LM317 Get Stable Output voltage at 1,25 V

Question

I am currently trying to use the LM317 with a stable output voltage of 1,25 V. The general formula is $$ Vo = Vref\cdot\left(1+\frac{R2}{R1}\right) + Iadj\cdot R2 $$

I used this circuit to create the 1,25 V output voltage:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

This circuit is directly according to the formula. R1 is infinitely high, R2 is zero. In general I measure the 1,25 V output voltage. But when the load is drawing current, everything gets unstable. Then I measure voltages higher and lower than 1,25 V...

I read some things regarding the minimum current in Adj Terminal. Therefore I thought about calculating R1 to create Iadj at ~50µA (according to ST LM317 datasheet). But whats with R2, should it be shortened to ground in this case?

I also thought about inserting a series resistor at the output, just to create a small load of ~1mA. Maybe the LM317 needs a minimum load... Are there any suggestions? Thanks for your help.

What load are you applying? Does it vary, is it constant? How unstable is it, do you have a waveform? What kind of output cap do you use (i.e. what ESR does it have) — PlasmaHH, Jul 21 '17 at 08:41
At the load are various resistors creating a load current between 0,5µA to 50 mA. Relays are used to switch between the load resistors. — Merki, Jul 21 '17 at 08:46
Output caps are normla electrolyt 10µ capacitors, ESR is neglectable.. — Merki, Jul 21 '17 at 08:47

score 3 · Accepted Answer · answered Jul 21 '17 at 09:06

Yes, as suggested, the LM317 DOES need a minimum load. The specification sheet suggests about 10 mA, LM317 see table 7.5 "minimum load current".

The bias current to run the internal error amplifier comes from the input pin (#3 of the TO220 package) and the circuit is completed by output current at the output pin (#2), while the sense pin (#1) carries only a small trickle of sense current. If the R1 resistor is 120 ohms, it will be an adequate load resistance (R2 = zero ohms). Lacking an adequate load, though, the formula is inapplicable, because the internal amplifiers are lacking the needed power supply current.

score 2 · Answer 2 · answered Jul 21 '17 at 08:51

Maybe the LM317 needs a minimum load

Indeed it does. It is specified in the datasheet for the exact version you are using, and is typically between 2-10 mA.

In fact, this is (one of?) the reasons behind the slightly unusual value for R1 in the example schematics. It creates a minimum load of 5 mA at all times.

Adding a 240 ohm resistor to your circuit should clean it up.

LM317 Get Stable Output voltage at 1,25 V

2 Answers2