Speakerguy
Active Member
Actually a good question. I don’t know what residual ripple is & never heard.
The frequency of course I have to calculate. What’s the best frequency will it suits?
Thanks
The graphic you used in this post is the way you want to do it. Connect the RC lowpass to the ADJ pin (point A connects to the ADJ pin, point B connects to ground). Use a 121 ohm resistor between OUT and ADJ.
The LM317 will fight to keep 1.25V between OUT and ADJ, so OUT will always be 1.25V above ADJ. Residual Ripple is the fact that the PWM signal will not be perfect DC but will have some of the PWM content on it at very low amplitude. It will look like little spikes at the switching frequency of less than 100mV on top of the DC output of the lowpass filter. This noise will translate directly onto OUT since the LM317 will just add 1.25V to whatever is on the ADJ pin.
Now, if you are using a 0-5V PWM signal, you will only be able to get 6.25V out of the regulator. You will need to apply gain to the signal if you want to get higher voltages. I suggest that you do this with an op amp configured as a second-order lowpass filter (better performance and less ripple induced from the PWM signal to the OUT voltage) with the gain factor that you need (kill two birds with one stone).
You are DONE. Do not waste data lines for 12 transistors and all that. Use the PWM.
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