SG3525 Working and experimentation

Question

I was trying to understand the SG3525A (from ST-Micro) working. So I built a circuit as shown below. I read various articles on this chip, it mentions that to know whether its working or not, it can be checked using the Ct pin which would show the Sawtooth wave.

In this circuit, I have the following questions.

1. The Vref pin is supposed to provide a 5V precision reference. Its not detectable in the oscilloscope. Is it normal?
2. What is the maximum input voltage that can be given to each pin.
3. In many reference circuits I found that the compensation pin is connected with a series capacitor and a resistor and it is fed to the inverting input. Why is it done and how is the values of capacitor and resistor is selected?
4. What is the work of the oscillator pin? Am I supposed to see square wave in the scope with the set frequency using RC?

Please help me out in finding the answers. Thank you!!

Answer 1

To help people like me who are wrestling first time with this SG3525 IC, I'm sharing the answers to the above questions.

1. "The Vref pin is supposed to provide a 5V precision reference. Its not detectable in the oscilloscope. Is it normal?"

Ans : The IC was damaged. With any SG3525 IC you are supposed to get 5V reference output from the Vref pin (16th pin) detectable using multimeter/oscilloscope

2. "What is the maximum input voltage that can be given to each pin."

Ans: The maximum input voltage to each pin is 5v unless otherwise specified. For eg. V+ and Vc pin accepts upto 35v.

3. "In many reference circuits I found that the compensation pin is connected with a series capacitor and a resistor and it is fed to the inverting input. Why is it done and how is the values of capacitor and resistor is selected?"

Ans. The compensation pin is actually the output of the error amplifier. So any change in input would almost instantaneously changes the output of the error amplifier. So to avoid those sudden changes normally RC series or parallel circuit is fedback to the Inverting pin of the Error amplifier (Pin-1).

Some more points to note for beginners.

• The maximum duty ratio of SG3525 is 50%. So if you want to run the IC standalone as an oscillator with 50% duty. Just connect Vref to Pin-2 and enjoy squrewave output with 50% duty.
• If you want to use this IC to drive mosfet using an optocouplor such as TLP250H, and the output squarewave must be of 5v, just connect 5v to the Vc pin (Pin-13) then you would see a square wave with 5v peak.

I'm still trying to understand the proportional change in duty ratio using feedback. If anyone is already an expert please share your experience.

Thank you