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ADT6401 Output Problem


We are using two ADT6401 temperature switches on our board. They are open-drain output. We use one for the high temperature (+85 degrees celcius) and one for the low temperature (-45 degrees celcius). Our schematic is attached.

Temperature switches can shutdown our board. While we are testing the board at room temperature, our board shuts down because one of the temperature switches pulls their output low. We could not understand which of them. But the temperature is very far away from both high and low limits. So temperature switches do not operate correctly, that's for sure.

Our low temp. switch inputs are Float - GND - Float. If the float on the S2 pin, is measured as GND some time, the temperature setting goes to +65 degrees celcius. Could this be a problem? Or could it be a soldering effect?



  • Hi, in an effort to isolate the problem, starting with U21, can you disconnect its output (keeping a pullup resistor on the output to Vdd in place) and confirm what the actual voltage readings are at ambient temperature on S2/S1/S0 pins? Because the output is active low, the output should read high, please confirm?

    With the outputs isolated between U21 and U18 can you confirm what the actual voltage readings are on U18 at ambient temperature on S2/S1/S0 pins? Because the output is active low, the output should read high, please confirm?

    Its best to measure the pin voltages carefully with a scope as this will show if there is any noise present on any of the input pins especially on the pins that are floating. Failing this can you send me a more complete schematic so that I can take a closer look. BTW a decoupling cap is missing from both Vdd pins and its not clear if the output is pulled up to Vdd?

  • Hi again,

    - There are 100nF caps very close to the IC's (one for each).

    - The output is pulled high to 4 volts with 100 uA current.

    To isolate the problem, we did some tests to the ADT6401 in a test board and we think we isolated the problem. Below are our findings:

    In the datasheet, ADT6401's operating supply voltage is given as 2,7 volts.

    1) 2,5 ms after powerup, ADT6401 checks its inputs (whether they are float, gnd, or Vcc)

    2) ADT6401 checks its inputs once and only once (2,5 ms after powerup).

    3) So, if anything goes wrong at powerup, the input states are latched.

    4) If Vcc rises slowly (1 volts per 25 ms), the float pins is not read correctly. They are read as GND's.

    So, in our case,

    S2/S1/S0 (Float-GND-Float) (-45 degrees with 2 degrees hysteresis) becomes

    S2/S1/S0 (GND-GND-GND) (+45 degrees with 2 degrees hysteresis).

    We tested ADT6401 for 2 full days. We tested with scope. We know it checks its inputs 2,5 ms after powerup, because we measured the FLOAT pin and saw that it rises to half of Vcc 2,5 ms after powerup.

    That is very critical for us. We need to make sure that ADT6401 reads FLOAT state correctly independent of the rise time of Vcc.

    Please reply ASAP.

    Thanks and sincerely,


  • Hi, there is an applications sensitivity with slow rise time with this part that can only be overcome by increasing the risetime. Is that an option for you?.

  • Hi,

    We use LDO for the supply so we cannot control the risetime.

    Is there anything we can do, to make the FLOAT pins appear correctly with the slow rise time?

    If not, is there any alternate parts that is pin compatible or same package?

    If there is nothing we can do, we have to cancel these parts.


  • Hi, we will look at this on the bench using your rise time settings and get back to you with the next steps.

  • Hi again,

    We have at least 800-1000 units in the field. For us, this is very critical.
    We will make a decision whether we call the units back or not by Monday.



  • Hi, apologies for delay in getting back. We have looked at ADT6401 on the bench and when the rise time is >10mS from 0->2.7V, the part isn’t entering the correct mode with the associated floating inputs.

    We also tied the S2 & S0 pins to Vdd/2 but to no avail as part interprets the pins as a high.

    The next nearest family is ADT6501, these have factory set trip points in a similar SOT23 package.

  • Hi,

    I am mallikarjun from honeywell. We are planning to use this component in one of our new design. Whether this dependency on VCC Rise time (2.5ms upto 0 to 2.7V) is still existing for this part.  Any device errata/ whether this problem rectified in latest part (ADT6401)?

    Whether same problem exists form ADT6402 also?

    My email:

    +91 8095950458