictly for the challenge then absolutely go for it, I can relate to that. Be sure to keep us updated with the results either way.

author: David Hess
author_rank: Super Contributor
timestamp: December 23, 2019, 12:07:31 am
content: There are a few things which should be done:1. Bootstrap startup from the high input voltage and then disconnect and operate only off of the output voltage. For 12 to 1.8 volts, this immediately reduces the quiescent power by 85%.2. Gate the control circuits in bursts so the only continuously active element is a comparator and reference. A hysteritic design does away with the oscillator.3. Miller capacitance in the power switch is going to hurt because of the high voltage swing at the drain so select the power switch appropriately.4. A low duty cycles means high peak currents. Use a transformer or tapped inductor to bring the duty cycle closer to 50%.

author: eddie1
author_rank: Contributor
timestamp: December 23, 2019, 02:35:06 am
content: Before it was brought up in this thread I hadn't considered the idea of building a buck converter from discrete components (instead of an off-the-shelf buck IC). I would have assumed it would be too complex, but the fundamental circuit looks to be easier than I thought. I was already able to get the basic concept working on a breadboard with a few spare components I have. (I used the pattern generator on my Analog Discovery to generate a 3.3V push-pull clock signal, at a frequency I can adjust, to simulate a PWM output. I wired that up to one side of an inductor, and a capacitor and oscilloscope probe to the other side. No diode was needed since the clock output was set to push-pull. Of course, I wouldn't actually power any load this way; this was just for experimentation.)I still have a lot more reading to do, but this sounds like a fun project. And I have a Joulescope for accurate nanoamp-level current measurements. (Other random thoughts: it would be cool if the PIC could control its own regulator (kind of similar tothis from Roman Black's site; thanks for that link!), though that means a bug in the regulation code could kill the PIC with 12V. I also have some Silego GreenPAK parts that might be good for this kind of application.) It looks like I will be placing another UPS overnight order at Digi-Key tomorrow, just in time to get it on Christmas Eve.Quote from: james_s on December 22, 2019, 11:40:42 pmAbsolutely! I'll open source it if I can get it to work. (Starting off with breadboarding for basic experimentation/prototyping, but once it's more concrete than that I will design a PCB.)

author: Marco
author_rank: Super Contributor
timestamp: December 23, 2019, 03:00:37 am
content: How much does an active comparator and voltage reference add to the sleep mode power consumption of the PIC? Can it compete with MAX964x?

author: mariush
author_rank: Super Contributor
timestamp: December 23, 2019, 03:26:55 am
content: I wonder if you could do something with a super capacitor and a schmitt trigger/inverter/logic something and maybe some mosfet or transistor... and I wonder if this would be more efficient.Charge the super capacitor up to ~2.5v and at that point you disconnect the super capacitor from the 12v input ... a ldo discharges the super cap to 1.8v at which point you connect the supercap back.Or maybe not even use a ldo, just ride it out on the super cap voltage, between 1.8v and 2.5v, your microcontroller or whatever you use shouldn't mind.Or maybe use a plain zener diode to limit to 1.8v-ish? Not sure how well zeners work at such low voltage...

author: eddie1
author_rank: Contributor
timestamp: December 23, 2019, 03:29:01 am
content: Quote from: Marco on December 23, 2019, 03:00:37 amNot competitive at all, going by Microchip's specs on the PIC16LF18324 (which my testing found to be a very power-efficient part for peripherals, ~500nA for a 50% duty cycle 1.5k output). Comparator power-down current is rated at 30 uA typical at 3V and voltage reference is 40 uA at 3V. However, I'll eventually test it myself to be sure because it's unclear exactly what their test conditions were. In the past, including with this part, I have found some of Microchip's more detailed numbers to be either overly conservative or overly optimistic. I'm already going to order the Maxim part to play with, since it does set a very low bar for power consumption (and same with that Micro Crystal oscillator, which is almost certainly the lowest-power external 32.768k oscillator around).(Oh, and Microchip doesn't say this anywhere that I can find, but package *does* make a difference in power consumption on the 14-pin and larger parts at least in the PIC16LF183xx series. SSOP power consumption is lowest, followed by SOIC, followed by DIP; I haven't tested QFN. I tested numerous parts within the family and couldn't figure out why I couldn't reproduce any of Microchip's advertised sleep current numbers, until a mistaken Digi-Key order lead me to accidentally this out.)

author: eddie1
author_rank: Contributor
timestamp: December 23, 2019, 09:39:38 am
content: Looking at it further, how would the MAX964x parts be used here? They have an internal reference voltage of 0.2V. I assume the comparator would be used to turn the switch on when the output voltage starts to fall below the intended 1.8V, so I don't see how the 0.2V reference can be used.I did find a TI part that looks interesting,TPS3840 voltage supervisor. 300 nA typical, 700 nA max (at VDD=1.5V < VDD < 10V). It looks like it could be used as a low-power comparator and has an adjustable reference voltage.The easiest way for me to approach this might be to basically sketch out the control circuit with an MCU and then reimplement the working circuit with lower-power discrete parts.

author: exe
author_rank: Unknown
timestamp: December 23, 2019, 11:28:50 am
content: If you can tolerate variation in output voltage, then you can charge a super-cap with a buck converter, then put buck into sleep until voltage falls below a threshold.

author: Marco
author_rank: Super Contributor
timestamp: December 23, 2019, 01:05:53 pm
content: Quote from: eddie1 on December 23, 2019, 09:39:38 amThe MAX9645 exposes the negative input, so the moment the input falls below 0.2 the output goes high impedance. So a 10 Meg/1.25 Meg divider from the 1.8V rail to the input and a 10 Meg pull up resistor on the output will give you a high output when the voltage drops below 1.8V.

author: eddie1
author_rank: Contributor
timestamp: December 23, 2019, 07:21:29 pm
content: Thanks! That is an obvious solution and I can't believe I didn't think of that. (In my defense, it was 2AM.)The specs on those Micro Crystal oscillators are amazing. Besides the 32.768 kHz oscillator at 300 nA, they have a few 100 kHz parts at 550 nA (OM-0100-C8, OV-0100-C8) and 650 nA (OM-0100-C7, OV-0100-C7). I'd add some to this order to play with but it looks like Digi-Key doesn't stock them. Mouser does though.(Unrelated, but EM Microelectronic, who is the manufacturer of that EM7604 oscillator IC used in at least the Ox-7604-C7 parts, also makes a nice low-power RTC, EM3028, rated at 40 nA @ 3V timekeeping current. Micro Crystal sells a variant packaged with a crystal. I've tested it and found the advertised power consumption to be realistic. EM and Micro Crystal are owned by the same company and tend to repackage each other's parts, but only Micro Crystal parts seem to be easily available at Digi-Key, Mouser, etc.)

author: eddie1
author_rank: Contributor
timestamp: December 23, 2019, 11:56:39 pm
content: The wonderful EFM32 MCUs (for EFM32TG11, which is a Series 1 part:data sheet;reference manual) might be useful here:Voltage monitor:62 nAsupply current at 3.3V! 1.62V lowest voltage threshold. Accuracy isn't specified, but other SiLabs documents say it's calibrated during production, and given the intended use of detecting low battery voltage I'd assume it's fairly accurate. Unfortunately, it doesn't look like this is a PRS producer (which should allow you to toggle a clock output without waking the CPU); it can only interrupt.Analog comparator:306 nAsupply current at 3.3V, plus 50 nA for a 0.625V reference. The bad side: the reference (which is 1.25V and divided), is specified as not being much of a reference: 1.0-1.47V. It may be that this isn't factory-calibrated like the voltage monitor, in which case it may be possible to calibrate it and get acceptable performance. (I might ask SiLabs; they're pretty responsive on their forum.) Response time at the 306 nA level is 3.7 us; if you go up to 30 us, it's 50 nA. This *is* a PRS producer.Voltage comparator: The older Series 0 parts have a voltage comparator peripheral that looks similar to the analog comparator in power consumption and the voltage monitor in purpose. It's also a PRS producer. Unspecified accuracy.EM2 power consumption is rated at 1.48 uA with full RAM retention and RTCC running from the integrated 32.768 kHz crystal oscillator. Dropping RAM retention to 8 kB should save ~200 nA or so, and I'm not sure what disabling RTCC saves. EM4H can go down to 820 nA with RTCC and the crystal. (EM3 is basically EM2 without the 32.768 kHz oscillator.) If PRS is used, that will only work down to EM3. The sleep current is high, but possibly could result in a lower overall consumption by using one MCU for everything.I would love to get to use the EFM32 for this. They're such a pleasure to develop for.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: December 24, 2019, 05:33:15 am
content: Quote from: exe on December 23, 2019, 11:28:50 amThat's a great idea.

author: eddie1
author_rank: Contributor
timestamp: December 24, 2019, 05:54:40 am
content: On the "self-regulating MCU" thought:TheAtmel/Microchip SAM L10/L11 familyis another option. The analog comparator is rated at 51 nA typical (25C), 118 nA max (85C) at 3.3V in its lowest-speed mode. The lowest-speed mode has a propagation delay between 4 (typical) and 12.3 us, which is even better than the discrete Maxim part. The comparators can run while the MCU is in standby mode (500 nA at 1.8V with 4K RAM retention). The internal bandgap reference is accurate enough, 1.086-1.128V (1.1V nominal). It's unclear if there is a separate current draw for the reference voltage or if it's included in the comparator consumption.I have an ATSAML10D14A on hand and will test it at some point.I tested the PIC16LF18324. Total power consumption with just the FVR enabled is about 7.3 uA at 1.8V. With the comparator alone (just comparing two pin inputs and doing nothing, didn't configure it any further) it's about 32 uA. For reference, base sleep current is between 30 and 40 nA, consistent with the spec. I looked at some data sheets in the PIC24, PIC32MM, and the latest ATtiny families, and the SAM is the only family in Microchip's catalog with competitive comparator power consumption.Also, here are some sleep mode numbers for the EFM32TG11 that are missing from the data sheet. No RTCC or any other peripherals enabled. All but one RAM block was powered down. Peripheral retention was disabled and EM2/EM3 voltage scaling was enabled (emuVScaleEM23_LowPower). Those are all the tricks I know of to reduce sleep current on this part.EM2, DC-to-DC enabled, no LFRCO or LFXO = ~950-980 nAEM2, DC-to-DC enabled, LFXO enabled = 1.2 uAEM2, DC-to-DC disabled, no LFRCO or LFXO = 1.17 uAEM2, DC-to-DC disabled, LFXO enabled = 1.46 uAEM4H, DC-to-DC enabled, LFXO enabled = ~500-540 nAEM4H, DC-to-DC disabled, LFXO enabled = ~480-500 nAEM3 is EM2 without LFRCO or LFXO, and I confirmed the power consumption is the same.LFXO is the 32.768k crystal oscillator and LFRCO is the 32.768k RC oscillator. LFXO actually uses less power (you can confirm in the data sheet).These were measured on the EFM32 dev board with the onboard energy profiler (AEM). The dev board supplies 3.3V and has the supporting circuitry to allow the EFM32's integrated DC-to-DC converter to be used. (An ultra-efficient low-load converter right there!) I would prefer to test with a straight 1.8V supply without the integrated converter, but I don't have a breakout board for this. The dev board's AEM is nice because it only measures the MCU power without any of the (dev-related) supporting circuitry.

author: eddie1
author_rank: Contributor
timestamp: December 25, 2019, 03:45:32 am
content: I tested the ATSAML10D14A. The power consumption of the comparator is as rated, but the bandgap reference brings the total up to about 27 uA at 1.8V. Not at all competitive with the Maxim part.The Digi-Key box arrived, so I'll start playing with that.

author: eddie1
author_rank: Contributor
timestamp: December 25, 2019, 10:13:55 pm
content: Random thought:There's little market for off-the-shelf converters with very-low-load efficiency from a 12V input supply, but there IS a market (smartphones and other portable devices) for the same from a lower input supply (~5V or under). As a result, those actually exist.TI has a new IC that claims 80% efficiency for 3.6V-1.8V at 1 uA load current:TPS62840. Operating quiescent current is rated at 60 nA, or 120 nA in 100% duty cycle mode where Vin = Vout. It has an integrated switch but not an integrated inductor. Is there any reason this (or a similar IC) couldn't be used as the control circuit (in PFM mode, at least) by putting a pair of logic-level MOSFETs (source 12V supply) between the switch pin and the inductor? I don't see any reason the IC's output voltage feedback circuit wouldn't work just because the inductor is being charged from a different supply via a MOSFET. Assuming this makes (some) sense, I suspect it will be more efficient than any possible solution with discrete components because it takes advantage of the large sums of R&D investments pouring into power efficiency for portable devices.I imagine PWM mode won't work due to the fact that the converter will be running off its own output (with a starter circuit to start from 12V and cut over to the converter output once in regulation), which would cause the duty cycle to be calculated wrong. While there isn't a force-PFM mode (just force-PWM), the power-save mode, which switches between PFM and PWM based on the load current, should stay in PFM for this.A more ideal solution would be a converter like that without the integrated switch (a controller IC, in other words), but none of the controller-only ICs are particularly low power. That, I assume, owes to the fact that the market for ultra-high low-load efficiency converters -- portable devices -- also demands more integration to reduce the total footprint.edit: Also, TI's efficiency numbers are using a very tiny (2.0 x 1.6 x 1.2 mm) and relatively lossy (116m DCR) inductor. Without the size constraint it should be possible to beat their numbers, at least unless the switching losses from the additional MOSFETs are high.

author: eddie1
author_rank: Contributor
timestamp: December 30, 2019, 12:33:39 am
content: The result of the experiment with the TPS62840 eval board was smoke and a destroyed IC (just starts smoking upon power on now, even with nothing else connected). I'm not sure why; presumably the cause was 12V hitting the IC, but the only way that could have happened is on the VOS (voltage output sense) pin which is connected to one side of the inductor, which should have been kept in regulation by the IC. I know this isn't a proper schematic at all, but I took the eval board, desoldered the inductor, connected to the SW pin: x-to-12V level shifter circuit -> one side of an inductor. The other side of the inductor was connected to the Vout/sense net (actually the same pad as the original inductor). It was meant to be a basic proof-of-concept, so I had a separate 5V power supply for the TPS62840; I didn't try to run the regulator off its own supply.I'm tired of breadboarding so I'm just going to design a power supply board using the Maxim comparator and have it fabbed. That adds a turnaround delay compared to being able to get components and breadboard adapters overnighted from Digi-Key, but it's way less annoying.edit: removed mention of the inverter since that was really just part of the level shifting circuit

author: eddie1
author_rank: Contributor
timestamp: January 04, 2020, 12:53:52 am
content: Is there any issue I'm missing (reason this idea wouldn't work) with using a low-IQ LDO as a reference voltage? The MCP1810 and TI TPS7A02 (new and doesn't seem to be available at distributors yet) are both around 20 nA and would seem suitable. Neither can be powered from 12V and for obvious reasons, couldn't be powered straight from the self-regulating supply, but aDickson voltage doublerlooks like it would be suitable to fix that. That would eliminate the need for the resistor divider (if comparing to a 0.2V reference) and associated losses.There are purpose-built voltage reference ICs as well but none appear to be as low-power as the ones in the Maxim or AD/Linear comparators with built-in reference. Another thought: design a circuit to turn on one of those non-low-power reference ICs (at the appropriate voltage or adjustable) every X (milli)second(s) to charge a suitably-sized capacitor then power down. The only loss would be the leakage currents of the comparator input pin and capacitor, which should be minimal. (Actually, it wouldn't surprise me if that's how the comparators with integrated reference work internally. The external reference ICs all seem to have output ratings in at least the microamps, while a comparator with integrated internal-only reference only has to deal with, at most, a few nanoamps of leakage current.)