The thermal protection behavior can be split up into 2 categories of events that can happen. Thermal behavior shows each of these categories.
- Relative thermal shutdown: The device is
enabled into an over current event. The DIAG_EN pin is high so that diagnostics
can be monitored on SNS and FLT. The output current rises up to the
IILIM level and the FLT goes low while the SNS goes to
VSNSFH. With this large amount of current going through the
junction temperature of the FET increases rapidly with respect to the controller
temperature. When the power FET temperature rises TREL amount above
the controller junction temperature ΔT = TFET – TCON >
TREL, the device shuts down. The faults are continually shown on
SNS and FLT and the part waits for the tRETRY timer to expire. When
tRETRY timer expires, since EN is still high, the device will
come back on into this IILIM condition.
- Absolute thermal shutdown: In this case,
the ambient temperature is now much higher than the previous case. The device is
still enabled in an over current event with DIAG_EN high. However, in this case
the junction temperature rises up and hits an absolute reference temperature,
TABS, and then shuts down. The device will not recover until both TJ
< TABS – Thys and the tRETRY timer has
expired.