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LM2731: Different results produced by same design for LM2731XMF/NOPB

Expert 5485 points

Replies: 12

Views: 175

Part Number: LM2731

Hi Team,

I am posting this on behalf of our customer regarding their situation with the different results in the circuit products by Webench design.

Here is the statement of the customer:
Same design was run on webench on 2 different dates this year (May and August). August report now produces much worse results. Pd of controller jumps from 18mW to 50mW for exact same design! efficiency drops from 90% to 82%.. We built this circuit and its efficiency is ~80%. not what we were designing for. what's going on? Are the specs for the FET in the controller worse than what's been modeled in webench? 
What is the source of this difference?
Is it a webench error or an error in the part’s datasheet?


Here's the comparison table of the customer in the same design:


Also, please see this pdf attached for the two the same designs of the customer:
WBDesign 6.5V 50mA 82.4%.pdfWBDesign_6.5V_50mA_85C (1).pdf

Please let me know if you have any questions for the customer.

Thanks,

Jonathan

12 Replies

  • In reply to tom greene1:

    Hi Tom,

    There is no change in datasheet specifications.

    The rise_time and fall_time specifications are not provided in the datasheet. These are the parameters used within WEBENCH model to calculate the internal FET switching losses, using below equation -

    FET Switching loss = 0.5*Vin*Iout*(rise_time + fall_time)*Fsw.

    We reviewed the Power dissipation model for this product and realized that there was significant error w.r.t bench data – due to incorrect internal FET parameters. We corrected the model in May – this lead to update of the Webench model.

    Thanks and Regards,

    Atul.


     

     

    If my reply answers your question please click the "This resolved my issue" button

  • In reply to tom greene1:

    Hi Tom,

    1. TPS61087 has same Iq, Rdson with TPS61085. You can selects it.

    2. What's the output voltage of the 5.5V design version? TPS61023 is a good choice but its maximum Vout is 5.5V. We would suggest you set Vout to 5.45V to leave some margin.

    3. For TPS61170 case, just a reminder, to measure the efficiency accurately, the input and output current should be measured by DMM. Normally the power supply monitor number is not accurate. Also measure the input and output voltage close to the IC input and output capacitor because the cables usually have voltage drop.

    Regards,
    Zack Liu
    Boost Converter Solution

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