LM317 output too low

Question

I am trying to figure out an issue that I have with LM317 regulator.

It is producing ~ 1.7V at VOUT instead of desired 5V.

I've used this design in several other PCBs with no issues.

The schematic is following:

The layout from both sides:

Couple of observations:

Vin measures stable 12V at first pin of LM317 as well as other places of the system - F1 fuse is not damaged (measured resistance between pin 5 of J7 and pin 1 of U10 is 0.9Ohm)
Voltage at ADJ pin is ~ 0.4V while powered on
There is no short between VOUT and GND: measured resistance is 898Ohm as expected - individual resistors R31 and R32 also measure according to the schematic
5V is connected to supply of several of CMOS ICs each of which has 0.1uF capacitor near its power supply - none of the ICs are heating while powered on
Replacement of LM317 with brand new IC did not produce any observable change

I would appreciate if you could point out what may cause this behavior in my prototype and what additional measurements I should do.

Justme · Accepted Answer · 2023-03-28 07:45:53Z

6

Based on the resistor values, there should be 0.4V on ADJ pin which you have, and there should be 1.65V on OUT pin, which you have.

The circuit works as per the schematics. The resistors are just incorrect for what you want.

answered Mar 28 at 7:45

Justme

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\$\begingroup\$ Just had a look at the datasheet closely and noticed that the resistor R31 and R32 should be swapped. After doing this the prototype works just fine. This is clearly a mistake on my part - in my mind I expected R31 and R32 to function as voltage divider. I will leave this post as a reference for other users \$\endgroup\$
– Ivan Demyachenko
Mar 28 at 7:56
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\$\begingroup\$ They do work as a voltage divider. It just needs to divide the voltage so that there's 1.25V of the total 5V between OUT and ADJ. \$\endgroup\$
– Justme
Mar 28 at 8:03
\$\begingroup\$ @IvanDemyachenko Pay also attention that for the LM317 to work properly, there should be a minimum current load of 10 mA (see in this data sheet, the minimum load current in table 7.5). It might work with lower currents, but it's not guaranteed. So, it's better to have the resistor between the terminals VOUT and ADJ of 120 Ω rather than 220 Ω. \$\endgroup\$
– Massimo Ortolano
Mar 28 at 18:27
\$\begingroup\$ @MassimoOrtolano thank you, I am aware of this - I've calculated that the load elsewhere in the system (couple of LEDs) is sufficient - therefore value of 220 Ohm was chosen. \$\endgroup\$
– Ivan Demyachenko
Mar 28 at 19:38

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