This fan controller is using an electronic thermostat LM56 produced by National Semiconductor.
The LM56 is a precision low power thermostat. Two stable temperature trip points (VT1 and VT2) are generated by dividing down the LM56 1.250V bandgap voltage reference using 3 external resistors. The LM56 has two digital outputs.
OUT1 goes LOW when the temperature exceeds T1 and goes HIGH when the the temperature goes below (T1–THYST). Similarly, OUT2 goes LOW when the temperature exceeds T2 and goes HIGH when the temperature goes below (T2–THYST). THYST is an internally set 5°C typical hysteresis.
The electronic thermostat circuit will turn on the fan when the temperature rise above 50 Celsius degree, and this threshold is set by the voltage at VT1. When the temperature rise above 70 Celsius degree then the OUT2 will go high. This signal then can be used to trigger system shutdown or just to turn on a warning LED. This threshold is set by voltage at VT2.