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LM25085A / about Webench simulation

Tsukui Yusuke
Guru 21930 points
Other Parts Discussed in Thread: LM25085, LM25085A

Hi,

Could you tell me about feasibility of customer specification?
Customer condition is;
・Vin=12.4 to 19.5V
・Vout=12V
・Iout=6.7A
・L=6.8uH
・Fsw(target)=600kHz
・100% Duty operate assumed

Is it possible to achieve using the LM25085?
Webench simulation could not be the operating frequency to 600kHz.



Could you give me your advice?

Best regards,

Yusuke/Japan Disty

  • 0
    TI__Intellectual 805 points

    Hi Yusuke,


    The LM25085A has a minimum on time for the PFET of 150nS. This applies an upper limit the switching frequency that also depends on duty cycle. The upper limit of the switching frequency can be estimated via the following calculation FSW_Ceiling =  (Vout / Vin ) / 150nS.

    More information on this minimum on time can be found in the LM25085 data sheet in section 7.3.4.

    So to answer your question on feasibility, This device is not well suitable for the design that you describe above.

    I hope this helps resolve your issue.


    Regards

    -Jason

  • 0 in reply to Jason
    Guru 21930 points

    Jason-san,

    Thank you for your support.
    I want to question about expression.

    ◇FSW_Ceiling =  (Vout / Vin ) / 150nS.

    ex)　Vin=19.5V、Vout=12V
    Fsw ceiling
    =(12V/19.5V)/150ns
    =4102kHz

    Could you give me your advice?

    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    Guru 21930 points

    Hi,

    Can I get the information?

    If you need more information or if  I should  clarify my comment ,

    please let me know.

    I’m so sorry to rush you but please support this.

    Ｃustomer is going to compromise the operating frequency.
    Is it possible to achieve if Fsw=500kHz?

    ◇Vin=12.4V to 19.5V、Vout=12V
    ◇Ｔａｒｇｅｔ Fsw=500kHz

    Could you give me ａｄｖｉｃｅ about Max duty cycle?

    Best regards,

    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Intellectual 2155 points

    Hi Yusuke-san,

    In your customer's design case (Vin=12.4-19.5V, Vout=12, Iout=6.7A), the max frequency in WEBENCH is being limited by the estimated power dissipation in the FET selection, not the Ton_min specification. 

    You can 'trick' WEBENCH into creating a higher frequency design by lowering the Ta (ambient temperature), but your customer needs to take care in the thermal management to make sure the FET and IC do not overheat.

  • 0 in reply to Kevin Ip
    Guru 21930 points

    Kevin-san,Jason-san-san,

    Thank you for your kind support.
    Let me ask you one more thing.
    Customers want to know the direction of design.
    Please tell me the simulation or design examples of the following conditions.

    Target spec and parts：
    -Vin=12.4 to 19.5V
    -Vout=12V
    -Iout=6.7A
    -Fsw=500kHz to 600kHz
    -PFET:TPCA8128
    -Diode:RBQ30NS45A
    -Inductor:4.7uH

    Customers want to know specifically about PFET and diode information.
    (For FETs, parallel driving is also assumed.)

    Could you tell me your advice?

    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Intellectual 2155 points

    Hi Yusuke-san,

    I created a shared public design as an example with your specifications:

    https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=CE025DB6A19CFEDD

    Fsw=554kHz

    D1= STPS20M100SG-TR

    M1= Si7149DP

    We do not have your customer's preferred diode or FET component, but the chosen components have similar specifcations.