Does high voltage on LM339 output cause a problem?

[tyilgin]

Hi all,

I am trying to detect High side current control with LM339.
I think I should use Scheme1 to reduce the comparator's output to the level of the mictocontroller.

However, since I use a lot of this process in my project, I'm thinking of using the Scheme2 style in order to both take up less space on the card and not have extra delays.
I am aware that the output of the LM339 in schematic 2 is not a pull-up resistor. I'm also undecided whether the absence of this resistor would be a downside for the LM339. Because it says in the datasheet that the output current should be Min.6mA, but while researching, I found many example schematics designed with 500uA (5v/10k). I'm a little confused about this too.

The LM339 datasheet says "Output voltage max.36v" but in my application it produces a voltage of around 281v.
Although there is a lower value (36v x 20mA=720mW == 281v x 0.3mA=85mW) in terms of power, unfortunately I am not educated enough to decide whether this usage will cause problems.

Can I have your opinion on this please

If it is not applicable, if it can be made usable with minor changes, I would be happy if you could help.

Thinking that I can benefit from the zener diode, I also thought of using a 270V zener diode in series with the 1M5 resistor in Scheme1, but I gave up because I could not find it in a small size.

 

[tyilgin]

No, but you can look at the output voltage and see if it exceeds the devices rating.

Thank you for your interest

--- Updated Jul 29, 2022 ---

Hi,

Datasheets for LM339 and LM2903, etc., group of comparators show simplified or quite complete schematics of internal structures: NPN open collector or NMOS open drain output.

LM339 output device is an NPN bjt, so a pull-up is necessary for the comparator's high-z output (when In+ < In-) to have a positive voltage.

Surely chancing a PD calculation as the reasoning for violating a voltage-limited device's voltage limit is wonky maths. For example, I'll bet you can't use a CD4000 or SN74xx at 100V and much less at 300V so long as you limit the current to a tiny amount.

When In+ > In-, the comparator will try to sink current by becoming a low impedance path to 'ground'. The internal open collector NPN will ideally only have (I_Rpull-up x R_ce) voltage across its emitter-collector - will that be what the datasheet says to expect (a few hundred mV at most) or will the 40V to 60V maximum Vce of the internal NPN get fried by trying to sink 300V?

Which begs the next question: If your idea works, and only a few mV to a few hundred mV develop across the internal NPN and no voltage spikes damage it, where will the other 299.9V disappear to, or not disappear?

Last, Schmitt triggers, whatever, bla bla bla, and the supposed confusion and a somewhat frivolous attitude - hysteresis, hysteresis, hysteresis... put some hysteresis on the comparators, it's easy to understand why, for God's sake, man! If you don't know what hysteresis is, here's a quick introduction:

Figures 3 and 4

https://www.analog.com/en/analog-dialogue/articles/curing-comparator-instability-with-hyisteresis.html

Has a calculator and brief explanation

https://www.allaboutcircuits.com/tools/hysteresis-comparator-calculator/

Thank you for your interest

The example of the link I gave in message #16 is already telling about this issue.

 

[FvM]

Referring to your latest schematics, voltage translation in #7 can basically work (despite of discussed KSP voltage rating), #9 does not work, it misses a suitable voltage shift solution.

There are additional faults in all schematics
- connecting the same Vref to low side and high side comparator. High side comparator needs a 300V referenced Vref.
- high side comparator can only work if its common mode range include the positive rail. LM339 does not.