0
\$\begingroup\$

I have seen the same issue posted here with TL494, but The feedback pin set up in that IC, seems to be different- so that I cannot adapt it here. Block Diagram Application

I got an idea about how to implement a closed loop system. Vref to non inverting pin and scaled output to inverting pin. But I need a open loop system where I can Vary the PWM duty cycle within the range using a potentiometer without taking a sample from output. Is it possible with this chip?

\$\endgroup\$
1
  • \$\begingroup\$ Sorry something happened to post. images not legible. \$\endgroup\$
    – UB_Roy
    Jul 25, 2020 at 14:33

2 Answers 2

2
\$\begingroup\$

I need a open loop system where I can Vary the PWM duty cycle within the range using a potentiometer without taking a sample from output. Is it possible with this chip?

Clues are in the data sheet such as this lab test fixture: -

enter image description here

Look bottom left and you'll see a switch that can be set to position 1 - this applies a 10 kohm feedback resistor to the error amplifier and then allows you to set the PWM output with the potentiometer shown top left.

\$\endgroup\$
4
  • \$\begingroup\$ Thanks. Basically got the idea. But in this method, RT is 0, that pin is grounded. it may adversely effects the existing frequency formula. Any thoughts about that? Current formula is Freq = 1/{( 0.7RT+3RD) * CT} \$\endgroup\$
    – UB_Roy
    Jul 25, 2020 at 17:05
  • \$\begingroup\$ @user2454516 hmm that looks like an error. I would just assume it's an error and fit RT as per the formula. \$\endgroup\$
    – Andy aka
    Jul 25, 2020 at 17:48
  • \$\begingroup\$ @ Andy aka Thanks for the input. Will try out and update. \$\endgroup\$
    – UB_Roy
    Jul 26, 2020 at 2:37
  • \$\begingroup\$ @UroBorosTech if you are done with this and have no further questions then please formally accept my answer. \$\endgroup\$
    – Andy aka
    Jul 28, 2020 at 17:15
0
\$\begingroup\$

@ Andy aka Thank you very much for the information. I have tried the circuit and your assumption was correct regarding the Rt. I have wired values as per the original formula and the frequency was close to intended value. I need 25Khz PWM so need 50Khz oscillator. enter image description here In this circuit which I tried, RV1 gives control over PWM close to 50% which is maximum in this, and R17 controls frequency around 20-25Khz. The dead time resistor which was 100 Ohms in the test set up seems to be too high considering my Ct and which resulted in considerable duty cycle loss because of high dead time. So I reduced the value to 10 Ohms here.

Thanks again!

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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