LM6172: Phase Margin

Sadanori Kato

Guru 36490 points

Part Number: LM6172
Other Parts Discussed in Thread: TINA-TI,

Hi,

Could you please tell me whether "LM6172 PSPICE Model" can support an open loop analysis with TINA-TI ?

LM6172 PSPICE Model
http://www.ti.com/lit/zip/snom234

Best regards,
Kato

over 4 years ago

0 Michael Steffes over 4 years ago

Guru 47485 points

Morning Kato-san,

Yes, the V11 TINA library had the 1999 model in it, no apparent update, about 139MHz true GBP

Here is this file as a V9

LM6172 Aol sim.TSC

This recent article talks about getting the true GBP,

https://www.planetanalog.com/why-is-amplifier-gbp-so-confusing-insight-12/

0 Sadanori Kato over 4 years ago in reply to Michael Steffes

Guru 36490 points

Hi Michael-san,

Thank you for your prompt reply.

I understood.

Best regards,
Kato

0 Sadanori Kato over 4 years ago in reply to Sadanori Kato

Guru 36490 points

Hi Michael-san,

I just modified the schematic as follows, so could you please double-check it just in case ?

LM6172 Aol sim test.TSC

Best regards,
Kato

0 Michael Steffes over 4 years ago in reply to Sadanori Kato

Guru 47485 points

Looks good, I went on to closed loop gain of 1, the model does not match the data sheet at all,

The model looks transistor level, almost would trust it more than the PDS plots.

That 1.57dB peaking implies about 52deg phase margin from Figure 2 in this article - they broke the links to enlarge figures in this article, if you want the original PDF let me know

https://www.planetanalog.com/stability-issues-for-high-speed-amplifiers-introductory-background-and-improved-analysis-insight-5/

To run a LG for this gain of 1, would need to add the input C - not in the data sheet, but this lower phase margin at crossover obviously extends the closed loop BW by that about 1.6X in the latest article, The original Aol curve (not quite right for the gain of 1 as it is not including the input C) showed a unity gain of 109MHz with 50deg phase margin (again, this is missing the effect of the input C), which taking that times the 1.6X BW extension in that latest article predicts a closed loop F-3dB of about 174Mhz - pretty close to actual sim of 182MHz. The data sheet plots are way off on this, not sure what their errors are, but not uncommon on this kind of thing.

0 Sadanori Kato over 4 years ago in reply to Michael Steffes

Guru 36490 points

Hi Michael-san,

Thank you for the detailed information.

Could you please provide me the original PDF ?
Additionally, I would like to confirm just in case, so is my understanding correct that the frequency characteristics mentioned in the data sheet are incorrect and the simulation results are correct ?

Best regards,
Kato

0 Sadanori Kato over 4 years ago in reply to Sadanori Kato

Guru 36490 points

Hi Michael-san,

Sorry for rushing you, but please share the original PDF file with me.

Best regards,
Kato

0 Michael Steffes over 4 years ago in reply to Sadanori Kato

Guru 47485 points

morning Sadanori-san

I was working with Planet Analog to see if they are going to fix those broken links in the older Insights? They are not, here is the pdf original for you

The SSU part 5 Stability issues for high speed op amps_introductory issues article text Feb. 2_2019.pdf

0 Michael Steffes over 4 years ago in reply to Sadanori Kato

Guru 47485 points

For some reason I was not seeing these followup questions

Datasheet vs TINA? Well, gain of 1 SSBW in the datasheet is very low vs simulation. I would be guessing they ran into a slew limit using too high a test output Vpp. I cannot be sure, perhaps the product group can run a SSBW test with 100mVpp output on the EVM board to get some guidance here.

One clue here, LM parts are done by the older precision group at NSM - LMH (Linear Monolithic High Speed) are done by more experienced high speed folks. LMH datasheets are usually a better, but not perfect either.

As for alternates,the parametric cross reference here shows these options,

http://www.ti.com/amplifier-circuit/op-amps/cross-reference.html

oops, this says that part number is not in the database -

0 Sadanori Kato over 4 years ago in reply to Michael Steffes

Guru 36490 points

Hi Michael-san,

Thank you for the information.

I have one more request, so could you please provide me the following pdf format file ?

https://www.planetanalog.com/stability-issues-for-high-speed-amplifiers-introductory-background-and-improved-analysis-insight-5/

Best regards,
Kato

0 Michael Steffes over 4 years ago in reply to Sadanori Kato

Guru 47485 points

I am not seeing these in my email, here you go

4760.The SSU part 5 Stability issues for high speed op amps_introductory issues article text Feb. 2_2019.pdf

0 Sadanori Kato over 4 years ago in reply to Michael Steffes

Guru 36490 points

Hi Michael-san,

Thank you for the information.

I got it.

Best regards,
Kato

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LM6172: Phase Margin