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# WEBENCH® Tools/LM3481-Q1: AC simulation

Part Number: LM3481-Q1

Tool/software: WEBENCH® Design Tools

I would like to design a boost converter using LM3481 from texas instruments. To de the job, I have selected TINA TI circuit simulator, where the average model of LM3481 is given. So I have used this model to get the open loop bode plot of the power stage without the compensator.My objective is to get a plot that will help me to decide where to put poles and zeros of the compensator. The circuit is given bellow with (L=6uH, Rsense=20m, Fsw=227kHz)

My probleme is that I do not know how to built a circuit for AC simulation, for exmample, I do not know how to isolate the COMPENSATION PIN or where to connect it.

*How do I get Vout/Vcomp bode plot (the open loop gain of the power stage) in order to decide where to place zeors and poles of the compensator

• Hi Kirous Bachi,

You can use TINA-TI "post-processor" to plot any transfer function. The same AC setup can be used to plot power stage as Vout(s)/Vc(s). Attached screen shot shows the details. Also please use resistor load - calculate load resistor based on your output voltage and current requirement and replace the current sink load with a resistor in your schematic.

Best Regards,

Srikanth Pam

• In reply to Srikanth Pam:

In case I want to remove the compensation part, is it right to inject the perturbation signal from COMP pin? in order to get a transfer function without a compensator

• In reply to kirous bachi:

Kirous Bachi,

It is recommended to measure power stage in closed loop so that converter is operating at the right duty to provide output regulation.

If you want to measure by completely removing the compensation - then you will need to manually bias comp pin to the right DC voltage and then inject the perturbation signal at comp pin. Both methods should give you the same power stage transfer function if they both have same operating point and power stage components. You can check the operating point by DC operating point simulation and looking at the output node voltage. Note that any change in load resistance / input voltage you need to update your bias on comp pin so output is at desired voltage level.

Below attached both methods with identical results.

Best Regards,

Srikanth Pam

• In reply to Srikanth Pam:

Dear,

I have simulate the above circuit with the same values but i get a different bode plot  for Vout/Vc. I do not really understand where is the probleme, Please can you figure out what I have missed ?

Bellow is my circuit

• In reply to kirous bachi:

Hi Kirous Bachi,

I have downloaded the TSC file you have shared above and simulated it. I am getting the same results as Srikanth got above. Could you please follow below steps to run the simulation? Let me know it this helps.

1. Click on AC Transfer Characteristics.

2. Please keep below settings and click OK to run the simulation.

3. Result

Best Regards,

Bhushan

• In reply to Bhushan Waghmare:

Hi Bhushan

I am really stack, I have redesigned the circuit from the begening, and simulated again, unfortunately I got a different plot (The same that I have already posted).  I don't understand anything!

Please how did you configure ERC ?? (Step 1)

Is there a configuration for the graph viewer ?

May it be the difference between version of TINA TI ? Me I'm using v9.2

• In reply to kirous bachi:

Hi Kirous,

We are able to simulate the schematic you provided as is with out any change / new configuration. So I believe there is no issue with ERC or graph viewer configuration.

I am assuming the model may be behaving erroneously in older TINA-TI v9.2. Can you please download the latest version TINA-TI v9.3 (9.3.200.277 SF-TI) from https://www.ti.com/design-resources/design-tools-simulation/models-simulators/overview.html and try to simulate the schematic ?

Best Regards,

Srikanth Pam

• In reply to Srikanth Pam:

Hi

Exactely, this is what I have noticed, because I have already downloaded v9.3 and I got the right graph, In another words:

*The model does not respond correctely in "Industrial edion".

*But it works well in basic edition "v9.3"

So I have contacted TI technicians and they have confirmed to me that, and I'm waiting to fix it for me

Thank you a lot