Integrated-circuit linear regulators: Under what condition does an IC regulator invoke thermal shutdown protection?

Introduction / Context:
Most IC voltage regulators (linear or switching controllers) include protection features such as current limiting, safe operating area protection, and thermal shutdown. Knowing precisely what triggers thermal shutdown prevents misdiagnosis when a regulator shuts off under heavy load or insufficient heatsinking.

Given Data / Assumptions:

• Standard monolithic regulator with on-die temperature sensor.
• Heatsink and ambient conditions can raise junction temperature during operation.
• Thermal shutdown is distinct from current limiting or foldback.

Concept / Approach:
Thermal shutdown is activated by an internal temperature threshold (junction temperature), not directly by power, current, or load resistance. While high dissipation or short circuits may indirectly raise temperature, the shutdown comparator monitors temperature and latches or cycles the device off once the threshold is exceeded, protecting the silicon from damage.

Step-by-Step Solution:

Verification / Alternative check:
Device datasheets specify a thermal shutdown threshold (for example, around 150–175 °C) and a hysteresis value for auto-restart. Load and ambient tests confirm cycling when T_j crosses this threshold, regardless of the specific current value at that instant.

Why Other Options Are Wrong:

• Power/current too high: Can contribute to heating, but shutdown trip is based on temperature, not directly on power or current.
• Load resistance increases: Usually reduces current and lowers die temperature, the opposite of causing shutdown.

Common Pitfalls:
Assuming a fixed current limit is the cause when a regulator turns off under thermal stress; in fact, many devices hit current limit first, then heat rises and thermal shutdown follows later.

Final Answer:
internal temperature is too high