Summary of Circuit adds foldback-current protection
This article describes adding foldback-current limiting to a three-terminal adjustable linear regulator (LM317) using a MOSFET, a second transistor, and a few resistors to provide high-speed short-circuit protection and reduce power dissipation during faults. By making the limiter dependent on output voltage via resistor R4, the circuit folds back output current under overload, protecting the pass transistor. Example values produce IKNEE = 0.7 A and ISC = 0.05 A, with RSC = 0.73 Ω and R4 = 4.3 kΩ for R3A = R3B = 100 Ω; measured and simulated VOUT vs IOUT agree closely.
Parts used in the Foldback-Current-Protected LM317 Regulator:
- LM317 three-terminal adjustable regulator (IC1)
- MOSFET Q1 (pass transistor)
- Transistor Q2 (current-sense/control transistor)
- Resistor R1
- Resistor R2
- Resistor R3A (100 Ω in example)
- Resistor R3B (100 Ω in example)
- Resistor R4 (4.3 kΩ in example)
- Resistor RSC (0.73 Ω in example)
- Variable-load resistor (0 to 200 Ω for testing)
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, IMAX, that doesn’t damage the regulator (Reference 1). When a fault condition occurs, the power dissipated in the pass transistor equals approximately VIN×IMAX. 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 (Reference 2).
The circuit in Figure 1 incorporates foldback-current limiting to protect the pass transistor by adding feedback resistor R4. Under normal conditions, transistor Q2 doesn’t conduct, and resistors R1 and R2 bias MOSFET Q1 into conduction. When an output overload occurs, Q2 conducts, reducing the on-state bias applied to Q1 and thus increasing its drain-source resistance and limiting the current flowing into regulator IC1, an LM317. Adding R4 makes Q2‘s bias current dependent on the output voltage, VOUT, which decreases under overload conditions.
For the circuit in Figure 1, you can calculate the maximum foldover and short-circuit currents, IKNEE and ISC, respectively, as follows:
n a practical design, you select values for IKNEE and ISC and equal values for R3A and R3B and then use equations 1 and 2 to calculate resistors RSC and R4. For the circuit in Figure 1, the output’s maximum and short-circuit currents are fixed at 0.7 and 0.05A, respectively. With R3A and R3B set to 100Ω, solving the equations yields values of 0.73Ω for RSC and 4.3 kΩ for R4. You can demonstrate the circuit’s performance by applying a variable-load resistor that’s adjustable from 0 to 200Ω. As Figure 2 shows, the output’s simulated and measured voltage-versus-current characteristics, VOUT and IOUT, respectively, are in close agreement.
For more detail: Circuit adds foldback-current protection
- What is the purpose of adding foldback-current limiting to an LM317 regulator?
To protect the pass transistor by reducing output current during overloads and thus lower power dissipation in fault conditions. - Can the circuit provide short-circuit current limiting?
Yes; the design fixes the short-circuit current ISC (0.05 A in the example) to protect the regulator. - How does resistor R4 affect the limiter behavior?
R4 makes Q2's bias current dependent on output voltage so the output current folds back as VOUT decreases during overloads. - What components are added to the regulator to implement foldback limiting?
A MOSFET pass transistor Q1, a control transistor Q2, and resistors including RSC and R4 (plus R1, R2, R3A, R3B). - How are RSC and R4 calculated?
You select desired IKNEE and ISC and equal R3A and R3B, then use the provided equations (equations 1 and 2) to solve for RSC and R4. - What example values produce IKNEE = 0.7 A and ISC = 0.05 A?
With R3A = R3B = 100 Ω, RSC = 0.73 Ω and R4 = 4.3 kΩ produce those currents. - Does Q2 conduct under normal operating conditions?
No; under normal conditions Q2 does not conduct and Q1 is biased on by R1 and R2. - How was the circuit performance demonstrated?
By applying a variable-load resistor adjustable from 0 to 200 Ω and comparing simulated and measured VOUT versus IOUT characteristics.
