Linear and Switching Power supply fundamentals (Part 6)

VOLUME 4, MARCH 2011

Linear and Switching Power supply fundamentals (Part 6)

By: Alireza Nazem

Protection Circuits Built Into IC Linear Regulators

Linear IC regulators contain built-in protection circuits, which make them virtually immune to damage from either excessive load current or high operating temperature. The two protection circuits found in nearly all linear IC regulators are:

a) Thermal Shutdown
b) Current Limiting

CHAIN OF COMMAND

The thermal shutdown, current limiter, and voltage error amplifier make up three distinct and separate control loops that have a definite hierarchy (pecking order) which allows one to "override" the other. The order of command of the loops is:

1) Thermal Limit (IC is regulating junction temperature/power dissipation)
2) Current Limit (IC is regulating load current)
3) Voltage Control (IC is regulating output voltage)

This hierarchy means that a linear regulator will normally try to operate in "constant voltage" mode, where the voltage error amplifier is regulating the output voltage to a fixed value. However, this assumes that both the load current and junction temperature are below their limit threshold values. If the load current increases to the limiting value, the current limiting circuitry will take control and force the load current to the set limiting value (overriding the voltage error amplifier). The voltage error amplifier can resume control only if the load current is reduced sufficiently to cause the current limiting circuits to release control.

A rise in dying temperature approaching the limit threshold (about 160°C for the non-military range) will cause the thermal shutdown and cut drive to the power transistor, thereby reducing the load current and internal power dissipation. Note that the thermal limiter can override both the current limit circuits and the voltage error amplifier. It is important to understand that a regulator holds its output voltage fixed only when it is in constant voltage mode. In current limiting, the output voltage will be reduced as required to hold the load current at the set limiting value.

In thermal limiting, the output voltage drops and the load current can be reduced to any value (including zero). No performance characteristic specifications apply when a part is operating in thermal shutdown mode.