Electrical – LM2596 Cin requirements

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I'm going to use an LM2596.

My maximum input will be 40V to a fixed output 5V 3A.

I've read in the datasheet that for Cin you need 1.5x for an electrolytic capacitor. So 40V x 1.5 will be some 60V. I've seen design and images from google of DC-DC Buck Converters featuring LM2596. They seem not to use 60+ V for Cin but can handle upto 40V input. What is going on here?

I've seen some schematic of that also only using 50V. Link.

Best Answer

I've read in the datasheet that for Cin you need 1.5x for an electrolytic capacitor.

The data sheet says this on page 15: -

The capacitor voltage rating must be at least 1.25 times greater than the maximum input voltage, and often a much higher voltage capacitor is required to satisfy the RMS current requirements.

So for the design you linked shown below: -

enter image description here

The 50 volt rating on the capacitor is adequate for an input voltage that might peak at 40 volts. But maybe you read about the output capacitor Cout by mistake: -

The capacitor voltage rating for electrolytic capacitors should be at least 1.5 times greater than the output voltage, and often require much higher voltage ratings to satisfy the low ESR requirements for low output ripple voltage.

The general reason for both Cin and Cout sometimes requiring "much higher voltage ratings" is because with a higher voltage rating you usually get a significant improvement in current handling and lower ESR.

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