4
\$\begingroup\$

A circuit uses the RN5RT33A voltage regulator to generate the 3.3V rail. According to its datasheet the abs. max output current is 150mA, typical output current is 60mA.

There's an add-on board that feeds on that 3.3V rail and draws additional current from the regulator. I've measured the approx. current used by the add-on board and it was why of 310mA max and around 200mA most of the time.

How is that possible? It seems that would exceed the ratings of the regulator, however the circuit seems to be working fine.

enter image description here

\$\endgroup\$
7
  • 8
    \$\begingroup\$ "Absolute maximum" doesn't mean they promise it will fail at that level. It means they don't promise it won't be damaged at that level. \$\endgroup\$
    – The Photon
    May 8, 2019 at 15:52
  • \$\begingroup\$ @ThePhoton However it seems to me risky to be running over that, sometimes as much as twice that. \$\endgroup\$
    – user733606
    May 8, 2019 at 15:59
  • 2
    \$\begingroup\$ Yes, it is risky. But you didn't ask if it was a good idea, you asked how it was possible. \$\endgroup\$
    – Elliot Alderson
    May 8, 2019 at 16:00
  • 1
    \$\begingroup\$ @user733606, yes it's risky and you shouldn't do it. But there's no promise it will fail right away. It could appear to work for minutes or days or hours or weeks before failing. \$\endgroup\$
    – The Photon
    May 8, 2019 at 16:01
  • \$\begingroup\$ Usually thermal death is the main cause of failure, if this had a heatsink or good thermal relief one could see it surviving past the absolute maximum ratings, which were probably tested with no heatsink. \$\endgroup\$
    – Voltage Spike ♦
    May 8, 2019 at 16:41

2 Answers 2

Reset to default
5
\$\begingroup\$

The value given in the datasheet as "absolute maximum" is the maximum value at which the manufacturer guarantees the device will not break. That doesn't mean it will break, just that it may break and if it does it's definitely your fault, so they're not legally liable for damages caused.

Most of the time, devices can survive conditions outside their absolute maximum, as long as they aren't too far outside it and don't stay outside it for too long. But you shouldn't depend on that, and you definitely shouldn't depend on them actually doing what they're meant to do under those conditions.

\$\endgroup\$
2
  • \$\begingroup\$ Worse, it may not "break" instantly. It may break at 150.1 mA if left in that condition long enough. And you may not be around to put out the fire when it finally burns out. \$\endgroup\$
    – scorpdaddy
    May 8, 2019 at 16:12
  • \$\begingroup\$ Good point, I should add that to the answer. \$\endgroup\$
    – Hearth
    May 8, 2019 at 16:41
1
\$\begingroup\$

@Hearth answers the question correctly, I just want to point out that on the RN5RT33A's datasheet it does say there is a "current limit circuit".

This will not limit the output current to a known value, merely that in case of a Vout short to ground the IC will only output 30mA of current in order to protect itself from immediate destruction.

\$\endgroup\$
2
  • \$\begingroup\$ I tried to find a datasheet for this thing and couldn't! Thanks for digging one up. \$\endgroup\$
    – Hearth
    May 8, 2019 at 19:13
  • \$\begingroup\$ @Hearth i had to use a round about way to find it: i searched the part on octopart, then followed their links to a few distributors, and finally found the datasheet on Rochester Electronic's website. \$\endgroup\$
    – Hatman
    May 8, 2019 at 20:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.