MOSFET enhances voltage regulator ' s overcurrent protection

the classic LM317 Adjustable-output linear voltage regulator offers a relatively high, if package-de Pendent, current-handling capability. In addition, the LM317 features current limiting and thermal-overload protection. With the addition of a few, you can enhance an lm317-based voltage regulator by adding a high-speed SHORT-CIRCU It current limiter (Figure 1). Under normal operation, resistors r_{2 and r3 apply V< Sub id= "Id2152039-47-sub" >gs bias to power MOSFET q1, an irf4905s, which fully conducts a nd presents an on-resistance of a few milliohms. The voltage drop across current-sampling resistor r1 is proportional to Ic1" 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 AP Proximately 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, your can adjust the circuit ' s limiting current from milliamperes to the LM317 ' s maximum -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.

MOSFET enhances voltage regulator ' s overcurrent protection