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LM675: Weird behavior at certain input voltages

Alex Acevedo
Prodigy 10 points

Part Number: LM675
Other Parts Discussed in Thread: OPA548, LM3886

I am trying to drive a proportioning valve with this op amp. For this purpose, I have designed a non-inverting op amp circuit to amplify a 0-5 VDC value to 0-30 VDC value with an output current of about an amp. An issue I am having is that at a certain threshold the op amp circuit saturates and fully outputs the positive supply voltage. I am thinking it is a problem with my resistors being too low of a resistance. However, I would like to know if this is a common issue with these op amps. Perhaps there is a higher precision, high voltage and high current power op amp for my design. I will attach my circuit.

  • 0
    TI__Guru* 77501 points

    Hi Alex, 

    LM675 is a non-compensated op amp. The LM675's gains have to be ≥ 10V/V in order to work properly. 

    Another limitation is that the input common mode range. Since you Vee = GND, your input common mode voltage range has to be configured at approx. 20V_min. Otherwise, the output vs. input in the part may not behave linearly or operate in a linear mode. Please use the dual supply rails if you are not sure. 

    We do not have a good LM675 Spice model to demonstrate. I will use OPA548 to demonstrate behavior. The Vcm input voltage in OPA548 is different from LM675. Please also watch out for the output swing requirements vs. output current. 

    OPA548 vs LM675 03072024.TSC

    How much current are you driving for your application? What is your application? We do have other options. If you have other questions, please let us know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 10 points

    My application is that I have to control a proportioning valve that runs on 0-30 VDC using a microcontroller that can only output an analog 0-5 V DC. The output current is at maximum 1 A. I thought this would be a good application for a non-inverting amplifier circuit, however it is not giving the proper behavior. What would you recommend?

  • 0 in reply to Alex Acevedo
    TI__Guru* 77501 points

    Hi Alex,

    Would you want to try LM3886? It costs slightly higher than LM675, but it may work better for your application. OPA548 costs more in this case. 

    LM3886 Squarewave 03072024.TSC

    If you have other questions, please let me know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 10 points

    I'll definitely try it, I'll ask you if I have any trouble.