[HN Gopher] Cursed circuits: charge pump voltage halver
       ___________________________________________________________________
        
       Cursed circuits: charge pump voltage halver
        
       Author : surprisetalk
       Score  : 47 points
       Date   : 2025-12-02 18:47 UTC (4 hours ago)
        
 (HTM) web link (lcamtuf.substack.com)
 (TXT) w3m dump (lcamtuf.substack.com)
        
       | amelius wrote:
       | A simulator will certainly not like the floating center terminal.
        
         | doug_life wrote:
         | I just threw it into Micro-Cap and it surprisingly didn't throw
         | any errors with the floating node. https://imgbox.com/riKCyWI5
        
           | magicalhippo wrote:
           | 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.
        
       | ajross wrote:
       | 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.
        
         | jacquesm wrote:
         | 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.
        
           | dwattttt wrote:
           | 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?
        
             | addaon wrote:
             | 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.
        
       | addaon wrote:
       | 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...
        
       ___________________________________________________________________
       (page generated 2025-12-02 23:00 UTC)