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LM2595 Input Capacitor and Filter Capacitor

Henk Le Roux
Prodigy 20 points
Other Parts Discussed in Thread: LM2595

Hi, two questions please,

1) In the LM2595 datasheet, requirements of the input capacitor states "Use caution when using ceramic capacitors for input bypassing, because it may cause severe ringing at the VIN pin."  but WEBENCH suggests the Murata 1206 GRM 4.7μF Ceramic Multilayer Capacitor, 50 V dc X7R Dielectric as an input capacitor. What should I look out for if I want to use this capacitor as WEBENCH suggests?  Input voltage for my application is between 21-26V and output is 15V @ 1A max.

2) The datasheet specifies that an input capacitor must be rated at least 1.5x the input voltage. Will a filter capacitor after a full-wave bridge rectifier also need to be specified at 1.5x the input voltage if it feeds the LM2595 circuit? I'm planning on using a 4700uF 35V filter cap for 26V max input voltage.

  • 0
    TI__Guru 72035 points

    The ringing you might see on the input, with a ceramic cap, will come about because of the inductance in the line between your input supply and the regulator.  If you input supply is close to the regulator, you will probably not have a problem.  Also, using an electrolytic capacitor on the input will damp out any ringing and/or spikes.  Since you are feeding the regulator from a line supply and you have a 4700uF on the input, you should be OK.  A 35V cap on a 26V line should be OK.  I would monitor the input voltage to the regulator while starting up the line supply with no load to be sure that there is no overvoltage or ringing.

    FD

  • 0 in reply to Frank De Stasi
    Prodigy 20 points
    Hi Frank, thanks for the quick and helpful response!

    Henk