MOSFET enhances voltage regulator's overcurrent protection
Edited by Brad Thompson
Herb Hulseman, Eagle Test Systems, Mundelein, IL -- EDN, March 3, 2005
The classic LM317 adjustable-output linear voltage regulator offers a relatively high, if package-dependent, current-handling capability. In addition, the LM317 features current limiting and thermal-overload protection. With the addition of a few components, you can enhance an LM317-based voltage regulator by adding a high-speed short-circuit current limiter (Figure 1). Under normal operation, resistors R2 and R3 apply VGS bias to power MOSFET Q1, an IRF4905S, which fully conducts and presents an on-resistance of a few milliohms. The voltage drop across current-sampling resistor R1 is proportional to IC1's input current and provides base drive for bipolar transistor Q2.
As load current increases, the voltage across R1 increases, biasing Q2 into conduction and decreasing Q1's gate bias. As Q1's gate bias decreases, its on-resistance increases, limiting the current into IC1, according to IMAX=VBEQ2/R1, or approximately 0.6V/1Ω.
Resistors R5 and R6 set IC1's output voltage, as the LM317's application notes describe. By varying the value of R1, you can adjust the circuit's limiting current from milliamperes to the LM317's maximum current-handling capability. Diodes D1 and D2, respectively, protect against capacitive-load discharge and polarity reversal. Depending on the circuit's requirements, IC1 and Q1 may require heat dissipators.
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