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WEBENCH® Tools/LM3478: Evaluation Design Not Reproducible on Webench & Large Ripple on Webench Designs

Chithambaram Veerappan1
Prodigy 50 points
Part Number: LM3478

Tool/software: WEBENCH® Design Tools

Hi All

I am having problems with a boost circuit created using the LM3478.

Firstly I have created a Webench Design with a 8-25 V input and a 31 V output at 1A.

When I test the circuit unloaded, I get almost a 1 V ripple (saw-tooth) with superimposed spikes which can reach more than 10 V peak-to-peak.

When I then attach a 75 ohm resistive load (current should then be about 413 mA) I get a awful amount of noise and even larger spikes.

I am using the circuit provided by Webench - apart from the fact that I am not using Rfilt - which seems to also be called R_SL in the datasheet.  I am also not using a Cbyp - will this really have a big effect. I have tried smaller and larger Cin and Cout values. 

Second issue. I thought I might be wise to purchase the Evaluation Board shortly and then slowly change components on that - at least this way I will not the PCB layout is good. However when I try to recreate the design in Webench (based on the Evaluation Datasheet/Application Note) the component values are completely different. Can anyone explain what is going on?

Thanks

Anand

  • 0
    TI__Guru* 78060 points

    Hi Anand,

    Thank you for considering the LM3478.  First, please note that any optimal circuit design is a results of iteration and tradeoffs.  The Webench just gives you a good start point, and you need to fine tune the circuit by testing.  The Evaluation Board seems to be optimized by its designer for the targeted specifications given in the user guide, and it is not surprise the Webench may not create an exactly same design. 

    It is a good idea to start with the EVM.  Your prototype may have layout issues.  Switching power supply requires careful PCB placement and routing of critical traces.  There is a layout guideline given in the datasheet, and also the Evaluation Board layout can be referenced when you design your own PCB.

    Thanks,

    Youhao Xi, Applications Engineering