Cursed circuits: charge pump voltage halver
lcamtuf.substack.com52 points by surprisetalk 6 hours ago
52 points by surprisetalk 6 hours ago
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...
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.
Inductors are large, expensive, hot, and unreliable.
Not on modern high-frequency switchers.
GaN stuff can be 99%+ efficiency. The frequencies are multiple MHz which shrinks the inductors significantly--sometimes allowing PCB based coils (See Anker 120W teardowns).
I’m with you, I’m not sure the volume or cost would be less once you factor in capacitors that are high enough quality for the application.
The datasheet mentions low profile a lot. That does make sense as one can make a flat, high quality, capacitor. Making a flat high quality inductor is harder and probably more expensive and likely consumes more volume overall. I can imagine some applications where being flat is important, like the back of a panel.
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.
A simulator will certainly not like the floating center terminal.
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.
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.
I was trying to figure out where the cursed part comes into it; I assume it's the switching, that typical circuit analysis just doesn't involve discrete states that are swapped between?
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.