Cursed circuits: charge pump voltage halver
(lcamtuf.substack.com)80 points by surprisetalk 16 hours ago
80 points by surprisetalk 16 hours ago
The '7660 is good for low-power and is my go-to DIP-8 part when I need a half or double voltage supply on a breadboard.
But, why use those parts?
These circuits take a lot of parts to do a job that you can do with modern high frequency stuff with a lot lower cost and parts count.
The normal point of a capacitive doubler is either to give you a voltage you need without a lot of extra parts count (often negative) or to generate a very high voltage.
The app notes and data sheets of related parts suggest that the target application is large conversion ratios, where the duty cycle in an inductive converter is close to 0 (or 1). That forces tradeoffs, like a lower switching frequency (lower efficiency), a larger inductor (more weight and/or cost) or very short T_on for one of the FETs (lower efficiency because transition times become important). So you can use the charge pump as the first stage of a hybrid converter to get a higher system efficiency.
My recommendation to anyone who finds themself stuck in this corner of the design space is to consider a tapped inductor converter.
I did a tapped inductor boost last year to take 3V input to 80V output (at not-much output current, I forget exactly what it was but it was mostly a bias voltage; also, the actual output voltage was DAC-set and could be quite low, so the loop dynamics were unpleasant). It was definitely annoying to wrap my head around, and very annoying to select the inductor (Würth has a nice OTS series, at the usual Würth prices; HVM would likely want a custom or semicustom design) but it just plain worked the first try and continued working through the usual stress tests and also the unusual stress tests of the Very Expensive Load™ getting itself Very Expensively Killed™ (for non-power-supply reasons). I was really happy with that converter, that kind of step-up ratio isn't easy and it just worked.
Is this really even a little cursed? It's a perfectly nice device, and in fact has been manufactured as a chip before. Enpirion has made this under the name of EC2650QI 6A Voltage Divider:
https://cdrdv2-public.intel.com/632833/ec2650qi-datasheet.pd...
Enpirion's first products - integrated-inductor DC/DC converters - were limited to fairly low input voltages. This allowed them to run very fast with the technology of the time; they needed to run fast, because the integrated inductor was not very large. But this was severely limiting Enpirion's market: if you wanted to make something 5V-powered, they were great. But a PC motherboard application with 12V input?
This device was the answer: convert 12V to 6V with this switched capacitor halver, and then use other Enpirion parts. I don't think it was super successful, though, because at this point you were no longer winning on board complexity by using integrated inductors.
It seems as if you could get a range of voltages at the point between C1 and C2 with a closed negative feedback loop that controls the duty cycle of the flying capacitor.
If you charge the bottom cap more and the top one less, you can jack the voltage toward the power rail.
A buck as well as boost-buck converter could be produced without inductors.
Indeeed, I found an article about exactly this: https://www.allaboutcircuits.com/technical-articles/boosting...
The article references the LTC3265 IC, whose datasheet says "The LDO output voltages can be adjusted using external resistor dividers" (connected to the ADJ pins).
If I remember correctly, some kind of highly efficient (and low cost) voltage halver (not sure if this design or a different one) is AFAIK used with the PPS ("programmable power supply") protocol to let phones charge efficiently:
2x the battery charge voltage is requested from the power supply, e.g. 8.6V if the phone is trying to apply 4.3V to the battery. This way, the phone doesn't need to run any complicated and heat-generating voltage regulation, just the halver, while still being able to request more than the standard 5V over the cable (allowing it to draw more than 15W of power over a standard cable).
Fascinating design I haven't tried, I have made inductor based designs but a pure capacitor design combined with a high speed mos might make for a fun micro psu design.
These types of switching circuits are very common inside ASIC where the high speed isn’t an issue, you don’t need to move all that much charge, and one can’t easily (if at all) support inductors.
I think I (a long time software nerd) am finally getting over the hump with analog stuff. The article says the circuit is complicated and hard to understand, yet I got it instantly. Feels sort of like learning a musical instrument and realizing that one of the early pieces you struggled with is easy now.
FWIW: the didactic trick of imagining the floating capacitor "carrying" charge from one "place" to another was really good. That's not the way most treatments talk about charge pumps, and I think it's a lot cleaner.
It's not complicated. It's just very basic capacitor behavior. If there is a tricky part here then it is in the bit that is glossed over: the switches. But congrats on getting it! Analog is fun, you can get incredibly complex behavior out of a handful of components.
Yep. The state where the capacitor is fully floating relies on non-obvious details. Recall that a (real) transistor has a rated maximum voltage that it can block. But when the (ideal) transistors are opened, there's no defined voltage relationship between (either side of) the floating capacitor and the rest of the circuit; so if analyzed at this level, its not clear if the transistors are within spec or not. You need to at least think about the parasitics to convince yourself that this is sane... or build it with real parts and see that it works.
Indeed. That's why the MAX232 is such an impressive little chip, it was the first time I found an on-chip charge pump in the wild. One of the more interesting uses for charge pumps is loss-free balancing of battery cells. I really like it because contrary to resistive balancing it works with the charge already in the cells by redistributing it.
> It's not complicated. It's just very basic capacitor behavior.
All circuits are just very basic circuit element behavior. In fact a charge pump is a decidedly counter-intuitive thing, up there with things like "why a long-tailed pair makes a differential amplifier" or "how tf does a buck/boost regulator work?".
This is like looking at a 30-line function implementing a FFT and announcing "it's just very basic C code".
I don't know about that. I was building inverters from scratch on intuition when I was 17 or so and I started with electronics properly when I was 11 reading library books, taking stuff apart and putting it (or something else) back together again.
Compared to an FFT, which would require a lot of math insight it seems to me a charge pump is trivial because charging and discharging is what capacitors do, it is the most basic thing you can do with a cap. In programming it would not be the equivalent of an FFT, but more like the equivalent of putting a value in a variable and expecting that value to still be there a while later.
Now, I came into software from hardware, to me software is having an infinite parts box so what's trivial to someone that started in hardware is probably entirely different from the view a software person has of the hardware world. Which is one reason why analog is such a barrier to programmers: they tend to look for the place where the state of the system is stored, rather than that the whole circuit is a continuous function of its input and what happened before. Getting analog circuitry to behave predictably can be quite tricky because of that and just hanging a probe of it will influence the circuit.
Resistors are easy, for the most part grade school math. Capacitors harder but not a lot harder, high school math, some basic integration required. Coils are where it gets more interesting and difficult math wise. And of course parasitic components are everywhere and can make your life harder, especially at higher frequencies or, in the case of single pulses shorter rise and fall times. A piece of wire is a resistor, capacitor and a coil for free all at once. Stripline takes advantage of that fact but without tooling it is next to impossible to work out the math and get predictable results (again, for me, I know people that can do this intuitively).
Multi-pole filters, high gain amplifiers, clean oscillators, those rise to the level of that FFT (for me).
The long tailed pair I agree with, that's a puzzler.
If you don't have one yet (and I assume you do, but just in case): get a scope. They're pretty cheap nowadays and a good scope will do wonders for such insights. I only got one when I was an adult and finally could afford it and it was an absolute game changer. If I had had one earlier I would have gotten a lot more mileage out of my time, especially when troubleshooting broken hardware. You don't need a fancy one, 100 MHz dual trace is more than good enough but that's table stakes now.
One big insight for me is that hardware 'is' and software 'does'. You don't tell a piece of hardware what to do, you design it to 'be' something and a computer simply (ok, maybe not so simply) needs to be told what to 'do'. This took me a long time to really grok after making the jump from hardware to software.
Could be also done with 2 switches and some PWM closed loop control. Which you probably want to do anyway as the halving won't hold under load.
I just threw it into Micro-Cap and it surprisingly didn't throw any errors with the floating node. https://imgbox.com/riKCyWI5
I can't recall if it was Falstad or which other simulator code I read, but it had "connect this one terminal to ground via multi-Gigaohm resistor for stability" sprinkled throughout the code for capacitors and similar components.
A relevant part that changed my view of charge pumps is the LTC7820 [0]. This is an inductorless charge pump that can be used as a an unregulated voltage doubler or halver... at 500+ W and 98%+ efficiency. I used to think of charge pumps as designed for generating bias voltages where the actual power is quite small... but this shows that they scale quite well. (There's also the LTC7821 that combines the unreglated inductorless halver of the '7820 with a regulated, nominally-2:1 buck to give a regulated 48V -> 12V converter with some impressive efficiency numbers.)
[0] https://www.analog.com/media/en/technical-documentation/data...