0
\$\begingroup\$

For a school project I am building an analog PWM generator with 0.1 to full-scale linearity and a duty cycle 0-100%. Powersupply is 5 Volt and 100% is equal to 5V at the output. The frequency 1 Khz.

My circuit is done for like 80%. The problem I have is with the reference voltage of my error amplifier. My research so far has shown that this gives you the ability to have the 0-100% duty cycle.

When I apply a reference of 2.5 Volt, I see that my dutycycle is 50%. When I change Vin to change the duty cycle , it changes with 0.5V to 0% or 100% from reference voltage.

So, 2V = 0%

2,5V = 50%

3V = 100%

Sidenote:

I used the slau508 design and theory, but my design differs slighty because mine is not going to be used for audio, hence why I need to adjust the reference voltage according to my teacher.

How can I fix this so that I can change the dutycycle from 0 to 100%?

I need to have a reference voltage for my error amplifier so that I can have a duty cycle from 0-100% with a voltage change of Vin from 0-5V.

With the reference of 2,5V that I have right now I can only go from 0 to 100 procent with an increase or decrease of 0,5V. This should be 5V in order to change between 0 and 100% dutycycle.

My question is:

How can I calculate or determine what I have to do with the reference voltage in order to have the 0-100% duty cycle with a voltage change of Vin from 0-5V?

enter image description here

\$\endgroup\$
  • \$\begingroup\$ It's not clear what help you are asking for. \$\endgroup\$ – Andy aka Oct 22 '16 at 16:00
  • \$\begingroup\$ Sorry , i thought i was clear. i need to have a reference voltage for my error amplifier so that i can have an duty cycle from 0-100%. with the reference of 2,5V that i have right now i can only go from 0 to 100 procent with an increase or decrease of 0,5V. This should be 5V in order to change between 0 and 100% dutycycle. my question, is how can calculate or determine what i have to do with the reference voltage in order to have th 0-100% duty cylce with a voltage change of Vin from 0-5V?.... \$\endgroup\$ – D-Antonio Oct 22 '16 at 16:01
0
\$\begingroup\$

Currently with R3 at 10k you get +/-50% duty cycle changes for +/- 0.5 volts centred on 2.5 volts. You want this to be +/- 2.5 volts so, make R3 five times bigger at 50 kohm.

\$\endgroup\$
  • \$\begingroup\$ What has R3 to do with the reference voltage? If you make R3 bigger the frequency of my integrator will change from 53 Hz to (still have to calculate). Making R3 will have the capacitor charge more slowly due the fact that there is less current, referring to Ohms law? \$\endgroup\$ – D-Antonio Oct 22 '16 at 16:14
  • \$\begingroup\$ C1 and R4 integrate out the PWM and create "analogue" negative feedback. I said adjust R3. Why not try it or simulate it because I'm not going to get into a long discussion about this. \$\endgroup\$ – Andy aka Oct 22 '16 at 16:21
  • \$\begingroup\$ I'm just trying to understand why you give me that advice. I dont have the packages or components in my simulation tool. \$\endgroup\$ – D-Antonio Oct 22 '16 at 16:24
  • \$\begingroup\$ Increasing R3 lowers the gain and hence you require more volts at the input for the same duty cycle. \$\endgroup\$ – Andy aka Oct 22 '16 at 16:37
  • \$\begingroup\$ Okéy, im going to try that. Also i'm checking if I can find op amp models that I can download for lt Spice to simulatie . Thnx for the advice. \$\endgroup\$ – D-Antonio Oct 22 '16 at 16:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.