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LM317: Data sheet SNVS774P circuit problem

Part Number: LM317

Data sheet SNVS774P (October 2015 revision) for the LM317-N includes a number of typical applications for this device. Section 9.2.11 describes a current-limited voltage regulator which does not function as intended.

Firstly, there is a simple typo – with the component values given in Fig. 29, the designed short circuit current is around 120mA, not 210mA

More seriously, although the circuit works well at most output levels, at low voltage outputs the current limiter fails. Consider Vout = 1.25V, R2 =0. When the output is shorted, Out is connected to Adjust and the transistor collector has no load.

Two solutions present themselves:

  1. Accept a higher minimum output voltage by including a fixed resistor R5 in series with the variable resistor R2. With R1 =220 ohms, R5 = 220 ohms and Vout min =2.5 volts

  2. Alternatively, a load for the current limiting transistor can be provided by inserting a resistor, say 120 ohms, between Adjust and the junction of R1, R2 thus providing a current limiting load for all overload conditions. Obviously this extra resistor will cause a second small error in the output voltage equation but this will not matter in many applications as Iadjust is very stable.

I opted for the second solution and the circuit works well.

This isn’t really a question, just a suggestion that the data sheet be revised.