Electronics > Projects, Designs, and Technical Stuff
Self-Oscillating Analog-to-PWM Converter for Class-D
Evan.Cornell:
Can anyone explain to me how the attached circuit works? This comes from https://www.psemi.com/pdf/app_notes/an72.pdf
I am interested in how to set PWM frequency, how component selection affects accuracy of the PWM pulse width (and thus, measurable distortion), etc.
iMo:
LT1016 comparator is wired as an oscillator where the R33/R34 with C29 set the frequency. The voltage at emitter changes the outputs duty. The transistor is a buffer only.
Evan.Cornell:
Purpose of D1-D3, D7? They would seem to limit the input voltage range, at first glance.
Is C6 there for Q1 buffer amp stability or something like that?
iMo:
Above is the LTSpice simulation you may play with.
Diodes are limiters (2.8Vp-p), the voltage at emitter should be at 2.5V idle, imho.
C6 - perhaps stability or low pass..
SiliconWizard:
To better understand this, if you haven't already, the key would be to take a look at LT1016's datasheet. (Don't you love it when symbols used on schematics bear no meaning at all, such as here with this LT1016 represented as a box? It would look a lot clearer to me if they had used the typical vendor's symbol... it's basically a fast comparator with complementary outputs.) https://www.analog.com/en/products/lt1016.html
I think the main parameter that will influence the average oscillation frequency is R35. R35 basically provides the positive feedback that allows the circuit to oscillate. Frequency will typically increase as R35 is increased. Keep in mind the oscillation frequency will not be quite fixed (thus I talked about average) here, so this is not exactly pure PWM. More like some kind of PDM.
Attached is a quick simulation I made (LTSpice) so you can try things. I've added a very crude low-pass filtering of the output so you get an idea of the reconstructed signal. Of course it will require some more serious filtering. I suppose it's going to feed a power output stage if it's for a class-D amp, so you'd need to take this into account to get a realistic of the end distortion. You can further add your power stage and a model of speaker into this simulation and make some simulated measurements.
Navigation
[0] Message Index
[#] Next page
Go to full version