solved: Does the LM317 even need feedback? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: June 19, 2025, 01:14:00 pm
content: Does the LM317 need feedback from the out pin to the adjust pin to output a fixed voltage?For example, if you apply a fixed 10V signal to the adjust pin (from a supply that can source and sink current). Will you get 11.25V on the output pin with a load of 0 to 1 amp?When I look at the internal diagram, it appears to already have negative feedback for regulating the output voltage with respect to the adjust pin.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 19, 2025, 01:25:20 pm
content: Right, the LM317 has internal feedback regulating the difference between Vout and Vadj. If you have the right reference voltage available, you can tie that directly to Adj. But usually that's not the case and the resistor feedback network is used instead.

author: David Hess
author_rank: Super Contributor
timestamp: June 19, 2025, 01:44:19 pm
content: Quote from: incf on June 19, 2025, 01:14:00 pmExactly, no feedback is required. The output pin follows the adjustment pin with a +1.25 volt offset.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 19, 2025, 03:02:20 pm
content: Yes, the feedback is not 100% needed, but output voltage stability will be totally different from datasheet values (because voltage likes to drop on any resistance if there is a current flow).

author: ejeffrey
author_rank: Super Contributor
timestamp: June 19, 2025, 03:46:52 pm
content: Quote from: Vovk_Z on June 19, 2025, 03:02:20 pmNah. The way the 317 works is not like a proper remote sense. It doesn't reduce the output impedance or improve load regulation. Quite the opposite, any load output resistance will be amplified by the gain of the feedback circuit.However the minimum load current must still be satisfied. If you were counting on the feedback network to provide some or all of that you need to make other arrangements.

author: floobydust
author_rank: Super Contributor
timestamp: June 19, 2025, 04:27:01 pm
content: It's the cost for not having a GND pin.

author: David Hess
author_rank: Super Contributor
timestamp: June 19, 2025, 04:35:20 pm
content: Quote from: ejeffrey on June 19, 2025, 03:46:52 pmThen you put a 120 ohm resistor between the output and adjustment pins to draw a constant 10 milliamps from the output. Of course now whatever is driving the adjustment pin needs to be able to sink 10 milliamps.

author: PGPG
author_rank: Frequent Contributor
timestamp: June 19, 2025, 10:40:10 pm
content: Quote from: incf on June 19, 2025, 01:14:00 pmNo.But with a load of 0.01 to 1A yes.

author: Gyro
author_rank: Super Contributor
timestamp: June 20, 2025, 01:24:00 pm
content: Just as an aside, the LM317 behaves very similarly to an LM395 "Ultra reliable power transistor" when used in this mode. The LM395 has much lower 'Base' current and a little lower minimum 'VCE', but C-E quiescent current is very similar. The LM317 offsets this by being rather cheaper and more widely available. Both are handy, especially is small package, where you want quick and dirty overcurrent protection and thermal shutdown.Edit: you need to check how much positive voltage and current you're driving into the adjust pin in the LM317 case though - the limit is unspecified in this mode of operation, so a series resistor and clamping diode may be required.