Cursed circuits: charge pump voltage halver

Summary

In the spring of 2023, when this Substack had only a handful of subscribers, I posted a primer on voltage adjustment in electronic circuits. The article opened with a brief discussion of linear regulators, and then promptly threw them under the bus in favor of more efficient charge pumps and inductor-based topologies.The basic charge pump architecture — a voltage doubler — is quite elegant and easy to understand. It’s also far more common than many people suspect: the circuit can be constructed directly on a silicon die, so it shows up inside quite a few digital chips, from modern op-amps to MCUs. If you weren’t a subscriber back in 2023, or if you don’t have a photographic memory for random blog articles, a conceptual diagram of the pump is shown below:The operation of a rudimentary charge pump.In the panel on the left, we see a Cout capacitor that’s perched on top of the positive rail while a “flying” capacitance Cf is charging from the power supply. The charging process produces a voltage that’s internal to the component: we can unplug Cf, put it in our pocket, and then hook it up to another circuit to power it for a brief while.In the second panel (right), we see the second part of the cycle: Cf is disconnected from the supply and then hooked up to the terminals of Cout. This action transfers some of the charge from Cf to Cout, up until the voltages across the terminals of the capacitors are equalized. After several of these roundtrips, VAB should approach Vsupply. Of course, VBC is also equal to Vsupply; it follows that the voltage between A and C must be the sum of the two, or 2 · Vsupply.In other words, the circuit is a voltage doubler; the repeated motion of Cf ensures that the charge in Cout is continually replenished if we connect any load between the points A and C. There will be a bit of voltage ripple, but the amount can be controlled by sizing the capacitors and choosing the operating frequency to match the intended load.Naturally, practical charge pum...