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Comment by bsder | Hacker News Reader

And if your device spends a lot of time asleep, linear regulators may beat switching regulators by a LOT.

Leakage current can dominate your design, and switching regulators often have lousy leakage current specs (to be fair, so do a lot of old school linear regulators). Furthermore, switching regulators often have to "spin up" while a linear regulator is just sitting there ready to go as soon as your MCU switches on.

In addition, switching regulators tend to be designed for higher currents and tend to have terrible efficiency at small currents (< 1mA). If your circuit uses a very small amount of current even when active, a linear regulator may be superior even for constant-on systems.

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Nowadays you can pretty easily find switchers with sub-100nA quiescent current + leakage, e.g. TPS62840 (of course, that's assuming you're at room temperature... FETs at high temperature are all crap for leakage). I note you get 80% efficiency at 1uA out on this device with a 3.6V to 1.8V stepdown config; beats the LDO by a lot, particularly if you spend a lot of your time at uA load currents.

But spin-up time... Yeah that still sucks :)

Are there chips doing that but buck-boost ? I know there are for "normal" current range, but I haven't found anything super low power

Use case: I want to just have a solar panel + supercap into microcontroller for IoT stuff so I want to suck that cap dry and avoid batteries for temperature range reasons.

Maybe TPS61094 or something similar?

https://www.ti.com/lit/ds/symlink/tps61094.pdf

Oh, wow, it's even in stock, thanks. I was looking for something like that some time ago but back then only BQ25570 really fitted (and was overkill and a bit more pricy for that)

Issue with a lot of these and switching regulators is off chip inductors, which take up space. In small form factors, SOCs etc, you need to use charge pumps and LDOs for internal analog/mixed signal voltages.

I built all the low voltage MCU based electronics for my team's solar race cars in college. Power efficiency is a huge deal for a solar powered race car. One of the first pieces I designed was a standalone switching regulator PCB with a 3 pin interface, then made many dozens of them. I then used them in all of my other boards.

Years later after I graduated and left the team, I heard that the folk that took over went through and replaced all the switching regulators with linear ones. I asked them why, and they said someone told them the quiescent draw was lower for linear regulators. I asked them if it was really worth the significantly reduced efficiency dropping from 12V to 3.3V, and they didn't seem to understand what I was talking about. Oops!

> One of the first pieces I designed was a standalone switching regulator PCB with a 3 pin interface,

This sounds like a useful thing to learn from. Do you have the schematics?

The eagle file is probably rattling around somewhere on my hard drive. It was back in 2005 or so, though. I had very little idea what I was doing back then. I just followed the reference schematic from the switching IC's datasheet. It had the IC, an inductor, and some size 1206 resistors and capacitors.

One can purchase $0.15 TI parts have <0.4mA quiescent current and output 3A. These things draw so little under no/low load circumstances that my cheaper 4-digit readout benchtop supplies cannot detect any draw.

https://www.ti.com/lit/ds/symlink/tps563201.pdf

0.4 mA quiescent current is huge for very low-power designs though. If your system must spend the majority of its time consuming very little power, that level of constant current draw is a non-starter.

Well, typical $0.15 linear regulators will have similar or higher quiescent current

TPS7A20 is 6.5uA, $0.10/ku. TPS7A05 is 1uA, $0.19/ku. It's not that hard to do better than 400uA!

And if you don't mind coughing up when it counts... TPS7A02 is 25nA, $0.45/ku. Triple the price, but more than four orders of magnitude better leakage.

If money is no object, there are some interesting chips out there. The LTC3335 chip[0] is a buck-boost that claims to have a 680nA Iq. Not as low as the TPS7A02, but it looks like it could make up for that in efficiency if your design does anything other than sleep.

[0]: https://www.analog.com/media/en/technical-documentation/data...

I was looking for one for one of my project but this is above "I could have just 10 years worth of batteries instead of that" level of pricing

Do very low-power designs also involve "awake" current of 3A?) Asking unironically.)

Maybe 300mA order of magnitude is more common than 3A, but it's not inconceivable. Lots of designs wake up, burst some RF data at a few hundred mA transceiver load for a few tens of microseconds, then go back to sleep for another few seconds. There are some long-interval scientific instruments that need high-speed ADCs or DACs on battery power in remote locations, which might get into higher current than a few hundred mA. Some deep space crafts can sleep at very low power and run their transmitters at comparable currents. It's very situational.

Older cell phone standards used to burst to about 2.3A upon initial connection to a tower before the tower told the handset to dial it back. I don't know about modern cell phone standards.

400uA isn't "low"--that's roughly speaking a fully-awake MCU running at 10MHz.

Linear regulators also have a start-up time, and its usually somewhere in the datasheet. All feedback loops have some kind of settling response time depending on the bandwidth of the loop, and you'll see that behavior both on start-up and if there's any kind of disturbance, like the load changing.

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