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# LM293A: Schematic problem

Part Number: LM293A

Hi all:

Have a reply:"The first stage, U5A, is at the biggest risk. Both the input and reference voltage exceed the limit and has the possibility of an incorrect output."

However, IN my actual test, the voltage of U5A IN+ to GND IN the first stage is about 3.9v, and the voltage of IN- to GND is about 1~ 4.5v. My test results are all normal

So, is it ok to assume that I designed this circuit?

Or my input and reference voltage are over 3V, there are hidden dangers, how to improve?I've made thousands of them

Thank you!

• Hi Timsen,

e2e.ti.com/.../784395

The easiest way to cure the issue is to introduce a resistive voltage divider at the -input of U5A to prevent the input signal from exceeding 3.0V. Then, you should modify the resistors R40, R41 and R42 to accordingly set the threshold voltage lower.

Kai
• Hi Kai：
However, IN my actual test, the voltage of U5A IN+ to GND IN the first stage is about 3.9v, and the voltage of IN- to GND is about 1~ 4.5v. My test results are all normal

So, is it ok to assume that I designed this circuit?

Or my input and reference voltage are over 3V, there are hidden dangers, how to improve?I've made thousands of them
• Hi Timsen,

the datasheet clearly says that the input voltage must be within the input voltage range of 0...3.5V at room temperature and 0...3.0V at full range temperature, provided your supply voltage is 5.0V. See section 6.6 of datasheet.

That your circuit also works at 3.9V input voltage, does not mean that the datasheet is wrong. It only says that you are lucky. But with the next LM293A the situation may totally turn and the LM293A might stop working properly.

Kai

• Timsen,

Your circuit WILL FAIL if you go below room temperature. Also expect random failures with the output stuck "low". Input range reduces as temperature goes down. Hit it with freeze spray and see what happens - it wont take much..

You are operating at the very upper edge (~4V) of the input range "gray zone", where the comparator "appears" to function but does not meet datasheet specs. You are on the edge of failure.

As Kai said, you need to bring the inputs to within the specified input range (0 to 3V), or, increase the supply voltage to > 6.5V.