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LM2936: Issues Regulating

Ryan Savage
Prodigy 30 points

Part Number: LM2936

I am using the 5V fixed output version. I am having trouble regulating to 5V. It's typically around 8-9V. I have tried using a 10uF cap on the output, but not sure of the ESR. Is the output capacitor with the ESR no greater than 8 ohms that critical to get the correct 5V output?

  • 0
    TI__Genius 14245 points
    Hi Ryan,

    Yes if the ESR of the output cap (including trace resistance) is too high or too low the the LDO could become unstable and result in output oscillations. If you are measuring the output using a scope you'll see this oscillation as the output rings. If you are measuring the output voltage using a DMM, this measures a RMS voltage and the oscillations may not be noticeable.

    -Kyle
  • 0 in reply to Kyle Van Renterghem
    Prodigy 30 points
    Without a capacitor installed on the output, the scope is reading steady right around 8V with no noticeable oscillations. The DMM is reading roughly 8V RMS as well. This is with a 25V input.
  • 0 in reply to Ryan Savage
    TI__Genius 14245 points
    It's hard to know exactly how an LDO will operate when the correct value (capacitance and ESR) is not used, especially with these older devices. So yes the output may be at the wrong voltage. As for the ringing that I mentioned, you may see it ring more if you were to perform a load transient.

    -Kyle