LM317 & PWM Control As it turns out, if the upper resistor R1 is left in its place and lower resistor R2 is replaced with a voltage source, the output voltage of the LM317 is approximately 1.25V ...

Li-ion charging with an LM317 There are an awful lot of Li-ion charging chips out there, and you would have thought there was one for every eventuality. But it turns out there isn’t.