Now, this is a great Digi-Key article on diodes, in particular I find the coverage of Schottky diodes most interesting. Schottky diodes have a low forward voltage drop which makes them great for use in power supplies, for protecting against reverse polarity, and protective diodes in inductor/motor circuits. Also included is a great schenatic on how to use a voltage regulator chip, LM2595. Alas, that’s only a buck switching power supply, it doesn’t have boost capability. Nonetheless, it’s a high frequency switching regulator, which allows for smaller sized filter components.
20191127/https://www.digikey.com/en/articles/techzone/2019/nov/the-fundamentals-application-of-zener-pin-schottky-varactor-diodes
20191127/https://www.digikey.com/product-detail/en/texas-instruments/LM2595SX-ADJ-NOPB/LM2595SX-ADJ-NOPBCT-ND/3440112
Okay, so I think I’ve found a few good candidates. First of all, though, some design considerations on the Raspberry Pi Zero power supply. Although you can run the Raspberry Pi Zero off of 3.3 V power, keep in mind (at least to be safe) that the limit of 1 A of power consumption still applies. In other words, that means that you can only distribute about 66% of the maximum power when running 3.3 V power into the 5 V connector, effectively limiting the max power consumption to “660 mA at 5 V” (3.3 Watts). Yeah, that’s good enough to run the Raspberry Pi Zero at full CPU load plus Raspberry Pi camera (500 mA at 5 V), a good handful of LEDs (max 242 mA at 3.3 V), but basically nothing else. But, of course, you can’t run USB devices anyways unless you give a full 5 V power supply.
Point in hand, if you plan to consume a lot of power from your Raspberry Pi Zero board assembly, you’ve absolutely got to run off of a full 5 V, not 3.3 V.
So, let’s review some of the good PMICs I’ve found. The point in hand to keep in mind here when searching is that you generally want to search for a boost converter. If you need any buck functionality, often times you can achieve that adequately with simple linear voltage regulator circuits as you typically will not need to buck much voltage off of the supply… yeah, but don’t take my word, the Raspberry Pi Foundation recommends you use a good switch-mode power supply without linear waste heat style step-down conversion. Yeah, looks like there actually is some cleverness on how to do proper switched buck conversion even with a boost converter IC, the Linear Technologies specification sheet seems to show a good example.
So, here is for the best 3.3 V boost converter, surface mount package. Also claims to be able to output 5 V.
20191127/https://www.digikey.com/product-detail/en/texas-instruments/TPS613221ADBVR/296-50502-1-ND/9685641
Here is a second pick for a 5 V boost converter. Alas, it points straight into the same data sheet! Ah, I see, there are different part numbers for different output voltages.
20191127/https://www.digikey.com/product-detail/en/texas-instruments/TPS613222ADBVR/296-50503-1-ND/9685642
Here is a first pick for a 5 V boost converter, max 600 mA output. That’s lower than ideal, but good enough for Raspberry Pi Zero + Raspberry Pi Camera. By comparison, all of the Texas Instruments chips have pretty good current output capabilities. A second version of the chip is configurable to 3.3 V output.
20191127/https://www.digikey.com/product-detail/en/linear-technology-analog-devices/LT1302CN8-5-PBF/LT1302CN8-5-PBF-ND/889560
Regarding thermal shutdown, all of the PMICs referenced contain thermal shutdown protection. The LM2595SX-ADJ/NOPB additionally contains overcurrent protection.
Okay, so frm my recent search so far, some careful analysis, and reviewing some of my older notes, I think I’ve learned a lot of lessons thus far.
First of all, there is no need to shy away from configurable, variable output voltage PMICs. It is pretty easy to configure them to provide a fixed output voltage by means of a voltage divider resistor network using fixed resistors values. And indeed, if you truly want to be able to arbitrarily tweak the output voltage, you could connect a variable resistor with voltage divider configuration in place of fixed voltage divider resistors. The most important consequence of properly understanding this fact is that you can drastically widen your search on Digi-Key and compare a much wider selection of available PMICs that can suit your needs.
Second, it appears to be difficult to find PMICs that provide both buck and boost functions, and when you do find such PMICs, they tend to need to make compromises to be able to provide both functions. Or, they are new designs that only come in surface-mount packages. If you can think really carefully about your power supply approach, about what you really need to optimize, then you can opt to prefer to buy whichever of buck or boost provides better optimization for the common case, and wire up simple but less efficient circuits to handle the uncommon case.
UPDATE: Many early PMICs have three operating modes: buck, boost, and inverting. The inverting mode is a buck-boost converter, and by far the simplest one at that.
Third, if you really want the best PMIC available, you might need to concede with some surface-mount soldering. But if it is only on one part in your system, that’s not too bad a price to pay.
Fourth, the toroidal inductor “donut rings” have not actually disappeared from modern mobile devices! Take a look at the photos of this Adafruit “PowerBoost 1000 Basic - 5V USB Boost @ 1000mA from 1.8V+” board. It uses a standard fair Texas Instruments PMIC, which you may know typical such circuits require an inductor. Beyond the PMIC, there are many standard fair supporting resistors, tantalum capacitors, one transistor, and two LEDs, all very recognizable as such. But, you’ll notice that there is a large squat “tin can” on the board. Can you guess what that is? What else could it be, other than the required inductor, sneakily packaged into a “tin can”?
20191127/https://www.adafruit.com/products/2030
Likewise, take a real careful look at the Raspberry Pi Zero schematics. You’ll see that there are indeed two inductors wired up to the switch-mode power supply PAM2306AYPKE, labeled L1 and L2. Now, look around real carefully at some photos of the Raspberry Pi Zero. You’ll see the corresponding labeled components as what looks like rectangular “tin cans.” Yep, those are inductors packed away inside a metal can of shielding, the large toroidal inductors, which were already small compared to what came before them, have been further miniaturized an packed away in a metal shield. Now you could easily miss that they even exist on the circuit board.
20191127/https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/Raspberry-Pi-Zero-V1.3-Schematics.pdf
20191127/https://en.wikipedia.org/wiki/File:Raspberry-Pi-Zero-FL.jpg
Fifth, revisiting my previous notes on Adafruit switch-mode power supply boards for easy battery power off of AA batteries, etc. Most of the boards aren’t all that great because they can’t supply the full 1 A of power ot the limit of the Raspberry Pi’s power supply. Second, only the best one that can supply to the full 1 A limit, the PowerBoost 1000 Basic, can also boost up to 5 V from a battery voltage down to 1.8 V. But when you think about it, this isn’t actually that much better than wiring directly to your AA batteries. Why? Some simple voltage comparison equations can explain this easily.
The Raspberry Pi Zero can run down to a voltage supply of 2.9 V on the 5 V power input, or 58% of a nominal 5 V power supply. PowerBoost buys you the ability to run down to 1.8 V, or 36%. Yeah, although that is an improvement, to be honest, assuming voltage varries linearly with remaining battery charge (it doesn’t), PowerBoost only buys you 1.5x running time over wiring directly to your AA batteries. That’s not even better by a factor of two. And you’ve got to shell out about $15 to get that board. You’re better off wiring directly to either 3 AA Alkaline batteries or 4 AA Nickel Metal-Hydride batteries. 4 AA batteries would be more ideal for getting more run-time and better run-down of the full battery capacity. If you really want to use 4 AA Alkaline batteries, a buck PMIC is not a bad investment, or even just a linear voltage regulator that wastes the extra voltage for the period of time that it lasts, if you want to fly in the face of the Raspberry Pi Foundation’s recommendations.
Is there any justification for the PowerBoost? Yes, if you want to power a USB device that is finicky and will not run unless it gets a full 5 V power supply, then you really do need the PowerBoost.
Okay, so armed with that knowledge, let’s try again on the search for my ideal PMIC. Oh, wow, this one is pretty good. Well, I mean… at first glance, if I primarily want a buck converter, but with some ability to also boost for the sake of 5 V USB devices via the “inverting” configuration, this works great. In terms of pure Raspberry Pi Zero use, it breaks even with powering straight from batteries. This also supports current limiting, so a polyfuse can be obviated.
20191127/https://www.digikey.com/product-detail/en/linear-technology-analog-devices/LT1172CN8-PBF/LT1172CN8-PBF-ND/891731
Let’s go searching for more information. Surely, Digi-Key has one of those blog articles to better explain buck-boost power supplies, don’t they? Yes, indeed they do.
20191127/DuckDuckGo digikey buck boost power supply design
20191127/https://www.digikey.com/en/articles/techzone/2012/apr/buck-boost-design-solutions
Alas, the nice H-bridge buck-boost design in the newer devices is pretty much only available in surface-mount packages. Luckily, the simplest inverting buck-boost design can also be made to work quite well with the older series PMICs.
20191127/DuckDuckGo digikey inverting buck-boost
20191127/https://www.digikey.com/en/articles/techzone/2015/aug/using-an-inverting-regulator-for-buck-boost-dc-to-dc-voltage-conversion
Such as this one, for example. Yes, an almost great find. This one runs down to 2 V and can supply 750 mA of switch current… which means that the load can be 375 mA. Okay, bummer, but almost great. Lots of great documentation on how to set it up in the inverting design too, and plenty good stocking. Yes, it is a bit expensive, as are all the PMICs. It has built-in current limiting, which may obviate the need for a polyfuse.
20191127/https://www.digikey.com/product-detail/en/texas-instruments/LM3578AN-NOPB/LM3578AN-NOPB-ND/32487
Okay, so there’s another lesson I learned that you must beware about, looking further into the data sheets for the switch-mode power supplies. If you do use higher frequency switching, you must also be more careful about the length of your wires connecting your PMIC to its external inductor and capacitor components. Keep the length of the wires as short as possible, and use wide traces. Especially, we’re talking small components and short distances if we’re using a surface-mount PMIC. That means that the highest switching frequencies are basically off limits when doing basic novice electronics work. For this reason, there is a benefit to lower frequency switching power supplies: they can be built on a breadboard, though this comes at the expense of larger components, possibly incurring more noise and less efficiency.
This is where if you do want to use a high frequency switch-mode power supply, it may be beneficial to buy one of those pre-built Adafruit boards, even though it does come at a higher price. But that’s the price you pay for buying lower-volume manufactured boards.
Do you really need a PMIC for a switch-mode power supply? Yes, you do, if you are powering integrated circuits or the like. Take a look at the Wikipedia article on “Joule thief.” The Joule thief is a very simple circuit for a switch-mode power supply, but it results in flickering of the output. The various techniques to improve the output quality ultimately result in a lot of complexity that is better expressed in an integrated circuit package than with discrete circuits.
20191127/https://en.wikipedia.org/wiki/Joule_thief
What’s the difference between a PMIC switching regulator and a DC-DC converter? A PMIC requires other components for a complete switch-mode power supply, whereas DC-DC converter basically gives you the full power regulation module: PMIC, inductor, capacitors, protective diodes, and all. With a DC-DC converter, it may be designed so that it will not be damaged if there is no load connected. However, there are of course disadvantages to a DC-DC converter, mainly that the commercially available modules cover a more limited selection of voltage/power options than what is possible when designing your own power module. Furthermore, designing your own power module allows you to integrate the same components on a smaller amount of board space and shorter vertical height.
That being said, I’ve found a selection of some good 5 V and 9 V DC-DC converters to include here for representation.
5 V:
20191219/https://www.digikey.com/product-detail/en/recom-power/R-78AA5.0-1.0SMD-R/945-1044-1-ND/2256550
9 V:
20191219/https://www.digikey.com/product-detail/en/xp-power/ITQ2409SA/1470-2859-5-ND/5225652
20191219/https://www.digikey.com/product-detail/en/recom-power/R-789.0-0.5/945-1040-ND/2256220
20191219/https://www.digikey.com/product-detail/en/recom-power/REC5-1209SRW-H4-A/945-1874-5-ND/2318781
20191219/https://www.digikey.com/product-detail/en/xp-power/JCD0512D09/1470-1886-5-ND/4488266
Note that a switching controller is another step forward in customizability than a switching regulator: the switching FET is external to the chip. Therefore, they can handle higher currents, but they require more complex external circuit design to do so.
20191220/DuckDuckGo dc-dc converter versus pmic
20191220/https://electronics.stackexchange.com/questions/174892/difference-between-dc-dc-switching-controllers-and-regulators-and-converter
Point of Load converter? That is a DC-DC converter that is designed to be used directly at a load, rather than transferred over a distance. I’m not sure what exactly makes these special compared to a regular DC-DC converter, will they still work okay if you use a PoL converter for powering a larger circuit?
20191221/DuckDuckGo point of load converter
20191221/https://electronics.stackexchange.com/questions/231325/what-is-a-point-of-load-converter
UPDATE 2020-03-05: Okay, okay, searching yet again for the ideal DC-DC converter modules. And I’ve found the ideal 5 V module, except that it is a BLGA surface mount package. If only I can get the right adapter, it would be great. But 25 BLGA pins, why so many?
20200305/https://www.digikey.com/product-detail/en/recom-power/RPM5-0-1-0/945-3250-1-ND/9644702
Second up, narrower voltage range and lesser current supply. But unlike the previous one, this one boosts from below 5 V, not just bucks from above 5 V.
20200305/https://www.digikey.com/product-detail/en/recom-power/REC3-0505SR-H1/945-1458-5-ND/2304706
Here are some 3.3 V converters that go from 4.5 V to 9 V, in case you need more 3.3 V current than the 800 mA that the Raspberry Pi gives you.
20200305/https://www.digikey.com/product-detail/en/tdk-lambda-americas-inc/CC1R5-0503SF-E/445-2444-ND/920404
20200305/https://www.digikey.com/product-detail/en/tdk-lambda-americas-inc/CC6-0503SF-E/445-2478-ND/920438