Safe to run an MCU requiring minimum 1.8V from a 1.8V regulator? - Page 1
Forum section: electronics / beginners

author: bourbaki
author_rank: Newbie
timestamp: May 09, 2023, 09:55:58 pm
content: Hi,Many microcontrollers require a 1.8V minimum power supply for active operation. For example, Microchip's PIC18F04's (datasheet:https://www.mouser.com/datasheet/2/268/PIC18F04_05_14_15Q40_Data_Sheet_40002236C-2634603.pdf) says:QuoteIt seems that 1.8 volts is also a popular output voltage for fixed voltage regulators, such as Microchip's MIC5317-1.8YMT (datasheet:https://www.mouser.com/datasheet/2/268/MIC5317_High_Performance_Single_150mA_LDO_DS200061-1891237.pdf).If I power this MCU from this voltage regulator then the actual voltage could be slightly less than 1.8 volts due to regulator inaccuracy, trace resistance, and supply power noise. Do the "Standard Operating Conditions" take these conditions into account -- meaning that the MCU should operate reliably up to a few % below VDDMIN -- or do I need to use a voltage higher than 1.8 volts in order to leave some margin?Thanks!J

author: ataradov
author_rank: Super Contributor
timestamp: May 09, 2023, 10:41:27 pm
content: There is some margin and it should be fine to operate with 1.8V nominal. The usual assumption is that power supply is within +/-5%. In case MIC5317 it is +/-3%, which is well within reasonable tolerances.

author: bourbaki
author_rank: Newbie
timestamp: May 10, 2023, 03:41:58 am
content: Makes sense, thanks for the confirmation!

author: coppice
author_rank: Super Contributor
timestamp: May 10, 2023, 03:53:40 am
content: Most MCUs specified to run at 1.8V mean 1.8V nominal. The tolerance they allow varies from device to device. It should be in the data sheet. Watch out for things like brownout and ADC modules. Some data sheets indicate a wide tolerance on the supply voltage, but when you check in detail some modules may not perform properly at the minimum specified voltage.

author: bourbaki
author_rank: Newbie
timestamp: May 10, 2023, 05:08:00 am
content: @Coppice:Thanks for flagging that not all features will work at lower voltages. You said "Most MCUs specified to run at 1.8V mean 1.8V nominal. The tolerance they allow varies from device to device. It should be in the data sheet."Here are two data sheets from two different manufacturers:https://www.ti.com/lit/ds/symlink/msp430f2131.pdf?HQS=dis-mous-null-mousermode-dsf-pf-null-wwe&ts=1683694867210https://www.mouser.com/datasheet/2/268/PIC18F04_05_14_15Q40_Data_Sheet_40002236C-2634603.pdfThey specify input voltage ranges of 1.8V - 3.6V and 1.8V - 5.5 V respectively. I could not find any tolerance information beyond this. Are you suggesting that I shouldnotrun these MCUs off of a 1.8V regulator? Can you show me a datasheet that does specify 1.8V nominal and a tolerance around it so that I can compare?Thanks!J

author: ataradov
author_rank: Super Contributor
timestamp: May 10, 2023, 05:10:14 am
content: It is extremely rare to see tolerances, I can't remember any off the top of my head. This is just too much information, just use common sense.Plus if you start being nitpicky, the accuracy of your measurement setup would also come into play. It will be a never-ending nightmare of overthinking.Plus the voltage is not truly stable, there is some noise. What voltage will you use? RMS? Minimum?

author: Berni
author_rank: Super Contributor
timestamp: May 10, 2023, 05:20:37 am
content: Quote from: bourbaki on May 10, 2023, 05:08:00 am1.8V to 5.5V range IS the tolerance.The MCU is designed to work on a nominal 3.3V or 5V but has a tolerance of working within that range.In theory you need a MCU that can go to say 1.5V so that any tolerance on the regulator, voltage drop on traces, dips from load surges etc.. stay above the minimum operating voltage.In practice the MCU probably works fine at even 1.7V since the values in the datasheet are guaranteed by the manufacturer, so the chips are designed to work a little bit outside that range to make sure they meet spec.For personal one off projects this is fine. For production you want to be careful with this since when you make thousands of boards you might get a unlucky combination of a MCU with a particularly high minimum voltage and a voltage regulator chip that has a its output voltage towards the bottom of the tolerance. So you might get boards that don't work, or even worse are glitchy or work only when hot or cold etc.

author: Miha
author_rank: Contributor
timestamp: May 10, 2023, 05:26:19 am
content: Just read the DS's.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 10, 2023, 06:47:33 am
content: Quote from: Berni on May 10, 2023, 05:20:37 amThis. Very surprisingly, ataradov is wrong this time. The idea about adding some hidden assumption regarding a non-specified tolerance is possible, but definitely not guaranteed. Microcontroller datasheet is like any other component datasheet, with minimum and maximum ratings between which operation is guaranteed. If minimum is specified at 1.8V, then it is the minimum, period. Operating at 1.74V is not specified.Of course,practicallyataradov is right, running it with 1.8V-3% is extremely likely to work in practice, especially because as OP has noticed, 1.8V regulators are very common, so thousands and thousands of designers have looked at the ratings and designed this combination in; I have never heard about anyone having problems with that. You would still be running outside of ratings, but only very slightly so. Given poor availability of parts and difficulty in second sourcing, I would not hesitate to design in a 1.8V regulator if that happens to be the optimal solution otherwise, just realize you are taking a controlled risk.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 10, 2023, 07:08:49 am
content: Quote from: ataradov on May 10, 2023, 05:10:14 amIn presence of ripple or noise, you have to use minimum/maximum voltages of your supply. You have to add the inaccuracy of your measurement setup (although if you calculate using worst-case regulator specs, then measurement is not part of design.)This is electronics design 101, we do this all day long. I personally use Excel to combine worst case ratings of all parts involved (note: for a regulator with external feedback resistor, their tolerances could act at extreme ends, adding to the regulator inaccuracy). If you feel like it's a "nightmare of overthinking", the easiest way out is to add generous safety margins; we all leavesomeinsignificantly small details out and swamp them into orders of magnitude larger generic margins. However, it's important to realize the task for adding those marginsis the designer's job, not the IC manufacturer's. Theymayhave added some extra margin, but also may have not and no one knows how much if any.There is a reason why electronics designers are pretty much needed, and why we are getting paid for what we do.

author: james_s
author_rank: Super Contributor
timestamp: May 10, 2023, 07:27:13 am
content: I'd bet it will work just fine, especially if you choose a relatively low clock speed. IIRC a lot of the 8 bit AVRs say they require 5V to run at the full 16MHz but in practice I've run them at 3.3V a number of times without any issues.

author: nali
author_rank: Frequent Contributor
timestamp: May 10, 2023, 07:42:53 am
content: Quote from: Miha on May 10, 2023, 05:26:19 amYup. The 18F04 DS linked in OP has a whole section detailing supply voltage!

author: Miha
author_rank: Contributor
timestamp: May 10, 2023, 08:21:09 am
content: Also, one should not forget that any LDO has negative and positive spikes in the output voltage when the conditions of the input voltage and load current change. The selected LDO according to LSH has dips of +/-30 mV max. So together with the -3% possible deviation, we get a minimum output of 1.7 V.Also, the use of an external supervisor (possibly, also built into the MCU) will become difficult in this application.Of course, if this is a DIY product for yourself, then can try it, but if you plan to release multiple products, then there may be problems.

author: iMo
author_rank: Super Contributor
timestamp: May 10, 2023, 08:45:26 am
content: Those voltage ranges are typically given for the entire temperature range the device may work at (when not stated in DS otherwise) and clock freq. The temperature is usually the limiting factor when going down with voltage and keeping a clock frequency. Except the potential voltage reg output fluctuation described above the temperature and clock frequency is the most important factor when talking minimal working voltage, imho.

author: bourbaki
author_rank: Newbie
timestamp: May 10, 2023, 01:05:39 pm
content: Thank you all for your inputs. Right now I am just doing hobby projects but I am trying to learn how to design things that would also be suitable for small scale manufacturing in case I decide to do that in a future life.Right now I am working on a battery operated device which, when activated by a button press or via a signal read by polling a low power RF receiver such ashttps://www.ti.com/product/CC2500, will reverse the magnetization of a small Alnico magnet.I intend the micro controller to turn on a boost converter which will slowly (0.1 - 1 seconds) charge a ~150uF Al Polymer capacitor to about 25 volts. Once the capacitor is charged, I intend to discharge it at a peak current of 5-10 Amps through an H-Bridge such ashttps://www.ti.com/lit/ds/symlink/drv8244-q1.pdfinto the magnetic coil.Because of the large current draw when the boost converter is on I intend to use a specialty battery (datasheet:https://www.nichicon.co.jp/_assets/pdf/products/slb/datasheet0811_2108_e.pdf) that drops to 1.8 volts when fully discharged. I only intend to discharge it half way at which the voltage is typically 2.4 volts but I am designing for 2.1 volts.This means that the power rail for my MCU and RF receiver is somewhere between 2.1 volts and 2.8 volts. Moreover the boost converter will put massive ripple on it. I had thought to use an LDO regulator to smooth the ripple but outside of 1.8 volts my options are limited -- particularly because I need low quiescent current and (obviously) low max dropout.Maybe I would do better to just use an RC filter to decrease the ripple going to the MCU and RF receiver and forget about the voltage regulator altogether?

author: Siwastaja
author_rank: Super Contributor
timestamp: May 10, 2023, 02:39:16 pm
content: If you run the boost at high enough frequency, such as 1-1.5MHz, and you definitely should, PSRR of most linear regulators won't be that great for filtering anyway. e.g. for MIC5317, below 20dB at 1MHz, and I bet large part of this number is thanks to Cout and Cin plus R and L of wiring in the test circuit.Boost running in CCM isn't that bad on input, as it's a nice triangle wave (output is harsh, though; just like buck input). By choosing larger L for your boost, you make the ripple smaller, without any extra filters.In any case, what's bad for the microcontroller are those small dips that go beyond ratings. If you have ripple dropping below minimum rating, linear regulator won't do anything to fix that; actually it's the opposite, the regulator just adds drop-out voltage of its own. So I would start with decently designed, high f_sw boost, with large enough L, and large enough input caps separately for the boost converter and the MCU. Then test and measure to see if you have any problem to begin with.

author: ataradov
author_rank: Super Contributor
timestamp: May 10, 2023, 02:52:33 pm
content: Quote from: Siwastaja on May 10, 2023, 06:47:33 amI'm not sure I'm wrong. In practice for a big project, I'd contact support and ask about that.There is no reason to characterize the device down to 1.8 V if you are not going to let it work in a practical 1.8 V system. Nobody is running 1.9 V systems. And if it is only targeting 3.3 V systems, then characterizing it down to 2.7V-3.0 V would make everyone's life way easier and increase yields.I personally take is as a signal that it is designed to work at 1.8 V with typical deviations of the power supplies.For ARM devices this is solved in a different way. All devices that are designed to work at 1.8 V are characterized down to 1.65 V (-10%). Same for the upper limit, it is usually 3.6 V, which is 3.3 + 10%.

author: coppice
author_rank: Super Contributor
timestamp: May 10, 2023, 03:49:07 pm
content: Quote from: Berni on May 10, 2023, 05:20:37 amSome MCUs are designed for a specific voltage, but most modern ones are designed to operate over a wide range. The ones designed for a specific voltage are typically specified to work over a +-5% or +-10% range. When you see a wide range the device is usually designed to be used across that range. As I said before, sometimes peripherals, especially mixed signal ones, don't work over the whole range. Sometimes you need to keep the clock rate below some limit at lower voltages. Flash MCUs often won't self program below some specified voltage. You need to check data sheets carefully. However, the device is expected to be used across that wide specified voltage range. Many MCU applications work at a low voltage in a sleepy state, and crank up the voltage for full performance when woken up. You'll find numerous app notes, especially for ULP MCUs, which advocate doing this. Its what the vendors expect to get the best energy performance from their products.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 10, 2023, 05:19:33 pm
content: Quote from: ataradov on May 10, 2023, 02:52:33 pmNow this I can fully agree with. But rating the product down to 1.8V isan alarmingthing to do, exactly because 1.8V is so common voltage, and it leaves the question: does it work, or does it not? Characterizing the device at 1.65V (for example) is the RTDT, this is what most manufacturers of any components designed to run in real-world 1.8V systems do. Even Microchip did that, in this very same datasheet: the product is rated to 5.5V and not just 5.0V (the usual regulator voltage). Why would they change their logic at low side? This suggests it might not be reliable with 1.8V regulator after all. I would contact Microchip if I were going to use it at 1.8V in large scale production. Or just take the risk.

author: ataradov
author_rank: Super Contributor
timestamp: May 10, 2023, 05:55:23 pm
content: I've got a clarification on this and I was wrong here. 1.8v is a hard stop, so this device is not compatible with typical 1.8v systems and voltage regulators.The reason for the low voltage is possible operation from the batteries/supercaps, where the voltage may dip lower than a typical voltage.

author: bourbaki
author_rank: Newbie
timestamp: May 10, 2023, 06:32:59 pm
content: Thanks ataradov for the confirmation on that. Good to have some closure! I'll take a look at those ARM MCUs specified down to 1.65V.Thanks Siwastaja for the tips!QuoteMy boost converter is unusual because its output is held low by the large output capacitor for at least 10s of milliseconds. I'd like the flexibility to also power it from a higher impedance power source in which case this could grow to hundreds of milliseconds. I need the boost converter to limit its current input to a programmable level (20mA - 100mA) during this entire "soft start" time. I'm also limiting myself to parts I can hand solder (no BGA or QFN). My current prototype uses an Analog Devices LT1316 (datasheet:https://www.analog.com/media/en/technical-documentation/data-sheets/1316f.pdf) because it comes in an SO-8 package and has a programmable input current limit (fixed time off regulation).Unfortunately, for this voltage ratio it runs only indiscontinuousmode and with my parameters at a frequency of ~ 100 kHz.Is this a dumb design -- should I open a new thread about how to do this better?QuoteEven at 100 kHz I don't expect more than 100mA * 10 uS = 1uC to be drawn by the converter within a 10uS cycle so 20uF of capacitance (e.g. derated 47uF X7R) at the input should still limit my input voltage spikes to 50mV. Given that my power rail is >= 2.1V this should be well within spec for the MCU.I was worried that the MCU and RF receiver might not like a choppy ~ 50mV peak-to-peak 100kHz AC but maybe that is just paranoia? If I forgo the voltage regulator, I could still make a low pass filter by using large bypass capacitor(s) for the MCU and RF transceiver for a total capacitance of ~25 uF after derating and then inserting a 5 ohm resister in front of both. This should have a cutoff frequency of ~ 1.3 kHz and my peak current draw on these parts is far less than 50 mA so the resistor won't drop out more than 250mV. But maybe this is unnecessary?

author: ataradov
author_rank: Super Contributor
timestamp: May 10, 2023, 06:47:21 pm
content: PICs are probably more resilient to voltage ripple, but 50 mV is quite a lot. This is enough to make most ARM MCUs to not work correctly. I would find a way to filter that.And RF parts for sure don't like unstable supply, it would show up in the transmitted signal and may get you in trouble with FCC. For receive-only operation it would likely affect sensetivity.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 11, 2023, 01:39:13 pm
content: Remember that a boost converter supplying to an ideal output capacitor initially consumes infinite current, and no amount of control logic can do anything; the diode is in conduction as long as output voltage meets input voltage! Only after that event boosting starts. Therefore you'd really want a different topology, e.g. buck-boost or sepic. Or at very least boost with some primitive linear current limiting for the initial surge.

author: bourbaki
author_rank: Newbie
timestamp: May 11, 2023, 06:36:33 pm
content: Thanks Siwastaja.My plan for now is to keep the boost converter connected to my main power rails, but with the boost controller in sleep mode. This way all the inrush current will happen on startup before the MCU and RF transceiver have even powered on. I'm gambling that < 250 uF of capacitance won't damage the battery (I cannot find specifications on this).This keeps the parts count low and the topology simple although it is wasteful of power. The output capacitor is rated to leak < 160uA at 35V. Hopefully it will be less than this at 3V! Once I can get this simple design to work then I'll start working on a more elegant version and will doubtless have more questions for this forum!Thanks for mentioning the SEPIC converter topology. This is new to me and might indeed be just what I need to improve the design!I'm more worried right now about getting the MCU and RF chip startup sequence right given that they will be behind a low-pass filter and see very slow VDD voltage rise. The RF chip in particular does not like this and has a complicated reset sequence you have to use to make it work. I'm tempted to just put it behind a MOSFET and let the MCU turn it on when ready!Regards,J

author: Siwastaja
author_rank: Super Contributor
timestamp: May 11, 2023, 07:12:05 pm
content: Electrolytic capacitors leak orders of magnitude less than specified maximum. Maybe they leak to spec (or even exceeding it) after a long storage time unused, but they will reform within minutes to hours of use and settle to nearly zero leakage current. I remember measuring leakage in some MASSIVE like 10000uF 100V capacitor bank made of ages old electrolytics and it was below the reading threshold of the multimeter, 1µA! Didn't have a datasheet but I guess rated value would have been many milliamps.I would not worry about it, sounds good to me. As long as the boost chip has decent current limitation and high enough f_sw, I don't think power quality becomes a problem.