LM317 + PWM needs less voltage at the top end and 0V at the low end

I'm a digital guy so it's a bit of a dredge to get through the analog aspects of electronics. I can do it but I need a few clues and a little reassurance that I'm going in the right direction(s.)

I have an inductive device with a 9V limit. I have 5V and 12V available.

I'm looking at a really nice thread about controlling LM317 with PWM at http://forum.arduino.cc/index.php/topic,201154.0.html I had already seen the EDN page that that thread references. For example the R/C to smooth the PWM, I'm OK with that.

I understand that the low end of the voltage range of the 317 can be brought to 0V by using a -V on the 358. I understand how to use LM337 to do that. That's cool.

I understand that with a 12V input the LM317 has an 'unavailable' voltage drop, but I don't know what the name of the spec is in the data sheet. Regardless, if it's in the 1.5-2V that brings me closer to the target of 9V.

( I know I could limit the PWM but that costs resolution.)

I understand that using the correct -V on the 358 will bring the output down to 0V. That part I can figure out.

Rather than just asking for 'an answer,' the best I'd like to see is a web page that describes clearly and most importantly simply how to adjust the gain on the 358. The only pages I've found assume the reader has a lot more background than I have.

Just to be clear, your end goal is to vary the output voltage of an LM317 digitally? If you're just trying to achieve a single voltage then all you need is a resistor.

jbarchuk: I understand that with a 12V input the LM317 has an 'unavailable' voltage drop, but I don't know what the name of the spec is in the data sheet. Regardless, if it's in the 1.5-2V that brings me closer to the target of 9V.

I believe you're referring to the "dropout voltage". The input voltage - dropout voltage specifies the highest voltage that the regulator can regulate. It varies with current passing through it: http://www.fairchildsemi.com/ds/LM/LM317.pdf -- see Figure 4. So for 1A of current the dropout voltage is stated as 2.2V. With an input voltage of 12V the highest output voltage you can configure the regulator for is 12V - 2.2V = 9.8V.

Chagrin: Just to be clear, your end goal is to vary the output voltage of an LM317 digitally?

Yep I want to control it by the full range it's capable of. (Voltage dividers are one of the few things I'm clear on. :))

jbarchuk: I understand that with a 12V input the LM317 has an 'unavailable' voltage drop, but I don't know what the name of the spec is in the data sheet. Regardless, if it's in the 1.5-2V that brings me closer to the target of 9V.

I believe you're referring to the "dropout voltage". ... With an input voltage of 12V the highest output voltage you can configure the regulator for is 12V - 2.2V = 9.8V.

Excellent, understood, thanks. Now I know how to read that graph at the device's rated 1A.

jbarchuk: Rather than just asking for 'an answer,' the best I'd like to see is a web page that describes clearly and most importantly simply how to adjust the gain on the 358. The only pages I've found assume the reader has a lot more background than I have.

In the EDN link it's using the Op Amp in an active low pass filter. Putting that another way, the R/C filter alone does not smooth the PWM output well enough to control the LM317 stably. TI's "Op Amps for Everyone" describes active filters in section 16.

Back to the LM317, in a classic arrangement with a resistor (R1) between OUT and ADJ and second (R2) between ADJ and GND, the LM317 works by increasing voltage at the output until the voltage on it's ADJ pin reaches 1.25V. The two resistors act as a voltage divider. So the OUT voltage is increased until it equals R2 / (R1 + R2) * 1.25. When you apply current to the ADJ pin externally, with the active filter mentioned above, it causes the LM317 to adjust its output.