LM393: LM393 issue

Zhang Liang

Part Number: LM393
Other Parts Discussed in Thread: LM339

Hi,

We use the LM393DR to replace the LM393DRG4, and the output logic is reverse, and we test the input voltage that the VCC – 2 V range is well, please kindly review below schematic diagram and advise how to solve below issue, thanks.

over 1 year ago

0 Clemens Ladisch over 1 year ago

Guru 281760 points

There is quite a large dip in the power supply. Apparently, it is capacitively coupled into one of the inputs.

Where does the noise come from, and why is it not filtered out by the decoupling capacitors? Can you check whether there is also noise on GND?

0 Chi Nguyen1 over 1 year ago in reply to Clemens Ladisch

TI__Genius 11701 points

Thanks for your support as usual Clemens.

Zhang,

In addition to Clemens' questions, my only other comment would be to remove C18 as this would add a delay and negate the hysteresis. This may not be what is affecting your issue but should be removed nonetheless.

0 Zhang Liang over 1 year ago in reply to Chi Nguyen1

Expert 2935 points

Hi Clemens/ Chi,

We still have the Logically opposite output issue when we change the LM393DRG4 to LM393DR, so we have no idea what these two IC has any difference, thanks.

0 Paul Grohe over 1 year ago in reply to Zhang Liang

TI__Guru 53826 points

Hello Zhang,

You are seeing the difference in the out of common mode range behavior between the new and old die.

You are violating the input range. The input range is Vcc-2V, or 1.3V max on a 3.3V supply. You have the inputs tied up to 5V, which is way beyond the input range. When both inputs are pulled above the input range (>1.3V), the output will flip high on the new die, and low on the old die.

Please see section 2 of the LM339 family application note:

Application Design Guidelines for LM339, LM393, TL331 Family Comparators

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LM393: LM393 issue