Circuit adds foldback-current protection

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_IN×I_MAX. 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, V_OUT, which decreases under overload conditions.

For the circuit in Figure 1, you can calculate the maximum foldover and short-circuit currents, I_KNEE and I_SC, respectively, as follows:

In a practical design, you select values for I_KNEE and I_SC and equal values for R_3A and R_3B and then use equations 1 et 2 to calculate resistors R_SC 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 R_3A and R_3B set to 100Ω, solving the equations yields values of 0.73Ω for R_SC 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, V_OUT and I_OUT, respectively, are in close agreement.

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