Why do the MCP9509 or MCP9510 temperature switches not trip when operated over temperature?

The most common reason for this is that the wrong part is installed on the board. The MCP9509 and MCP9510 have an "H" or a "C" option at the end of the part number indicating a "hot" or a "cold" option, which indicates which direction the temperature has to pass over the trip point to get a trip on the output of the device. Refer to the device datasheet for more clarification. Other possibilities:

  • The wrong resistor is populated on the board to set the trip point. Again, refer to the device datasheet.
  • The resistor to set the trip point is low precision, or it varies over temperature too much.
  • The hysteresis of the device is set to 10°C when it should be set to 2°C for the given application. The HYST pin may need a change of state.
  • The OUT pin is loaded in such a way that it cannot drive a trigger indication to the host MCU.
  • The device is damaged. Swap it with a replacement device.

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