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I'm planning to use the MAX 6644 for a simple fan control application. All in all the IC is very straight forward to use. However after analyzing the typical application circuits in the datasheet I have doubts as to the circuit can actually work.

The problem I see is that the PWM signal switches a transistor in the negative supply wire of the fan. As far as I know, the TACHO output of most fans is open-collector and therefore references to the negative supply. If now during operation the transistor is switched off, the open-collector TACHO signal of the fan would now be at positive supply of the fan (e.g. 12V) correct? That would mean the IC can detect the short TACHO signal pulse only by chance if it occurs during the active phase of the PWM transistor.

Did someone already use this IC in the typical application circuit as provided by maxim and can confirm that it works?

Svenito
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    I suggest you read the "Fan-Fail Sensing" section on page 7 of the [datasheet](https://datasheets.maximintegrated.com/en/ds/MAX6643-MAX6645.pdf) – Finbarr Jun 16 '22 at 10:31
  • Thank you very much for the hint! I already read over the whole datasheet briefly. But it seems I missed something here. So if I understand correctly, the tacho signal is observerd only during 100% PWM conditions, anyway? – Svenito Jun 16 '22 at 10:46
  • That's what it says. Note that an "open collector" output doesn't have a pullup of its own, you will need to provide that yourself. – Finbarr Jun 16 '22 at 11:15
  • Yes I'm aware of that, thank you. But in the typical application circuits, no pull up is provided. I think I can assume, the pull-up is inside the MAX6644. However I'll probably plan for mounting one myself just in case. – Svenito Jun 16 '22 at 12:24

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Seems like the tacho signal is only evaluated when PWM is in 100% state. That means the open-collector output should be working properly in that state.

Svenito
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