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LM63 sensor is abnormal

pan rt
Prodigy 20 points
Other Parts Discussed in Thread: LM63

Hi all:

We're using LM63 as temperature and fan control sensor.

But we found a strange situation, i.e, when we unconnect the fan for a long while, its rotational speed (RPM) will sometimes be read as an non-zero value. I don't know what happens to our H/W design. May someone help me?

The following is our schematic for FAN and LM63.

Thank you all.

  • 0
    TI__Expert 4905 points

    Hi,

    What exactly do you mean by a "non-zero value" for the RPM. The RPM value register will read zero when the tach speed is way too fast for the internal counter to register a reading. So if the fan is spinning you should allways get a non-zero value that is if everything is working properly.

    As for your schematic it looks ok except for the fan tach resistors R8, R9 and R12. If pin 3 of the fan does not drive any voltage high (that is it's open-drain) then the input to the LM63 would be at about 2.16V and that might be a bit  just too close to the threshold level of the LM63 tach input - not enough noise margin. Perhaps that is what is causing your inconsistent tach readings.

    Take care,

  • 0 in reply to Emmy Denton
    Prodigy 20 points

    Hi Emmy:

    As I described before, we have unplugged the fan from the socket and kept this situation for a long time. And during this situation, we will sometimes read a non-zero RPM value(but fan is unplugged). I expect the tach should always be zero when fan is unplugged, isn't it?

    I'll check our schematic again. Thank you.

  • 0 in reply to pan rt
    TI__Expert 4905 points

    Hi vv,

    So sorry that I missed that part of your question. But the answer is the same if you have noise coupling into the tach input that could cause it.

    Take care,