Part Number: TPS50601-SP
Other Parts Discussed in Thread: TINA-TI,
Tool/software: TINA-TI or Spice Models
hello, I'm running the TPS50601-SP average model simulation to determine the phase margin of my design. I've noticed that the lower bound of the phase output converges towards -90d rather than the usual -180d. is there a +90d offset in the result? should I subtract the phase margin by 90d to get the correct result?
Mike,
What is important is the phase shift from DC not the absolute number for phase. If you reduce the lower bound of the frequency to 1mHz in the sim, you will see the phase starts from 180 degrees. So for Phase Margin calculations, you need to see how much margin you have above 0 degrees.
The reason the phase starts from 180 degrees in this case rather than the theoretical 0 degrees is because of the LC injection circuit used for measurement of the AC response in the sim. You can see the phase is actually 0 if I go to very low frequencies like 1nHz, then jumps to 180 degrees due to the LC injection circuit.
Hello Mike,
The phase curve in AC simulations will be shifted by 180 degrees compared to the typical phase plot of the loop gain transfer function. This is because the loop gain transfer function does not include "-1" factor due to the negative feedback at the error amplifier.
When trying to measure Phase Margin from the AC bode plot simulation of the averaged model, you need to measure it from 0 degrees (as opposed to from -180 degrees in standard loop gain plots). Below image shows an example.
Regards,
Srikanth Pam | Online Design Tools
