How many EEs know this basic thing? - Page 1
Forum section: electronics / general

author: terrance
author_rank: Contributor
timestamp: May 08, 2024, 09:28:44 pm
content: A device can't reach multiple maximum ratings at the same time, and ratings have their conditions.eg.LM1117 ratings800mA,15VTJ 125degCDoes not mean1. It can work at 15V input and regulate 800mA output at the same time.2. I can regulate 800mA at 125C ambient temperature

author: Andy Chee
author_rank: Super Contributor
timestamp: May 08, 2024, 09:55:20 pm
content: Isn’t that why we study the device data sheet’s “safe operating area” specification?

author: Sensorcat
author_rank: Regular Contributor
timestamp: May 08, 2024, 10:01:56 pm
content: The "absolute maximum ratings" section of a semiconductor datasheet tells nothing about how to operate the device. It tells that you have to expect destruction if you exceed any of the limits. In order to find the limits for correct operation, you have to look at all the other sections. The tables usually specify the conditions for each specification value in one of the columns. There's no magic in this, the datasheets follow the same conventions industry-wide, but you must be familiar with the way specifications are given, otherwise:

author: Someone
author_rank: Super Contributor
timestamp: May 08, 2024, 11:05:19 pm
content: I see you've been working with similar colleagues to me. Even ignoring the footnotes and * against all the specifications.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 09, 2024, 12:14:21 am
content: Also applies to scope probes. 600V at DC doesn't hold at 100s of KHz. Poof!

author: Whales
author_rank: Super Contributor
timestamp: May 09, 2024, 02:40:49 am
content: "Reaching" maximum ratings is misleading.If you exceed maximum ratings: permanent chip damagewilloccur, preventing normal function when conditions return to normal again.If you are less than maximum ratings: permanent chip damage or misbehaviourmayoccur.

author: Someone
author_rank: Super Contributor
timestamp: May 09, 2024, 05:17:23 am
content: Quote from: Whales on May 09, 2024, 02:40:49 amI wouldn't put it that certainly as binned/selected parts can work indefinitely outside the "ABSOLUTE MAXIMUM". But AD do clarify their interpretation:https://www.analog.com/en/resources/analog-dialogue/raqs/raq-issue-50.html

author: Berni
author_rank: Super Contributor
timestamp: May 09, 2024, 05:26:31 am
content: That is NOT how you are supposed to use the Absolute maximum ratings!The purpose of that table is to set limits for each parameter where exceedinganyof them could cause permanent damage to the device. If your design causes the device to operate close to those parameters then you fucked up by designing it wrong. They give you this table just so you can make sure you can avoid these numbers (even for short peak pulses).The actual numbers you want to work with are in the table labeled Operating conditions. These are the numbers that are characterized by the manufacturer to be within spec. It puts limits on where you are allowed to operate the device for long periods of time and expect it to work correctly.For linear regulators you can't just take the max current. Not only does it depend on the input voltage, but it even depends on your PCB layout. This is why they specify a Tjmax and the thermal conductivity of the package. You have to calculate the thermal conductivity of your PCB, add it to the package conductivity, calculate the max ambient temperature inside the box, use that to calculate the Watts of disipation available, calculate the voltage drop.... and only now can you calculate the max current for your specific design. The datasheet gives you the numbers so that you can do these calculations.EDIT: But in practice you don't actually calculate this to exact numbers every time. You just take rough approximations to make sure the LDO is far from Tjmax. it doesn't matter if it runs at 50 °C or 90 °C at your design current. As long as it is not dangerously close to Tjmax

author: elektryk
author_rank: Regular Contributor
timestamp: May 09, 2024, 08:24:03 am
content: Quote from: Andy Chee on May 08, 2024, 09:55:20 pmBeside SOA sometimes there's also "power derating".

author: ebastler
author_rank: Super Contributor
timestamp: May 09, 2024, 08:52:25 am
content: To get back to the OP's question in the thread title, I sure hope the answer is "all of them".

author: pdenisowski
author_rank: Super Contributor
timestamp: May 09, 2024, 09:55:47 am
content: On a similar note: I made an entire video explaining why max power supply output does not equal max output voltage * max output current.And then when some people still weren't convinced, I wrote a MATLAB program to demonstrateEdit: we also turned this into a webpage:https://www.rohde-schwarz.com/us/products/test-and-measurement/essentials-test-equipment/dc-power-supplies/understanding-derating-curves-_256009.html

author: Gyro
author_rank: Super Contributor
timestamp: May 09, 2024, 11:41:15 am
content: Quote from: ebastler on May 09, 2024, 08:52:25 amAny that you would hire anyway!The thread title and post read as if the OP has just read their first datasheet and made a 'discovery'.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 09, 2024, 01:22:54 pm
content: Quote from: terrance on May 08, 2024, 09:28:44 pmFirst thing that came to mind was high voltage relays. Worse, as companies are bought and sold, marketing and managers strips details from the datasheets that EE's require. Don't get me started....

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 09, 2024, 08:49:36 pm
content: TLDR; RTFM.

author: terrance
author_rank: Contributor
timestamp: May 09, 2024, 09:40:40 pm
content: Actually it's not about read or not, it's about how people understand how components work.I saw too many circuit deigns ignoring some fundamental limits....too many "why my 800mA IC turned into smoke at 200mA?"

author: terrance
author_rank: Contributor
timestamp: May 09, 2024, 09:43:44 pm
content: didn't I mention 'Reaching "MULTIPLE" maximum ratings' ?

author: terrance
author_rank: Contributor
timestamp: May 09, 2024, 09:46:11 pm
content: My job was wiping these asses for a long while... And they always say "this should be fine, within specs".

author: terrance
author_rank: Contributor
timestamp: May 09, 2024, 09:50:32 pm
content: Sorry I did not mean "absolute maximum ratings", I should say "normal operating range"

author: terrance
author_rank: Contributor
timestamp: May 09, 2024, 09:53:36 pm
content: Thanks! I didn't mean "absolute maximum ratings". Just too many colleagues and some EEs just keep ignoring limitations they don't "care".

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 09, 2024, 09:57:07 pm
content: Quote from: terrance on May 09, 2024, 09:43:44 pmDid you not see multiple answers responding to how to deal with "SAFE OPERATING AREA". While many devices do not explicitly define a safe operating area, any EE worth his salt knows that operating under conditions different than those shown as characterized is terra incognita, and requires knowledge of the details of how devices work, what things can limit their operation, and operating environments ranging from heat, to light to EM fields and more that influence both operation and life.Is your question just a way of finding out how many others have learned the hard way? I am sure that in some sense or other many of us have. One of the ways to distinguish good engineers from bad is how often they repeat mistakes, and for those who don't repeat them, how broadly they apply the lessons learned. Unfortunately there are the other kind. I once saw documentation where an engineer repeated the same mistake 29 times, destroying that many copies of what was at that time a rare and expensive part. Never stopped himself, someone else saw what he was doing and put an end to it. Multiple lessons learned in that situation by folks around this guy, things about how to find and detect such stupidity earlier, and how to control the scope of its impact.

author: ebastler
author_rank: Super Contributor
timestamp: May 09, 2024, 10:01:59 pm
content: @terrance -- I see you are new to the forum. When you reply to a post which is a few messages back in the thread, it is advised to use the "Quote" link in that post, rather than the "Reply" link.This will automatically insert a quoted copy of the post you are replying to, allowing readers to understand which message you are referring to. And it avoids the impression that you are mumbling to yourself in a long series of posts...

author: Sensorcat
author_rank: Regular Contributor
timestamp: May 09, 2024, 10:06:24 pm
content: Quote from: Gyro on May 09, 2024, 11:41:15 amAs a supervisor of students doing their 1st larger project, you can see the traces of these discoveries in their reports. When they focus on something not worth mentioning for an entire page, it's a 'discovery'.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 09, 2024, 10:20:48 pm
content: Quote from: terrance on May 08, 2024, 09:28:44 pmThis question is at Arduino hobbyst level, copy&paste, magic happens.I guess it's your first steps in design so pretty understandable, there's a vast land out there!It's the Pmax and Tjmax what limits the operation area.A typical regulator won't be able to cope with more than 0.5-1W just naked, it will overheat.If you're able to keep it chill with a proper cooling method, you still have to take care of Pmax, the heat the package can transfer to the outside world is not infinite, past this it will overheat internally, even if you attach it to the Eiffel tower during freezing winter.

author: Berni
author_rank: Super Contributor
timestamp: May 10, 2024, 08:07:07 am
content: Quote from: terrance on May 09, 2024, 09:43:44 pmYes, it is normal for all of the rating in the "Operating conditions" to apply simultaneously (Unless specifically mentioned in the note to apply under some specific conditions)You can reach up to the maximum on any of those specs, as long as you don't also exceed the any other specs. This is because not every circuit designer will be running the device under the same conditions. Some need a LDO to drop 30V to 5V, some only do 6V to 5V. It doesn't make sense to limit the designer who only needs to drop 1V to the current to the current of a 30V drop.This is the same as buying a car with the following specs:Top speed: 210 km/hTowing capacity: 1400 kgThen hitching up a 1000 kg trailer to the car and flooring it up a 15% grade hill. Yet the car only managed to get to 110km/h despite flooring it all the way and not even being at max towing capacity. Does that mean the top speed of 210km/h is bullshit? Why would they sell you a car that claims to do 210km/h if it can't do it in all conditions? Such a rip off.

author: pdenisowski
author_rank: Super Contributor
timestamp: May 10, 2024, 09:13:42 am
content: Quote from: Berni on May 10, 2024, 08:07:07 amThat's a fantastic example - thanks!!!(Although I'm going to have to change it to mph and pounds / tons before I can share it with my friends and coworkers)

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: May 10, 2024, 12:42:01 pm
content: Quote from: Berni on May 10, 2024, 08:07:07 amUsed twice as much fuel as the fuel consumption sticker too