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LM25141-Q1: LM25141-Q1 stabiliy analysis with DCR current sensing

Piotr F.
Intellectual 2360 points
Part Number: LM25141-Q1
Other Parts Discussed in Thread: LM25141DESIGN-CALC, , LM5141-Q1, LM5140-Q1, LM5143-Q1, LM5143

Hello Team!

I'm not sure how to perform correct stability analysis for LM25141-Q1 using dedicated excel design calculator (LM25141DESIGN-CALC) in application with DCR current sensing.
My question applies also to other buck controller from the family: LM5141-Q1, LM5143-Q1, LM5140-Q1.

LM25141DESIGN-CALC only gives option to choose and calculate current sensing on shunt resistor.
Terry Allinder in this thread https://e2e.ti.com/support/power-management/f/196/t/553224 says DCR  current sensing does not affect control loop.
However, when I change Rshunt value in the excel calculator, loop compensation values and reference bode plot change.

So please help to understand/confirm:

1. Does Rshunt value change control loop/compensation?

2. How to use excel calculator to analyze design stability, if Rshunt is not used but DCR sensing?
Should we put DCR value as "Rshunt" or rather put some low Rshunt value (like 0,01mOhm) as is not really used?

Piotr

  • 0
    TI__Mastermind 27030 points

    1. Rshunt value will change the control loop, especially the DC Gain.

    you can refer LM5143 datasheet 9.2.1.2.7, if the Rshunt increase , the Rcomp will increase , which means larger Rshunt, low DC gain

    2. You can use DCR value as Rshunt, cause the DCR sense formula is L*diL/dt+Rdcr*IL=Rs*C*dvc/dt+Vc, when Rs*C=L/Rdcr, Vc=Rdcr*iL, which is equal to Rshunt*iL

    Thanks

  • 0 in reply to Daniel Li14
    TI__Intellectual 2360 points

    Thank you for clarifying,

    As I think about it more:

    If Rshunt value changes  control loop - the DC Gain, why changing DCR in excel calculator does not move bode plot/compensation values? 

    DCR and Rshunt are series resistances present at LM25141-Q1 output - so both affect DC gain, right?

    Is this limitation of LM25141-Q1 excel calc? 

  • 0 in reply to Piotr F.
    TI__Mastermind 27030 points

    Hi

    you need to change Rshunt value . only Rshunt will affect the loop. DCR only affect the power loss

    Thanks