Simple current limiting clamps the output current at a constant value during a fault. That sounds safe, but it's actually the worst case for the pass transistor: the regulator drops nearly the full input voltage across the device while still pushing the limit current through it. A 12 V regulator delivering 5 V at a 2 A limit dissipates 14 W during a short — but only 14 W during normal operation when delivering 2 A at 5 V from 12 V in. Same dissipation. The transistor doesn't care it's a fault; it just cooks.

Foldback limiting fixes this by reducing the current limit as the output voltage drops. Under a dead short (V_OUT = 0), the regulator allows only a small fraction of the normal limit — maybe 20–30%. The pass transistor sees high V_CE but tiny I_C, keeping dissipation manageable.

How it works: Add a resistor divider (R1, R2) from the output node to the base of the current-sense transistor (Q_SENSE), referenced to the sense resistor R_SC. The sense transistor turns on when:

• V_RSC + V_OUT·(R1/(R1+R2)) ≥ V_BE + V_OUT·(R2/(R1+R2))

Rearranged, the trip current depends on V_OUT. Higher output voltage raises the allowed current; lower output lowers it.

Design rule of thumb: Pick the full-load knee current I_K (say 2 A at nominal V_OUT = 5 V) and the short-circuit current I_SC (say 0.5 A at V_OUT = 0). Then:

• R_SC = (V_BE + V_OUT·R1/(R1+R2)) / I_K
• Foldback ratio I_SC/I_K ≈ V_BE / (V_BE + V_OUT·R1/(R1+R2))

For V_BE = 0.65 V and a 4:1 foldback, pick R1/(R1+R2) ≈ 0.4, giving R_SC ≈ 0.65/2 = 0.33 Ω.

Real example: Bench supplies from the LM723 era universally used foldback. The classic Heathkit IP-2718 and HP 6200-series linear supplies fold back to roughly 20% of rated current under short, letting them survive indefinite shorts with a modest heatsink instead of a brick-sized one. Modern lab supplies still use the technique in their linear post-regulator stages.

The catch: Foldback can latch into a low-current state with nonlinear loads. Motors, incandescent bulbs, and capacitor banks draw huge inrush — the regulator folds back, output voltage collapses, and the load never gets enough current to start. The foldback curve crosses the load line at a stable low-voltage point. Solution: avoid foldback for capacitive or motor loads, or add a soft-start to walk past the knee.