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LM321: LM321MF/NOPB op amp heating issue

Alon Chen
Intellectual 585 points

Part Number: LM321

Tool/software:

Hi Team,
As shown below, a customer's PWM is connected to the positive input of an LM321. When the MCU is not initialized, the PWM input signal level is uncertain. This caused some op amps to overheat, while others from the same batch did not. Furthermore, the overheating disappears when the PWM level changes. What could be the cause? Please help analyze this. Thank you.

  • 0
    Guru 322010 points

    With capacitors connected directly to the output, this opamp is likely to be unstable and to oscillate.

    What is the purpose of C20/C11? If you want a smooth output signal, just filter the input signal, or put a resistor between the opamp's output and the capacitors.

  • 0
    TI__Expert 4830 points

    Hi Alon,

    I agree with Clemens, a large output capacitance can generally cause stability issues in an opamp circuit. Can you explain more about the PWM levels the opamp is seeing? What is the PWM input when the overheating disappears? I simulated the circuit in TinaTI, and it does look like the circuit is unstable, which is likely the cause of the overheating. I have attached the simulation below. The LM321 datasheet also echoes Clemens inputs of adding a resistor to the opamp's output to increase the stability.

    LM321 stability.TSC

    Best,
    Carrie