Can STM32U585 embedded Op Amp could withstand a 150mA pulse current?

[YFANG.2]

Hello

I use the embedded Op Amp in STM32U585VIT3Q to charge a 47uF capacitor.

The input of the Op Amp is connected to the output of DAC. The voltage level is 3V.

So you can see when the Op Amp begin to charge the cap, the peak current is quite large.

I would like to know whether the Op Amp or the Micro could withstand this pulse current

 

Thanks in advance!

[LCE]

That is generally a bad idea: if it doesn't, you destroy the complete MCU.

So put in some current regulation or limiting, even if it is only a serial resistor (e.g. 100R -> 30mA).

[YFANG.2]

Hello LCE

Thanks for your feedback. Following your suggestion, I would like to add a 220R resistor. It could limit the peak to 13.6mA. I believe this would help.

Another question is about the load current of the Op Amp in the datasheet.

I would like to know what is the load current, is it max output current by Op Amp?

For me, I don't think it is the max output current. In the test, I could see the Op Amp could output a 150mA peak current. And if the output current of the Op Amp is only 0.5mA. I would say this value is really very low.

[Peter BENSCH]

Iload(max) is the maximum output current of the opamp, which has been specially optimised for ultra-low-power applications. You must not forget that the STM32U58x is an ultra-low-power MCU and is particularly suitable for battery operation. 

However, it is no problem to buffer the high output resistance of the opamp e.g. with a voltage follower and thus be able to drive arbitrarily high currents.

Regards

/Peter

[YFANG.2]

Hello Peter

Thanks a lot for your feedback. So is it OK for the embeded Op Amp to output a pluse current, start from 13mA, drop to 0 at 220ms. Maybe 3 or 4 times every 10s. I know the Op Amp has the capability. I wonder whether this will damage the embedded Op Amp

Thanks in advance!

[Peter BENSCH]

No, this is not permitted because you would exceed the maximum value of 0.5mA and the function of the STM32U585 can no longer be guaranteed permanently, even if you were able to draw larger currents from the opamp in tests.

As already written, you should use an external voltage follower as a buffer, which you can also connect directly behind the DAC.

Regards

/Peter

[YFANG.2]

Hello Peter

It is quite clear that the embedded Op Amp could not output current higher than 0.5mA.

The external Op Amp could be a solution. But I want to know if I turn on the DAC buffer, is it OK to provide the pulse current up to 13mA?

Thanks in advance!

[Peter BENSCH · Accepted solution]

The short answer to your question is: no.

The detailed one: according to the data sheet, the smallest load resistance that the DAC buffer can drive is 5k, resulting in a current of 0.62mA at VDD=3.3V (with buffer, the output voltage only reaches max VDDA-0.2V). With a series resistor of 5k to your 47µF capacitor you keep this maximum load, but have an additional time constant of ~235ms, where the capacitor voltage needs about 1200ms to reach the setpoint.

If you use an external voltage follower, you should switch off the internal buffer, because then you can use the full voltage range of the DAC of 0...VDDA.

Regards

/Peter

[YFANG.2]

Hello Peter

Thanks a lot!

It seems the external follower is the only solution.

Yun FANG

[contact details deleted by moderator]

[Peter BENSCH]

Correct.

But please do not use the email function to reply to the community thread, because then all your contact details of the sender are publicly visible to everyone worldwide. So for safety's sake, I'm deleting this info, OK?

If the problem is solved, please mark this thread as answered by selecting Select as best, as also explained here. This will help other users find that answer faster.

Regards

/Peter