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LM3481-Q1 SEPIC Schematics webench

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: LM3481-Q1, LM3481
Hello,

I simulated LM3481-Q1 by the following conditions.
Vin: 3V ~12V
Vout : 5V
Iout:250mA
Fsw:700kHz
Ta=30°C
 
As a result , I found some discrepancies between simulation result and datasheet of LM3481.
Please see the attached file as webench reports.
 
<Vcc capacitor>
Webench : 220nF  ,  Datasheet : 0.47uF ~ 4.7uF
<Inductor>
Webench : L1=22uH, L2=47uH    Datasheet : L1>33.5uH , L2>12.3uH
 
I believe that the datasheet is correct.
However , could you please let me know which result is correct ?
Even if webench result of Vcc cap and Inductor is incorrect , is  compensation circuit of Webench result correct ?
I would like to know I can believe the compensation circuit of Webench result.
 
Best Regards,
Ryuji Asaka
 
 
 
  • 0
    TI__Intellectual 2930 points

    Hi Ryuji,

    Yes, please go with at least 470nF on the Vcc cap. I will get this fixed in WEBENCH promptly.

    The datasheet inductor values are picked to be in CCM at full load.
    WEBENCH values actually use a different set of criteria. When using uncoupled inductors an instablility can occur at some operating points as ringing across the SEPIC capacitor that you don't see when using coupled inductors. You can get rid of this with a RC damping branch across the Cap, or there is a stability criteria in inductor selection that makes it so that this doesn't happen. WEBENCH uses this to do its inductor selection. (an simplification of the criteria is that L2 should be at least double L1)

    The WEBENCH compensation is then designed for the power stage components it has selected. I would stick with  the WEBENCH selected powerstage in this case.

    Best regards,
    Tommy