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Original question:

LP2985 is Not Working

LP2985A: LP2985 is Not Working

Mikhail Matusov
Intellectual 340 points
Part Number: LP2985A
Other Parts Discussed in Thread: LP2985-N

I am using a 3.3V fixed version of the LP2985A IC with +12V at the input. The load is less than 10 mA. However, the regulator produces over 9 V at the output for some reason. Please see my schematic below:

What am I doing wrong?

Thanks,

/Mikhail

  • 0
    TI__Guru 55940 points
    Hi Mikhail,

    Your schematic looks to be okay. Often when an LDO output is biased higher than the set voltage it is due to an unintentional bias path such as residual flux that was not properly cleaned after soldering or even through the load itself (if the load requires multiple input rails). In particular for devices such as LP2985-N that have a BYPASS pin, it is important to check if there are any leakage paths to this pin as this pin is directly connected to the internal reference.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Intellectual 340 points
    Hi Ryan,

    I think there is a possibility of 1.8V leaking from the load through some pullup resistors to the output of the regulator but not to the BYPASS pin. Is that a concern?

    Thanks,
    /Mikhail
  • 0 in reply to Mikhail Matusov
    TI__Guru 55940 points
    Hi Mikhail,

    Due to topology a higher voltage leaking onto the output of the linear regulator could cause the output of the regulator to be biased high; however, a lower voltage will not as it will be driven by the output of the linear regulator. Since you are measuring 9 V on the output, the rail leaking onto the output pin would need to be at least 9 V.

    Since the output voltage will be equal to the reference voltage gained by the internal resistor divider, a rail above 1.23 V leaking onto the bypass pin would be cause the output voltage to be higher than the set voltage.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Intellectual 340 points
    Thank you Ryan. A higher voltage was indeed leaking from the load...