LM25118-Q1: Loop phase in calculator

Satoshi Morimoto

Part Number: LM25118-Q1
Other Parts Discussed in Thread: LM25118, LM5116

Hi Expert,

I am trying to estimate the gain and phase margin using "LM 5118/LM 25118 Quick Start Component Calculator (Rev.A)".

However, changing any parameter does not result in a Buck Loop Phase of less than 0 deg (Gain margin > 0dB). Also, the condition in Figure 19 Typical Application in the datasheet does not result in a Buck Loop Phase of less than 0 deg as shown in Figure 25.

/cfs-file/__key/communityserver-discussions-components-files/196/snvu065a_5F00_typical-application.xlsm

Is the Calculator's loop phase calculation correct? Could you please disclose the formula?

Best Regards,

Morimoto

over 2 years ago

0 dehuan Qi over 2 years ago

TI__Intellectual 2775 points

Thank you for your interest in the LM25118. It is need some time for response due to holiday.

0 Satoshi Morimoto over 2 years ago in reply to dehuan Qi

Prodigy 60 points

Could you answer this question?

Best Regards,

Morimoto

+1 Stefan Schauer over 2 years ago in reply to Satoshi Morimoto

TI__Mastermind 45101 points

Hi Morimoto,

sorry for the long delay.

I just had a look into the calculator and have seen that the tool only considers one Pole (dominant pole) for the modulator.

(I do not know why this has been implemented in that way as I have not designed that tool.)

You can see this in Bode Plots sheet cell w7. The 2nd pole is not considered therefore the phase goes down by 90 Degree and the up again (due to the Zero).
With considering the 2nd Pole the phase would finally go down by 90 Degree.

The used formula can be found in the datasheet of the LM25118 and LM5116.

If you would like to do more evaluation on the compensation you might also use the power stage designer: Power Stage Designer | Overview | Design Resources | TI.com

Best regards,

Stefan

Power management

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LM25118-Q1: Loop phase in calculator