LM5575: Generate closed loop Bode plot

Mario Ardizzone

Part Number: LM5575

Hi,

I'm working on measuring the closed loop Bode plots for the following regulator. Using one of our production units, I have inserted a 1k resistor to create a high Z / Low Z impedance nodes. Then using the Frequency Response Analyzer, I inject a signal via CH1 (1k and R50 node), and measure the response on CH2 (1k and C226 node). See the picture below.

I'm getting unusual phase measurements. Can anyone recommend what I'm doing wrong? Do I need to move the insertion point before the OUT (Pin 11) connection?

How does TI measure the closed loop performance that it shows in the Webench simulations?

Any help would be appreciated.

over 7 years ago

0 Katelyn Wiggenhorn over 7 years ago

TI__Genius 11040 points

Hello,

Your picture does not appear. Would you be able to reattach? This will help me be able to see if I understand your set up correctly.

The typical set up that we use is a 50-Ohm resistor between VOUT and FB. Then insert the the AC signal across this 50-Ohm resistor.

Once you reshare the image, I can advise further.

Thank you,
Katelyn

0 Mario Ardizzone over 7 years ago in reply to Katelyn Wiggenhorn

Prodigy 40 points

7450.Schematic.docx

0 Katelyn Wiggenhorn over 7 years ago in reply to Mario Ardizzone

TI__Genius 11040 points

Hi,

Thank you for resharing. The placement of the resistor and injection point looks good. I would recommend lowering the resistance to 50Ohm. What is the injection signal amplitude?

Also, below is a post which goes over the bode plot set up and measurement.

e2e.ti.com/.../power-tips-setting-up-bode-plotter-properly

Best Regards,
Katelyn

0 Mario Ardizzone over 7 years ago in reply to Katelyn Wiggenhorn

Prodigy 40 points

Thanks for the recommendation. We lowered the injection resistor to 42.2Ω (readily available value). The injection signal amplitude (@ 100 Hz) is approx CH1: 1.38 mVrms, CH2: 68.5mVrms.

I have attached a word doc with the schematic and the resulting bode plot. Would you please review the plot and comment? Do you see any areas of concern in the data?

thank you!

0 Mario Ardizzone over 7 years ago in reply to Mario Ardizzone

Prodigy 40 points

28V REG BODE PLOT.docx

0 Katelyn Wiggenhorn over 7 years ago in reply to Mario Ardizzone

TI__Genius 11040 points

Hello,

Thank you for sharing the bode plots and thank you for the discussion on Tuesday. Roman should be following up with you, but if you have any additional questions, please do not hesitate to reply to this thread as well.

Best Regards,
Katelyn

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LM5575: Generate closed loop Bode plot