STM32 Vref powerup secuence - Page 1
Forum section: electronics / general

author: AlexVlc
author_rank: Newbie
timestamp: April 15, 2021, 10:47:59 am
content: Hi all,I want to use an external reference voltage on the Vref pin but this is a problem because the powerup of the main Vdda regulator is different of the reference voltage regulator (one starts before the other), in the worst case one can give 3.3V while the other hasn't started yet giving zero volts, this situation only lasts a few milliseconds but enough to damage something.In the STM32F427 manual only appears that VDDA-VREF < 1.2 V, this means that if on powerup Vdda = 3.3 while Vref is starting and it's only 1V the device will me damaged? Or Vref should start before VDDA?.. this sounds rare.I've damaged devices is the past for not respecting this (STM32F373 have a very very special power up secuence because the Sigma Delta ADC).Thanks

author: peter-h
author_rank: Super Contributor
timestamp: April 17, 2021, 08:51:15 pm
content: I would make the reference source powered from the CPU VCC.The Vref is used for the 12 bit ADCs & DACs only, AFAIK, and they aren't that precise. Just a TL431 is likely to be good enough for many applications (with some FS calibration to take out the ~1% error, if needed) and there are way more accurate references one can get for a few $.

author: AlexVlc
author_rank: Newbie
timestamp: April 20, 2021, 05:25:48 pm
content: Hi peter-h,Yes, I know this ADC's are not precise, but, the reason to use the external Vref is because the rest of the board (the analog) is supplied with other regulator, not the same regulator that supplies the MCU, this is to avoid noise problems. Because I'm using different regulator for this I want to set Vref from this regulator to get the best precision, and this is the problem with Vref and Vdda.Thanks

author: harerod
author_rank: Frequent Contributor
timestamp: April 20, 2021, 07:47:30 pm
content: @AlexVlc: There is not much leeway for discussion. Always respect the power sequencing spec from the datasheet. Not doing so might damage your device, or worse, might lead to unreliable operation.How to implement the power sequencing depends on your actual design. Maybe a couple of passives will suffice, maybe you have to rely on integrated controllers for power sequencing.Another related topic would be specified power supply (and other signals, e.g. nRESET) risetimes. Design for both targets and check on the prototype.Getting either wrong may result in some nasty debugging. Don't ask why I know that.If a certain device doesn't require specific handling, the manufacturer will probably proudly announce that on page 1 of the datasheet (a.k.a. "the marketing section").

author: julian1
author_rank: Frequent Contributor
timestamp: April 20, 2021, 11:05:45 pm
content: Are you able to give the stm32 control over the power-up of the supply rails for the rest of thecircuitry (that includes vref)?Edit. scratch that. I misunderstand.

author: peter-h
author_rank: Super Contributor
timestamp: April 21, 2021, 07:01:30 am
content: The usual way to ensure one rail rises not above another is to put a diode between them, which becomes forward biased when the undesirable sequence occurs.It may have to be a schottky diode. I use normally ZHCS400 for this. The ~200mV Vf prevents turning on the input protection circuitry which most chips have.However, an analog regulator fed from the CPU +3.3V will generate a very clean output, especially if appropriate filtering is provided to keep any input noise within the regulator bandwidth.

author: AlexVlc
author_rank: Newbie
timestamp: April 22, 2021, 02:00:40 pm
content: @harerodThat's the problem, no power sequencing for VREF in the datasheet, only VDDA-VREF < 1.2 V, but this is not a sequence, this is a rule. The datasheet of the STM32F4 have near 2000 pages, no "marketing section" with specifications.Thanks

author: AlexVlc
author_rank: Newbie
timestamp: April 22, 2021, 02:02:11 pm
content: @peter-hTo put a diode from one to other regulator is a bad idea because not all regulators have reverse current protection, you can damage the device.Thanks

author: harerod
author_rank: Frequent Contributor
timestamp: April 22, 2021, 04:25:01 pm
content: Quote from: AlexVlc on April 22, 2021, 02:00:40 pmSo, why even post a question? You have found clear instructions regarding your design's behavior.Quote from: AlexVlc on April 22, 2021, 02:00:40 pmYou wish. The complete original docu is easily 10000 pages. As for "marketing section" - the first page of the datasheet tries to sell the product.

author: AlexVlc
author_rank: Newbie
timestamp: April 23, 2021, 07:43:33 am
content: @harerodI posted this question because this specification needs explanation.. on powerup "VDDA-VREF" may be lees than 1.2 if VREF needs for time to set, normally are different the power up rules than the normal operation, on some devices is not a problem but other devices like STM32F373 will be damaged.