Thermal shutdown is a safety mechanism for the device that limits total power dissipation by turning off the internal switches when the a temperature sensor on the IC exceeds 168°C (typical). After thermal shutdown occurs, hysteresis prevents the device from switching until the temperature sensor temperature drops to approximately 159°C (typical). When the sensor temperature falls below 159°C (typical), the LM644A2-Q1 attempts to soft start. While the sensor is close to the power FETs, the sensor must not be used to determine maximum junction temperature of the device under load as there can be temperature gradients across the device. Instead thermal couples and other methods must be used to characterize thermal design in application.

While the LM644A2-Q1 is shut down due to high junction temperature, power continues to be provided to VCC. To prevent overheating from a short circuit applied to VCC, the LDO providing power to VCC has reduced current limit while the part is disabled due to high junction temperature. The LDO only provides a few milliamperes during thermal shutdown.