Will micro be damaged by negative rail opamp?

[Flyback]

Hi,
Will the micro be damaged by the (non series resistor protected) opamp input?
LTspice and jpeg attached.
The micro has a -3v3 rail, and a 20V rail.
Micro has rail +3v3 to 0V.
Its a TL084C opamp.
What would be the worst circumstance for the micro?
The opamp output current is clamped to some 26mA, as you know....maybe micro can withstand a short blast at 26mA?

TL084C opamp

TL084 data sheet, product information and support | TI.com

TI’s TL084 is a Quad, 30-V, 3-MHz, 13-V/µs slew rate, In to V+, JFET-input op amp. Find parameters, ordering and quality information

www.ti.com

STM32F103V micro (100pin)

https://www.st.com/resource/en/datasheet/stm32f103c8.pdf

The opamp goes into pin 35 of the micro which is labelled as an Analog input. ("PB0/ ADC_IN8 /T3_C3")

 

[schmitt trigger]

A SMT resistor cost is fractions of a penny. And if you employ a 0201 or even a 0402 package, the required board real state is close to negligible.
Ultra cheap insurance. Use it.

 

[ronsimpson]

D2, D3 input protection diodes have a max current rating. Read the data sheet. Or some how (resistor) limit the current. I would pretend the op-amp output goes from rail to rail and pick a resistor based on that.

 

[Flyback]

There is a school of thought that the opamp is a buffer...and that since its non-inv pin is never <0V, then neither should be the output.....though is there something with offset voltages of certain batches?...that when inv and non-inv are both 0V......it could be that non-inv "looks like" it is less than 0V?...so opamp output goes to negative rail?

 

[alec_t]

Do you really need an op-amp buffer? What is providing the PWM signal?

 

[Flyback]

The attached explains it better...yes we need the buffer....its too high Z divider to send several cm's across the board back to the micro.

the attached sim is nearer what we are doing with the circuit........in the attached LTspice sim, after a few ms, the 20V rail goes down as the product is switched OFF......but the neg_3V3 rail stays about neg_3v3...its at this point where the big injection of current goes through the ADC pin 's ESD diodes....because the opamp output goes negative.

I think this is the danger point of the circuit...would you agree?

 

[Flyback]

A SMT resistor cost is fractions of a penny. And if you employ a 0201 or even a 0402 package, the required board real state is close to negligible.
Ultra cheap insurance. Use it.

thanks
[i must confess, i never encountered this before, as i always put a series resistor there...but this board has already been done and there's no room for the eight series resistors that would need adding......the whole board would have to be totally ripped up and started over. (a consultancy actually sent us this as they are overloaded at the mo)]

 

[audioguru]

The opamp output goes to-1.2V when the +3.3V and -3.3V go to 0V.

 

[schmitt trigger]

Haven’t you heard of dead bugging components?
I have been there, done that. Many, many times.

Or take the risk. Who knows, you may be lucky.

 

[danadak]

There are two primary problems with injection current into a CMOS device,
the input diode protection network and latchup. Latchup generally blows the internal
supply bond wires to the die, eg. catastrophic failure.

http://www.eeworld.com.cn/huodong/yunpan/nxpfile/5602f6a2aa3147b37b222acf3fb10e0e13517de1.pdf

https://www.ti.com/lit/wp/scaa124/scaa124.pdf?ts=1696227765458&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTMS570LS3134

https://www.ti.com/lit/an/slya014a/slya014a.pdf

http://large.stanford.edu/courses/2015/ph241/clark2/docs/AN-600.pdf

Regards, Dana.

 

[tumbleweed]

and the statement

The opamp output current is clamped to some 26mA

isn't accurate. The datasheet lists 26mA as the TYPICAL output short-circuit current... it's not the max nor is it "clamped"

Even if it doesn't blow anything up usually injecting a negative voltage causes all sorts of problems with any analog circuitry inside the uC. I wouldn't rely on just limiting the current with a resistor.

 

[Nigel Goodwin]

and the statement

isn't accurate. The datasheet lists 26mA as the TYPICAL output short-circuit current... it's not the max nor is it "clamped"

Even if it doesn't blow anything up usually injecting a negative voltage causes all sorts of problems with any analog circuitry inside the uC. I wouldn't rely on just limiting the current with a resistor.

There are internal protection diodes on a PIC, so it simply needs a suitable series resistor to limit the current.

 

[tumbleweed]

That depends on the device and the pin.
Microchip have a number of app notes addressing this where it is strongly discouraged.

 

[Nigel Goodwin]

That depends on the device and the pin.
Microchip have a number of app notes addressing this where it is strongly discouraged.

And have a great many app notes where they actually encourage such use.

 

[tumbleweed]

... which have been rebuked by the latter app notes.

Of course you're free to do as you like, but it's probably a good idea to warn people if you suggest they do it too.

 

[danadak]

Excellent ap note on injection current :

http://www.eeworld.com.cn/huodong/yunpan/nxpfile/5602f6a2aa3147b37b222acf3fb10e0e13517de1.pdf

Regards, Dana.