[SOLVED] lm339 positive input difference detection ?

[plouf]

Hi

i am testing a LM339, i see that it described to detect up to power supply with a few mV offset
however iwth a 5 Volt input i see that detection is triggered when i drop negative input to around 3,9 volt (and positive to 5)

while detection from 0 is nearly instand 10-50mV and triggered

do i uderstand something wrong about lm339 limits ?

 

[FvM]

Review common mode input voltage range in LM339 datasheet. It's 0 to Vcc-1.5V. If you operate LM339 with 5V supply, it doesn't work with 3.9V input.

 

[danadakk]

You can always, if you have to use LM339, divide the input down into the inputs to get
the CM range in spec. But easiest is to use a RRIO comparator (or RRIO OpAmp) if speed
not a problem.


Regards, Dana.

 

[plouf]

However because final project will eun with 14-16 volt input i see no rail-to-rail with than input (all rr are up to 5v) so voltage division is only option ..

 

[danadakk]

Multiple RRI parts over here to >> 5V, where input CM exceeds positive supply, one example :

https://www.analog.com/media/en/technical-documentation/data-sheets/ada4097-1-4097-2.pdf

Regards, Dana.

 

[FvM]

However because final project will eun with 14-16 volt input

Unclear. What is comparator supply and input voltage?

Anyway, once you understood the problem, you'll surely be able to choose components and design your circuit after reading datasheets.

 

[plouf]

thanx everyone,
original query was why lm339 does not detect positive rail which answer my mistake (tahnx)

best solution imho is voltage divider which should work perfect for this