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LM2674: Using a Ceramic Capacitors for CIN and COUT

Victor Salomon
Expert 3910 points
Part Number: LM2674

Hello team,

I am supporting an application where WEBENCH suggests using a 33 µF Panasonic POSCAP tantalum capacitor for the output capacitor for LM2674M-5.0, the output voltage will be 5.0 V and output current around 250 mA and none of the alternate capacitors options seem to be ceramic capacitors they are all are either tantalum or aluminum.

Would you have any recommendation or considerations on using the ceramic capacitors with LM2674?

Two (2) 22 µF, 16 V Ceramics on the Output (TDK 445-3955-2-ND).  
One (1) 4.7 µF, 25 V Ceramic on the Input (Murata GRM21BR71E475KA73L)

  • 0
    TI__Guru**** 191445 points

    For the input capacitor, it is possible to use a ceramic bypass capacitor.  In fact I always use ceramic input capacitors with my designs (not necessarily LM2674).  You just need to be careful about the possible ringing on the input if your connection to the input supply has very inductive long leads.  In that case some electrolytic capacitance may be required.

    For the output capacitors, Webench will sometimes pick ceramic types for certain designs, but not for your case. LM2674 is internally compensated and the ESR zero o the electrolytic capacitor helps to stabilize the control loop.  I tried 2 x 22 uF ceramic and Webench and was warned the ESR is too low.  The resulting design resulted in a phase margin of 38.2 degrees.  One common trick is to use a feed forward capacitor to boost phase at crossover.  To do that you would need to use the adjustable version.  The feed forward capacitor would be placed in parallel with the upper feedback resistor.  It should be easy to boost the phase margin above 50 degrees for a 5 V output voltage with this technique.  Another alternative would be to use an externally compensated device.  You could use the on line Webench tool to search for an appropriate one that fits your needs.