LM239: Both inputs same value

David Sanchez88

Part Number: LM239
Other Parts Discussed in Thread: LM339, LM339LV

Hi,

I'm using the LM239 quad-comparator in my design and I want to prevent unstability or unknown outputs in the scenario where the two inputs have the same value, for example = 0V. (This case is possible because I have two status signals connected at the inputs). Although it is a comparator I think there are no "internal protections" to avoid that.

What is your recommendation for that purpose?

My LM239 is connected to Vcc=3.3V and the input signals which can go from 0V to 2.5V (digital values)

Thanks for your support!

Best regards,

David

over 4 years ago

0 Paul Grohe over 4 years ago

TI__Guru 55906 points

Hi David,

Please see the LM339 family appnote, section 5.2.1.:

Application Design Guidelines for LM339, LM393, TL331 Family Comparators (Rev. A)

You also may wish to review section 2.3 on input voltage range.

Ideally you should use Vcc-2V for the limit, which would be 1.3V on a 3.3V supply.It is up to you if you wish to rely on the "feature".

You may wish to investigate the new LM339LV, which has a R-R input and can accept up to 5V on the inputs.

0 David Sanchez88 over 4 years ago in reply to Paul Grohe

Prodigy 50 points

Hi Paul,

Thanks for your answer.

I have simulated my circuit and finally decided to change the logic to avoid the scenario --> IN+ = 0V / IN- =0V

Regards.

0 Paul Grohe over 4 years ago in reply to David Sanchez88

TI__Guru 55906 points

Hi David,

Excellent. Glad we could help.

Equal inputs is (technically) a indeterminate condition and at the mercy of the devices unique offset voltage. Make sure any applied bias covers the range of possible maximum offset (±9mV for the LM239).

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LM239: Both inputs same value