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Cursed circuits: charge pump voltage halver

86 points by surprisetalk

by addaon

2 subcomments

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...

by summa_tech

0 subcomment

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.

by kazinator

0 subcomment

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).

by tgsovlerkhgsel

1 subcomments

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).

by chasing0entropy

1 subcomments

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.

by ajross

1 subcomments

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.

by amelius

1 subcomments

by PunchyHamster

0 subcomment

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.