Q: What is current limit foldback?
A: This is a current limiting technique to limit the peak power dissipation to prevent thermal destabilization or thermal runaway in the power transistor (which usually leads to the part destroying itself), to keep it in its SOA (Safe Operating Area).
As an example, see the current foldback characteristics of the LT3055 below:
Like many IC power regulators, the LT3055 has safe operating area protection. The safe area protection decreases current limit as input-to-output voltage increases, and keeps the power transistor inside a safe operating region for all values of input-to-output voltage. The LT3055 provides some output current at all values of input-to-output voltage up to the device breakdown.
The graph also shows how the LT3055 further reduces the output current limit as the junction temperature rises, adding another layer of protection versus part failure.