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IO pins current

Intellectual 260 points

Replies: 5

Views: 2709

Hi,

I have some simple questions which i can't find a clear answer for:

1. As i understand it's possible to draw/sink 2/4/8 mA from/to single GPIO pin. But that 18mA - is it meant for current sourcing, sinking or both? I reed that on sinking the low level raises above 0V, so when the sourcing is allowed, does the high level lower then drawing 18mA of current?

2. Is the sinking allowed in digital output mode too, or only in open-drain output mode?

3. What's the limit of total IO pins power consumption (for LMS3748) If i multiply the IO pins count (61) with 8mA i get almost 0.5A. That looks pretty awesome number. From datasheet's electrical maximums table i found this sentence: "Maximum current per output pins - 25 mA". This one, on the other hand, looks kind a small.

4. I don't dare to test it, but what happens when exceeding IO pin current limits - for example when shortening pin to ground or Vdd in digital output mode? You'll probably suggest me not to do that, but i'm interested whether the whole chip burns or only one pin.

Thanks in advance..

5 Replies

  • From 3748 datasheet:
    "With the GPIO pins configured as 8-mA output drivers, a total of four GPIO outputs may be used to sink current loads up to 18 mA each."

    Further - we're advised not to use more than 2 hi-current outputs "per side" and to use no more than 4 total - per ic package.

    Your questions:
    1) 18mA is current sinking - spec says voltage will rise to ~1.2V. Most always sourcing is less capable than sinking.

    2) Suspect that digital output would allow full sink.

    3) We've killed many an IC by running many/all pins "to the max!" Review LMI's conservative guideline - 4 at a time max!

    4)We did this often - rarely on purpose - and found some variation in the destruction. Depends upon density and routing w/in ic. Some chips continued to function on ALL but our destroyed output. Now if you're running a motor - and get a "shoot-thru" we've turned the chip into a "white star" - and launched it into our dropped ceiling. We "popped the top" without paying the usual "de-top" fee.

    Suggest that you investigate darlington ic packs for your current drive needs. These come in 7 & 8 channel versions - dip packages & provide higher voltage tolerance and up to 500mA Sink per output.
    Search for ULN2003 to start. Again - don't run all outputs at max together.

    We find it best to just use ARM I/O for their unique, specific functions & use latch or shift register to generate simple I/O. Added functions always helps your product sell...

    Post edited by: cb1, at: 2008/10/28 07:06
  • cb1 is correct. I also want to add that you can source 18mA on up to 4 pins total. The reason there is a limit is because of the way that the power rails are designed. If you have more than 2 "per side", you can overstress the rails and cause premature failures in devices due to electromigration.
  • Thank you for answers. I got clear answers for questions 2 and 4. The others:

    1) So, the conclusion is:
    18 mA is the maximum current of one pin for both sourcing and sinking (of course with 1 pin per side limit).

    Correct me if i'm wrong.

    3) Total IO pins current question:

    cb1 wrote:

    We've killed many an IC by running many/all pins "to the max!" Review LMI's conservative guideline - 4 at a time max!


    Do you mean 8 mA or 18 mA by "max"?

    I still can't understand what is this "Maximum current per output pins I - 25 mA" in Electrical Characteristics Maximum Ratings table. As Eric just said, it is allowed to source 18 mA from up to 4 pins. It sums up as 72 mA. 74 mA exceeds 25 mA three times and chip should be burning when believing what the datasheet says. It's written not only in LM3S3748 datasheet, but in other large pin count devices datasheets as well, so it shouldn't be a typo.

    So what is this 25 mA?
    Is it supposed to be "Maximum current per single output pin"?
    Or is it "Maximum current per output pins of one side"?

    The last option seems to be the closest to truth as 25 mA almost matches the instructions suggestion not to exceed 18 mA on one side.

    Please explain it.

    Post edited by: totenhose, at: 2008/10/31 15:14
  • I really can't & won't give you an absolute answer - more importantly I urge you/others NOT to squeeze "every drop" of power dissipation from any ic. Keep in mind there are chip to chip variations, power supply variances, and the likelihood of wide ambient temperature swings.

    I presented a safe alternative - use of devices specifically intended for higher power control. (Darlington 7/8 ch device)
    Other devices exist which sink in the 100mA per output range.

    Never use a chip at/close to its absolute maximum ratings. Saving the cost/size of a dedicated, higher current device will be quickly eroded by even one field failure.

    You're a smart guy - I would focus instead on adding unique features - making the product more user-friendly - and maximizing robustness & reliability. IMHO squeezing "every drop" should only be considered if/when your product achieves significant volume - and the 50-60 cents saving per unit starts to make sense. Good luck...
  • The maximum ratings list parameter limits which, if exceeded, will likely damage the device. It is not meant that the part should be operated at the maximum limits.

    The section titled "Recommended DC Operating Conditions" says the following:

    For special high-current applications, the GPIO output buffers may be used with the following restrictions. With the GPIO pins configured as 8-mA output drivers, a total of four GPIO outputs may be used to sink current loads up to 18 mA each. At 18-mA sink current loading, the VOL value is specified as 1.2 V. The high-current GPIO package pins must be selected such that there are only a maximum of two per side of the physical package or BGA pin group with the total number of high-current GPIO outputs not exceeding four for the entire package.

    This boils down to: 18 mA max for any single pin, a max of 2 of these on any side of the package, and a max of 4 of these total for the entire device.

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