LM56 is a monolithic  thermostat designed for simple temperature control. The chip has two programmable threshold that can be used to drive a fan and or alarm.  Here is the schematic diagram of the fan control circuit:

lm56 fan control circuit schematic

The 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.  [Schematic diagram source: National Semiconductor’s Temperature Sensor Handbook]