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LM25066: Noisy temperature and current values

Fiona Macpherson7
Prodigy 106 points
Part Number: LM25066

Hello,

I have a design using the LM25066.

When I read the temperature values the results are very noisy. (Varying from 21 to 28 degC)

I get similar noise when reading power and current values - even when I read the averaging registers.

The device is located between two switching power supplies - so perhaps it is picking up noise from those?

Do you have any advice?

For example, I have used a 1nF cap on the input to the DIODE pin. Would it be sensible to increase that capacitance?

Thanks, Fiona

  • 0
    TI__Guru 57737 points
    Hi Fiona,

    If you are reading wrong values on PMBus , then the controller itself is sensing wrong values of Voltage, Current and Temperature due to noise. The pins which sense these parameters are high impedance pins and they are more susceptible to noise. Now, you should be more worried about the operation of the controller with noise on its pins.
    Have you checked the functionality of the circuit in OverCurrent , OverVoltage and Fast Trip conditions? Is the circuit responding at the set thresholds ?
    Increasing the Capacitor at the Diode pin can reduce the accuracy of temperature measurement.
  • 0 in reply to Praveen GD
    Prodigy 106 points

    Hi Praveen,

    Thanks for getting back to me. 

    Yes, the controller works as I expect during hot-swap, power on - trip conditions. (although I have to admit to being not sure of the absolute accuracy of the thresholds)

    In this instance I'm using it more as a telemetry device - that also happens protects against major faults. 

    I just wondered if it was a common occurrence to have noise on these readings. I did take great care with the layout, but there are multiple switching power supplies around.

    And it may be that we have problems with the PMBus - since it's implemented on an FPGA and is still getting tweaked. However we can read back registers like the MFR_ID error free.

    Have you had other users finding that the readings are noisy? (I wondered if there are averaging registers because noise is expected)

    Regards, Fiona

  • 0 in reply to Fiona Macpherson7
    Prodigy 106 points
    Update - we had an error in our implementation of the PMBus. That's now fixed and the values look good and stable.
    Thanks for the support,
    Fiona