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PWM controller circuit

Thread starter #1
I am using Texas Instrument SG3524 PWM controller for 400Khz but I am getting constant output and not pulse (100% duty). Someone please tell where I am doing wrong or can share the circuit.
 

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ronsimpson

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#2
Do you have Vcc=10V ? Should have a capacitor from Vcc to GND.
Do you have "Vref on the left side connected to Vref on the right side?
Is the switch on pin 10 open or closed?
1k pot should be turned so pin 4 and pin 5 have almost the same voltage.
upload_2018-3-4_6-34-20.jpeg
 

alec_t

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#3
Are you using component values exactly as shown in that datasheet test circuit?
What are your Ct, Rt values?
 

ronsimpson

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#5
I am using 5K pot for Rt and 1nF for Ct.
Trouble shooting from 1/2 way around the world is hard. You must give much more information.
Do you have a oscilloscope?
What is happening on every pin? Voltage, wave form, etc
Example: CT should have a saw-tooth wave form. What about OSC OUT? Vref?
upload_2018-3-5_7-31-10.jpeg
 
Thread starter #6
Do you have Vcc=10V ? Should have a capacitor from Vcc to GND.
Do you have "Vref on the left side connected to Vref on the right side?
Is the switch on pin 10 open or closed?
1k pot should be turned so pin 4 and pin 5 have almost the same voltage.
View attachment 111319
I have every component as shown. I have connected as shown but I am not able to get full range of duty cycle but only 75% to 95%.
 
Last edited:

ronsimpson

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#9
but only 75% to 95%.
I think not. See picture below. The block "T" will keep duty cycle below 50%. (only one output can be on at a time) If you connect COL 1 and COL 2 together than yes you can get 90%.

Sawtooth ramp on CT is 0V to 3.5V (about) The voltage at COMP is compared to the sawtooth ramp. This is where duty cycle is made.

Inputs(-) and Input (+) only work right is they are from 1.8 to 3.4 volts. In your test circuit there is a 2k/10k Pot/2k. It limits the voltage on Input(+).
Because INPUT(-) is connected to COMP the gain of this amplifier is one. So the voltage on COMP will be the same as the arm of the pot.
To get less duty cycle the voltage on COMP needs to be lower. There are two ways.
1) pull down on COMP to force it lower.
2) increase the gain on the error amplifier.

upload_2018-3-5_12-23-39.png
Back to your circuit. I added two 10k resistors to make a 2.5V.
I cut open pins 1 and 9 and replaced with 100k resistor. Now the error amp has gain!
Now when you move the 10k pot; low voltage = 0%, high voltage = max duty cycle. When the pot output voltage is near 2.5V you will see the duty cycle change. Now the COMP voltage can change over a larger range.
upload_2018-3-5_12-36-39.png
 

alec_t

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#11
Here's an SG3524 model I created :
 

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alec_t

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#14
I don't know what your circuit is supposed to do, but the IC pins aren't being used in the normal way and the pins don't line up with connections to other components in the sim.
If it helps, here is a test file for the model.
 

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Ian Rogers

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#16
What sub-directory do I put these files in?
I put the .sub in the ltspice/lib/sub directory and just double clicked the .asy and ltspice installed the symbol...
 

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