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LM25141-Q1: LM25141 PSPICE unexpected results

user5987006
Prodigy 30 points
Part Number: LM25141-Q1
Other Parts Discussed in Thread: LM25141, LM5141,

Hello.

I am having problems with the PSPICE for TI model of the LM25141 which model is LM5141 and I get from www.ti.com/.../snvmau4

I tried with the example “Reference design-->LM5141-Q1RGE_TRANS” and I modified the FB pin by using a voltage divider with 100k to Vout and 11k to gnd, so I expect to see 12Vdc in the output (and 1.2V in FB pin). In the example I also remove the input filter and put an independent voltage source of 30Vdc and finally I put OSC pin to gnd to have 440kHz of SW frequency. (quite similar to the "Reference design-->LM5141-Q1RGE_TRANS")

Voltage in FB pin only reach 0.315V instead of 1.2V that the datashhet of LM25141 as regulation threshold indicates.

In addition in other simulation I connect OSC to the Vdda expecting a SW frequecy of 2.2MHz but I can not see that effect.

Is there any problem with PSPICE for TI or with the model, or I missing something?

I have a problem in a PCB with a buck based on the LM25141 (the duty is no stable) and I think that the simulation should help but now it does not.

Thank you very much.

Carlos.

  • 0
    TI__Mastermind 49317 points

    Hi Carlos,

    I recommend using the LM25141-Q1 Excel quickstart calculator to optimize component selection, especially compensation design. The file is downloadable from the product folder. Just enter the correct value for output capacitance (derated for voltage) and choose a crossover frequency such as 50kHz. Then install the compensation component values as recommended.

    In the meantime, we will check on the PSPICE model.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 30 points

    Hi, I do not know why but I can not send this emaikl with my excel attached. I already use this tool and I fix 44kHz of crossover frequency with 70º in phase margin, more than enough I guess.

    In addition I have doubts about "Just enter the correct value for output capacitance (derated for voltage)", could you give me more information about such derating.

    In the simulation i think my problem is related with the simulation configuration but I am not sure.Thank you very much.

    Carlos.

  • 0 in reply to user5987006
    TI__Mastermind 49317 points

    Hi Carlos,

    You can use the cap manufacturer tool to estimate derating of capacitance with applied voltage (e.g. simsurfing tool from Murata: https://ds.murata.co.jp/simsurfing/mlcc.html?lcid=en-us).

    Regards,

    Tim

  • 0
    TI__Intellectual 1440 points

    Hi Carlos,

    Looking at your simulations results, it looks like the model is operating in current limit condition and unable to reach to Steady state value.

    Due to smaller Soft-Start time and higher output load current demand, the device is not regulating.

    Please make appropriate changes in component selection and operating conditions to get proper simulation results.

    P.S: Please note that there is Rload = {5/5} = 1Ohm connected at VOUT in the reference design.

  • 0 in reply to Timothy Hegarty
    Prodigy 30 points

    Hi, thanks for the manufacturar tool...I did not know about that, seems useful.

    I am still having problems with the simulations PSPICE for TI ,  maybe is the simulation configuration but I do not know.

  • 0 in reply to user5987006
    TI__Mastermind 49317 points

    Hello,

    Please use the Excel quickstart file to check you component values. In the simulation, use a 0.1uF soft-start cap so that the startup is well controlled.

    Regards,

    Tim