What is thermal shutdown?
Automotive parts are subject to harsh conditions, and require more thorough validation than most applications call for. Within Maxim's automotive product line, datasheets guarantee electrical characteristics within ambient temperatures of -40°C to 125°C. If the junction temperature rises high enough to indicate thermal overload (typically 170-180°C), the IC shuts down to avoid irreversable thermal damage. The IC restarts when the junction temperature cools to below the thermal-shutdown hysteresis threshold (typically 15-20°C less than the thermal-shutdown temperature). These thresholds are specified in the datasheet and are accuate to within a degree.
How are thermal shutdown thresholds measured?
Datasheet thermal shutdown thresholds are measured during IC validation to ensure they match datasheet specs. The process is outlined below.
The IC evaluation kit is placed in an oven with two thermocouples (two for accuracy) measuring the ambient temperature inside the oven. Oscilloscope probes are placed on any output pins, and Enable pins are tied high to allow the IC to restart without manual intervention. A small current is drawn from the evaluation kit output (mA) such that the part is running, but not so large a load that the junction temperature is higher than the oven's ambient temperature. The oven is brought up to 10°C under the thermal shutdown threshold, and kept there for a minimum 10 minutes to 'settle'. This process is repeated for every 1°C up to the thermal shutdown threshold until an oscilloscope falling-edge trigger indicates the output voltage has collapsed. The thermal shutdown measurement is recorded.
The oven ambient temperature is decreased by the same process until an oscilloscope-rising edge trigger on the output pin indicates the IC has restarted. This is the thermal-shutdown hysteresis threshold.
Common mistakes when measuring thermal shutdown thresholds
-Too large a load on the IC output will increase the junction temperature above ambient and skew threshold measurements.
-Uncalibrated thermocouples can vary by several degrees, as can an oven's digital display. Use multiple thermocouples to measure oven ambient temperature, and ensure accuracy in case a thermocouple comes into contact with an oven wall or other surface.
-If the oven ambient temperature is increased too fast, the IC junction temperature will be cooler than the temperature indicated by the thermocouples.
-Though it matters less here than in applications with higher power dissipation, the IC should be properly soldered to the PCB for efficient heat transfer.