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WEBENCH® Tools/LM5176: LM5176 MODEL IN BOOST MODE

Francis Kimpel1
Prodigy 170 points
Part Number: LM5176
Other Parts Discussed in Thread: LM5118, TINA-TI

Tool/software: WEBENCH® Design Tools

Used Web Bench Designer to develop a buck-boost converter using the LM5176 device

8 < Vin < 36V

Vout = 12V

Iout = 5 amps

Ran the transient simulation with VIN = 22V and 8V.  The output voltage reached the 12 V with relatively low noise. 

The output capacitors were the default values from webbench:  3x 22 uF ceramic capacitors and 2x 180 uF capacitors

The BOM from the report file indicated the following components

22 uF part - TDK PN C2012X5R1V226M125AC

180 uF part - Samsung PN CL10C161JB8NNNC

When I replaced the 22uF parts with a simple R-L-C model from TDK data sheets( R=20 mohm, L=.480nH, C=22uF)  the 8Vin simulation(boost) indicated 1V of noise on the output.

The 22VIn buck result was still low noise.

Noticed the 22uF default mode6(MACRO)  was more complex and didn't appear to have the inductance directly in series with the 22uf capacitance.

Question:  For the boost simulations, which model do I believe? 

Any idea why there a difference in the models? 

How was the Coutx default values developed??

thanks,

  • 0
    TI__Expert 8365 points

     Hi Francis,

    Could you please let us know which macro model are you referring to? kindly let us know the source of this model.

    Thanks & Regards,
    Praveen

  • 0 in reply to Praveen Badiger
    Prodigy 170 points

    Praveen,

    SInce I used this original model, I used a more complex capacitor spice model from TDK with the same results.

    Reran the simulations with the original Tina schematics and a  modified schematic using the TDK spice model.

    I am new to the TI forum and would like to send these files to you but not sure how to do this.

    22uF model from TI webbench

    ***********************************
    * Created by WB Importer *
    ***********************************
    .SUBCKT LM5118_BUCKBOOST_BLOCK_Coutx3_WB_CAP_CERAMIC 2 4
    R1 2 3 0.00205
    C1 3 4 1.4E-5 IC=0.0
    R3 5 4 5
    R2 2 4 7.142857142857143E7
    R4 3 26 455555.5555555556
    R6 3 7 455.5555555555556
    C5 7 1 4.2E-7 IC=0.0
    R7 3 10 45.555555555555564
    C6 10 1 4.2E-7 IC=0.0
    R8 3 13 4.555555555555556
    C7 13 1 4.2E-7 IC=0.0
    C2 26 1 4.2E-7 IC=0.0
    R9 3 28 45555.55555555556
    C3 28 1 4.2E-7 IC=0.0
    R10 3 29 4555.555555555557
    C4 29 1 4.2E-7 IC=0.0
    L8 1 5 15p
    R24 1 5 0.006150000000000001
    L12 5 4 1n
    .ENDS

    TDK model from TDK website put into Webbench design

    .SUBCKT LM5118_BUCKBOOST_BLOCK_Coutx3_WB_CAP_CERAMIC 2 4
    C1 2 11 1.97558419E-05
    C2 2 12 5.46117538E-07
    C3 2 13 5.46117538E-07
    C4 2 14 5.46117538E-07
    C5 2 15 5.46117538E-07
    C6 2 16 5.46117538E-07
    C7 2 17 5.46117538E-07
    L1 4 21 4.80000000E-10
    R1 11 21 2.24990000E-03
    R2 11 12 2.64269036E+00
    R3 11 13 1.75788822E+01
    R4 11 14 1.16773848E+02
    R5 11 15 7.74709587E+02
    R6 11 16 5.13332487E+03
    R7 11 17 3.39743027E+04
    R8 2 11 2.20000000E+07
    .ENDS

    thanks,

    Frank 

  • 0 in reply to Francis Kimpel1
    TI__Expert 8365 points

    Hi Frank,

    This difference in the Vout results is observed due to ESL value. We are looking into it. We will keep you updated.

    Thanks & Regards,

    Praveen

  • 0 in reply to Praveen Badiger
    TI__Intellectual 1490 points

    Hi Frank,

    We use generic template based SPICE models for Passive components (Capacitors, Inductors) and Switches (MOSFETs, Diodes). While the key low frequency characteristics like dc bias derating for Cap,  Rdson for FET etc are included in the model, the high frequency behavior like ESL value is not very accurate in WEBENCH models.

    The vendor model (TDK in the above case) is probably more accurate. Note that the vendor model is provided for DC bias = 0 and you will need to update the capacitance value to match with the derated capacitor value when you use the vendor model.

    To capture high frequency effects like accurate noise on output and input node or ringing behavior on switch node - we recommend you download the vendor SPICE models and perform simulations on offline tools like OrCad / TINA-TI. You should also validate on bench as the high frequency effects are often difficult to model and may also depend on the layout or other factors. 

    Best Regards,

    Srikanth Pam

    Online Design Tools