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Is it safe to use only ceramic caps with the LM2674-5.0 ?

D. Johnson
Intellectual 700 points
Other Parts Discussed in Thread: LM2674

Webench Design

Vin = 22 to 26 V

Vout = 5.0 V

Iout = 0.5A max, 0.3 typical.

Note in datasheet(page 11): Use caution when using only ceramic capacitors for input bypassing, because it may cause severe ringing at the VIN pin.

Customer is very concerned about input ripple, EMI and supply stability issues.  Using Webench, only ceramic caps are selected for the design which seems to contradict the note in the datasheet.

Question: Will this design be stable with chosen ceramic capacitors ? or is there an issue with Webench ?

  • 0
    TI__Guru 72145 points

    Sometimes with long traces/leads going to the input to the regulator, the inductance of the wiriing can resonate with the input capacitance and cause

    ringing and/or overshoot on the input supply.  The low ESR of the ceramics provides very little damping for this ringing.  The electrolytic cap will have

    much larger ESR and therefore damp this ringing before it can cause a large overshoot on the input.  This can happen with any regulator, not just the LM2674. 

    The best practice is to parallel the recomended ceramic caps with a small electrolytic;  if the customer is concerned about this potential issue.

    Frank D