For many applications that require power-supply currents of a few amperes or less, three-terminal adjustable-output linear voltage regulators, such as National Semiconductor’s **LM317**, offer ease of use, low cost, and full on-chip overload protection. The addition of a few components can provide a three-terminal regulator with high-speed short-circuit current limiting for improved reliability. The current limiter protects the regulator from damage by holding the maximum output current at a constant level, **I _{MAX},** that doesn’t damage the regulator

**(Reference 1).**When a fault condition occurs, the power dissipated in the pass transistor equals approximately

**V**Designing a regulator to survive an overload requires conservatively rated—and often overdesigned—components unless you can reduce, or fold back, the output current when a fault occurs

_{IN}×I_{MAX}.**(Reference 2).**

The circuit in **Figure 1 **incorporates fold back-current limiting to protect the pass transistor by adding feedback resistor **R _{4}**. Under normal conditions, transistor

**Q**doesn’t conduct, and resistors R

_{2}_{1}and R

_{2}bias MOSFET

**Q**into conduction. When an output overload occurs,

_{1}**Q**conducts, reducing the on-state bias applied to Q

_{2}_{1}and thus increasing its drain-source resistance and limiting the current flowing into regulator

**IC**, an

_{1}**LM317**. Adding

**R**makes

_{4}**Q**‘s bias current dependent on the output voltage,

_{2}**V**which decreases under overload conditions.

_{OUT},For the circuit in **Figure 1****,** you can calculate the maximum fold over and short-circuit currents, **I _{KNEE}** and

**I**, respectively, as follows:

_{SC}n a practical design, you select values for **I _{KNEE}** and

**I**and equal values for

_{SC}**R**and

_{3A}**R**and then use

_{3B}**equations 1**and

**2**to calculate resistors

**R**and

_{SC}**R**. For the circuit in

_{4}**Figure**

**1**

**,**the output’s maximum and short-circuit currents are fixed at

**0.7**and

**0.05A**, respectively. With

**R**and R

_{3A}_{3B}set to

**100Ω,**solving the equations yields values of

**0.73Ω**for R

_{SC}and

**4.3 kΩ**for

**R**. You can demonstrate the circuit’s performance by applying a variable-load resistor that’s adjustable from 0 to

_{4}**200Ω.**

**Read More: Circuit adds fold back-current protection**