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LM3478 Output capacitor size

David Tiefenbrunn
Intellectual 785 points
Other Parts Discussed in Thread: LM3478, LMR62014, LM2731

I'm working on a 5V to 10 V boost regulator.  The normal load will only be 4mA.  I need short circuit protection / current limit.  So I set the I out to 10mA in the webench table.

The webench designer is requiring a 220uF Electrolytic capacitor for Cout.

I'm curious as to why Cout has to be so large.    I have a handful of other designs [granted, other regulator chips, and buck mode] that are converting amps of current that require less capacitance.

I'd prefer to use ceramic caps, but they are much lower ESR.  We have had trouble with the bigger SMT electrolytic caps - as far as having them broken off the PCB in shipping.  Unlike our usual products, this particular one will be installed in big trucks.  If we do have to use an electrolytic, we'll probably use a through hole part.

I'm thinking I'm going to have to do some experiments to see what will work.  Any hints about what's more likely to work would be helpful.

Thank you,

Dave

Another question,  The feedback resistors are 698K & 100K.  Is there any reason I shouldn't make both of them 10 times lower?  6.98K  10.0K

  • 0
    TI__Intellectual 2930 points

    Hi Dave,
    WEBENCH is designing the circuit to work in Constant Current Mode which leads it to choose the large output cap for stability reasons. If you decrease the inductor value and work in discontinuous mode you can probably also get away with decreasing the output cap (but you will get larger ripple). You would need to experiment with this though.
    Normally parts like the LM3478, with external FETs, are used for higher current designs. We offer many other parts that are better suited to your design case. If you enter you design requirements in WEBENCH you will see parts like the LMR62014 and the LM2731 that offer smaller, more efficient and cheaper designs. Is there a specific reason you wanted the LM3478 for this design?

    Regards,
    Tommy Jewell 

     

  • 0 in reply to Tommy Jewell - WEBENCH Design Center
    Intellectual 785 points

    We used one of the smaller fixed current limit ICs in another design where we needed only a fraction of it's capability.  The problem is, if you use a small inductor matched to the current required, [like say, 50ma] and the regulator I limit is an amp or so, a short on the output burns out the coil and then the regulator IC dies. 

    With the LM3478 I can set the current limit below the rating of the inductor.

  • 0 in reply to David Tiefenbrunn
    TI__Intellectual 2930 points

    Ok, then i would go with the LM3478 in DCM.

    You may want to check out this note; 
    http://focus.ti.com/download/trng/docs/seminar/Topic_3_Lynch.pdf

    Page 12 goes into Frequency response in DCM for current mode (with some hints on page 9)
    And after that it goes into help with power stage component selection. 

    -Tommy