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LM5122EVM-1PH: PMP7970_LM5122/ 12V to 54V@ 3.5A solution survey

Simon Chen
Prodigy 70 points
Part Number: LM5122EVM-1PH
Other Parts Discussed in Thread: PMP7970, LM5122

Hi TI,

I'm looking for 12V to 54V@ 3.5A solution for Fan power supply application. May I reference PMP7970 PMP7970 reference design | TI.com 12V to 54V@ 2.5A and enhance to Iout=3.5A? (keep using 1-phase)

eg. use 2H+2L MOSFET, choose better inductor spec.....

Please  give me some optimization guideline, thanks!

  • 0
    TI__Expert 5845 points

    Hi Simon,

    thanks for using E2E.

    Yes, you can enhance the reference design to 3.5A with on phase. You just have to make sure, like you said, to choose suitable MOSFETs and inductor.

    Best regards,

    Moritz

  • 0 in reply to Moritz
    Prodigy 70 points

    Hi Moritz,

    I'm drawinf the schematic, but I have a little confuse for AGND/ PGND connection.

    Q1: May I use 0 ohm for connection between AGND and PGND, refert to as below picture?

    Q2: which pin should use AGND for their GND? which pin should use PGND for their GND? Please comment, thanks!

    sch as below...

  • 0 in reply to Simon Chen
    TI__Expert 5845 points

    Hi Simon,

    AGND and PGND has to be connected at one point. We recommend to connect them over the groundplane of the IC.

    Sensitive pins like UVLO, RT, SS,FB should be connected to the analog ground plane. See picture attached:

    Best regards,

    Moritz

  • 0 in reply to Moritz
    Prodigy 70 points

    Hi Moritz,

    Q1: Let me confirm again, according to the LM5122-1PH EVM layout.

    VIN, UVLO, SS, SYN/RT, SLOPE, MODE, RES Pin connect to AGND

    VCC Pin  connect to PGND

    Am I right?

    Q2: when I using WEBENCH to caluate my design, after I set user preferred  Fsw=250kHz, and click "Redesign" , then it always show "99.01kHz"?

    unless I modify Iout from 4A to 2A, then the "Redesign result" could meet my quest: Fsw=246.57kHz

    Is there any limitation for my design spec ? since Fsw need >200kHz for esay select output inductor spec. ( High current + lower inductance spec)

    If you have LM5122 calculation sheet could share with me is better, thank you!

  • 0 in reply to Simon Chen
    TI__Expert 5845 points

    Hi Simon,

    The Vin capacitor should be connected to PGND. This seems to be not right in the EVM schematic.

    For the WEBENCH questions i will move the thread.

    Best regards,

    Moritz

  • 0 in reply to Moritz
    Prodigy 70 points

    Hi Moritz,

    Thanks for your confirmation of Q1!

    Regarding to the Q2, my concern is Fsw could set to 250kHz? Is it workable in 12V to 54V@ 4A application ? 

  • 0 in reply to Simon Chen
    TI__Intellectual 2630 points

    HI Moritz,

    We are looking into WEBENCH query related to frequency set for the WEBENCH design. Kidly give us sometime to look into the WEBENCH model for the device and get back to you. 

    Regards

    Shaheen S M

  • +1 in reply to Shaheen Makandar
    TI__Expert 5010 points

    Hi Moritz,

    In regards to the query pertaining to Webench frequency selection ,following are our observations.

    The WEBENCH FET selection algorithm calculates the power dissipation of FETs for given conditions(Vin, Vout, Iout & Frequency). If FET hits the junction temperature threshold (IC Tj Max) then WEBENCH rejects the FET and reduces the operating frequency in order to reduce power dissipation and selects the suitable  FET.

    For the below condition for user preferred  frequency of 250kHz , since a suitable FET couldn't be found at 250KHz the operating frequency is reduced to "99.01kHz" to select a suitable FET.

    When we reduce the Iout from 4A to 2A,power dissipation reduces and hence we are able to select the FETs for the frequency of 246.57kHz without frequency reduction.

    Kindly revert back to us for any further queries.

    Thanks and Regards,

    Umayal