Question on schematic snippit in Art of Electronics

[Dan Moos]

I'm looking at at figure 9.18 on page 610 of the third edition. Voltage regulator chapter.

What is the purpose/function of Rc and Cc. The 'c' in the part reference suggests to me that this is a compensation filter, but if so, how does it work? From my understanding of what an lm317 looks like internally,this wouldn't be a miller cap.

I also thought it might be a noise filter, which would make sense as it is a small signal controlling a large voltage, and noise error would be greatly amplified.

Thanks!

[barshatriplee]

Hi, I think you forgot to attach the picture.

[armandine2]

or thinks everyone has the pdf from the internet

[srb1954]

I'm looking at at figure 9.18 on page 610 of the third edition. Voltage regulator chapter.

What is the purpose/function of Rc and Cc. The 'c' in the part reference suggests to me that this is a compensation filter, but if so, how does it work? From my understanding of what an lm317 looks like internally,this wouldn't be a miller cap.

I also thought it might be a noise filter, which would make sense as it is a small signal controlling a large voltage, and noise error would be greatly amplified.

Thanks!

It is for compensation of the control loop. Although the LM317 normally doesn't need external compensation this circuit adds extra loop gain, from the LM358, and extra phase shift through the DC-DC convertor so additional compensation is required to stabilise the overall feedback loop.

The capacitor Cc rolls off the gain to reduce the loop gain at higher frequencies to give the feedback loop sufficient gain margin. Unfortunately, it also adds an additional 90 deg of phase shift which degrades the phase margin and hence the loop stability. The addition of Rc compensates for this by adding a zero in the compensation ultimately reducing the extra phase shift introduced by Cc and enhancing the phase margin.

[Dan Moos]

Perfect answer! Thank you!