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LM331: LM331 inversion logic

Michael Amoruso
Intellectual 275 points
Part Number: LM331
Other Parts Discussed in Thread: LMV331

Support Path: /Product/Help with Device Selection/Logic devices by function and voltage/

Hello,

The basic function of this part (LM331) has pins Vref, Vin, and Vout. In my design there will be a constant 2.8V on Vin+ (Vin), and Vref will alternate between 3.3V, 2.0V, and 0V.

The logic of this part is:

Vin < Vref -> Vout = Low

Vin > Vref -> Vout = High

The logic I need is the inverse of this so the following cases (vin = 2.8V):

Vref = 3.3V, Vout -> High

Vref = 2.0V, Vout -> Low

Vref = 0V, Vout -> Low

Do you have a part that I could use that has the inverse logic of the LM331. I did not think I could just put Vref on the non-inverting pin and Vin on the inverting pin so that the logic checks out. I also don't want to have to add an inverter to the out put as I have limited space on the PCB. What do you recommend?

Thanks,

Mike

  • 0
    TI__Mastermind 23050 points

    Hi Michael,

    The LM331 is a precision voltage to frequency converter. I believe you are actually referring to the LMV331. What you have defined as your Vin would typically be considered your reference voltage, since it isn't changing and is the voltage you are comparing to. To get the opposite logic, just switch the voltages you are applying to the input pins.

    Regards,

    Zak Kaye 

    Precision Amplifiers Applications

  • 0 in reply to Zak Kaye
    Intellectual 275 points

    Zak,

    My board has been developed and it seems that this LMV331 part is not working as I thought it would. My assumption right now is that there is a threshold that I need to abide by that I was unaware of. Here are the details:

    Part LMV331:

    Case 1:
    Vin+ = 3.3V

    Vin- = 0V

    Output = High

    Case 1 works as intended. Vin+ > Vin-, therefore the output should be high.

    Case 2:
    Vin+ = 3.3V

    Vin- = 2.8V

    Output = Low.

    Case 2 is not working as intended. Clearly Vin+ is still greater than Vin-, however the output is being saturated. Why is this happening?

    Thanks