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NDorn.1
NDorn.1
Associate II
June 16, 2021
Question

Input/Output currents from STM32F072RBT6-MCU for driving LEDs.

  • June 16, 2021
  • 2 replies
  • 1460 views

Hello,

we build a breadboard configuration with 20 LEDs. The LEDs have a current consumption of 20mA per LED. It worked properly. Now I read in the datasheet something about a maximum injection current of 25mA of all I/Os (screenshot is attached). So we should be a lot over the top. But why does it work then without any problems for a longer time? Do we damage the MCU or did i misunderstand the datasheet?

Thanks for your advice

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2 replies

TDK
TDK
June 16, 2021

You should look for current source or current sink capabilities, depending on how the LEDs are hooked up. Injected current is something else and not relevant here.

0693W00000BbAzvQAF.png 

Regardless, 400mA is way outside of the specification, so you'll need to rethink your scheme, or accept that the chip may die prematurely.

I'm surprised it even works at 400mA.

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Andrew Neil
Andrew Neil
Super User
June 16, 2021

"But why does it work then without any problems for a longer time?"

Have you done a complete characterisation over all operating conditions?

How long is "a long time" ?

There's a big difference between "no noticeable problems" in one single example under benign conditions on your bench - and a product that it guaranteed to work under all possible conditions throughout its entire life.

Datasheet limits tell you under what conditions the product is guaranteed to work as specified - that doesn't mean that it will fail completely & immediately the moment you exceed any 1 parameter.

What it means is that you are into the realms of undefined behaviour ...

As @TDK​  suggests, you are over-stressing the chip and it will likely die prematurely.

You may well also find that some other characteristics are out-of-spec - especially at extremes of temperature, etc ...

A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
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