I am wanting to build a custom power supply for myself, and am having trouble understanding how to drive the adjust pin with a microcontroller.
The datasheet for it is here: http://www.ti.com/lit/ds/symlink/lm117.pdf
The issue is that I can make a pseudo? DAC with a low pass filter to get 0-5 volts from my PWM output, however Vout is formed by \$1.25+V_{adj} \$ , meaning, I can only get 1.5-6.5 (or 0-5 if negatively offset) with my solution as is.
My questions are: How do I offset -1.25V? A negative regulator (i.e. LM337) seems a little overkill, however I have never seen a solution with my own eyes and it could be very normal. Are there any quick ways you can think of to offset it?
And more importantly, how do I drive the ADJ pin from ~0-12V if my uC can only output 0-5 with PWM? Is there an indirect method of doing this? A simple building block I can place in between to map this range?
I am quite open to alternatives, however fixing this has piqued my interest and there are not many similar adj. regulators available to me (LT3080 with ~0 offset not available on Mouser at the moment)
Answer
To scale your PWM output voltage (0 - 5 V) up to 0 - 12 V you'll just need a an op-amp in the standard non-inverting configuration with gain of 2.4. You could do this with just about any commodity op-amp. You can also add low-pass filtering to this circuit to (further) reduce the ripple from your PWM setup.
For this to give 0-12 V output from the '317, Vcc needs to be 14-15 V (to bias the LM4041 properly at 10.8 V out), and you also need a negative rail low to let the op-amp generate -1.2 V when you want the LM317 output at 0 V.
You also want to remember the LM317 can only source current, not sink. So if your load will require some current to be drawn out of it to reach 0 V, you'll need either some kind of pre-loading (a pull-down resistor from the output pin to the negative supply rail)
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