LM339: Simulation ok, but prototype doesn't work

Allen Du

Prodigy 60 points

Part Number: LM339

Hi TI team, merry christmas and happy new year!!

I am using a LM339 comparator to detect a LED circuit.

The input from LED circuit is 5V or 3V.

I hope when input is 3V, LM339 output low and MOSFET on.

and when input is 5V, LM339 output high and MOSFET OFF.

PSpice simulation result is same as my expection, but prototype board doesn't work, the LM339 output always low and MOSFET always on.

when i changed the LM339 to a OP-AMP LM6132B, the prototype board is working well.

1. Could you help advice is anything woring of the circuit.?

2. What's the difference between comparator and OP-AMP? thanks in advance.

over 4 years ago

+1 kai klaas69 over 4 years ago

Guru 140200 points

Hi Allen,

unfotunately, you have made a design mistake: The input voltages of LM339 must stay 1.5V (2.0V for the full temperature range) below the supply voltage of LM339. This means that input voltages up to only 3.0V are allowed here. Best add an additional resistive voltage divider for the signal input.

Kai

+1 Joe Vanacore over 4 years ago in reply to kai klaas69

TI__Mastermind 19245 points

Hi Kai,

Thank you for helping Allen with this.

Hi Allen,

I believe the info that Kai provided will help you get the answer you need. I will now temporarily close this thread as we are on holiday break. I you have any further questions please respond to this post or create a new thread. Please expect a response to any of your follow-ups on Monday January 3rd.

Best Regards,

Joe Vanacore

Applications Engineer

Comparators Product Line (CMPS)

All information in this correspondence and in any related correspondence is provided “as is” and “with all faults”, and is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml)

0 Allen Du over 4 years ago in reply to Joe Vanacore

Prodigy 60 points

Hi Kai, Joe, thanks for your reply and the suggestion.

0 Allen Du over 4 years ago in reply to Allen Du

Prodigy 60 points

Hi Kai, Joe,

I have another question.

Both +in and -in voltage need to be smaller then supply voltage 2V, or just +in?

Because i have another board, supply is 12V, +in is 9V, -in is 0 or 12V, this board can work as expected. does this circuit have any issue? thanks in advance.

0 kai klaas69 over 4 years ago in reply to Allen Du

Guru 140200 points

Hi Allen,

why not reading the very detailed discussion of this issue in section 9.2.2.1 of datasheet of LM339? And read the footnote 3 below the table in section 7.7 :-)

But keep in mind, that with these input levels merely the logical levels at the output are correct. Leaving the specified common mode input voltage range of a comparator is never a good idea, because the speed performance of comparator can be degraded. Provided the speed is no issue in your application, you can do this. But if you need the specified speed, better stay with both input voltages within the specified common mode input range.

Kai

0 Chuck Sins over 4 years ago in reply to kai klaas69

TI__Mastermind 26870 points

Thanks for your post Allen

I believe your question has been resolved, so I will close the post.

Chuck

Amplifiers

Amplifiers forum

LM339: Simulation ok, but prototype doesn't work