07-04-2012, 03:31 PM
Hi Al,
They should be pretty good, even at low voltages. In fact, the IC doesn't know anything about the output voltage; all it tries to do is maintain a fixed 1.25V between OUT and ADJ. Because the internal design is such that hardly any current flows at the ADJ pin, it means that simple Ohms Law applies; the current in the two resistors is equal, and equal to 1.25/R, where R is the resistor between OUT and ADJ. Datasheets recommend 240 ohms, which gives 5mA, near as dammit
. From there, knowing there is 5mA is the resistor between ADJ and ground, it's a simple task to work out which resistor gives a voltage drop of Vout-1.25.
You probably knew all of that
Anyway, you've either got faulty ICs or a mistake with the wiring, an outside chance of parasitic oscillation. The datasheet is excellent and should give plenty of hints about layout, decoupling, protection diodes, etc...
Good luck - and if you get stuck, you can come here and look after the kids, and I'll hide in your "shack" for the rest of the day
Mark
PS: There's plenty of LM317Ts here if necessary...
They should be pretty good, even at low voltages. In fact, the IC doesn't know anything about the output voltage; all it tries to do is maintain a fixed 1.25V between OUT and ADJ. Because the internal design is such that hardly any current flows at the ADJ pin, it means that simple Ohms Law applies; the current in the two resistors is equal, and equal to 1.25/R, where R is the resistor between OUT and ADJ. Datasheets recommend 240 ohms, which gives 5mA, near as dammit
. From there, knowing there is 5mA is the resistor between ADJ and ground, it's a simple task to work out which resistor gives a voltage drop of Vout-1.25.You probably knew all of that
Anyway, you've either got faulty ICs or a mistake with the wiring, an outside chance of parasitic oscillation. The datasheet is excellent and should give plenty of hints about layout, decoupling, protection diodes, etc...
Good luck - and if you get stuck, you can come here and look after the kids, and I'll hide in your "shack" for the rest of the day
Mark
PS: There's plenty of LM317Ts here if necessary...
