Is there a thumb rule for the input and output capacitor for DC-DC converters?

Question

If we are using a buck converter of say 5V 1A output, we definitely require input and output capacitors.

Just without making any calculations, is there a thumb rule on what the values of the input and output capacitors value need to be? Without considering any specification, just to arrive at the values.

Is it really necessary to have the electrolytic capacitors at the output? Can't we just use the ceramic capacitor itself? (I read that the ESR of the electrolytic capacitors helps to control loop stability of the feedback loop of the converter.)

Answer 1

You are trying to take too many shortcuts if you want to have a good result. Often you can copy a reference design, or other known-good design. Also, datasheets for the DC-DC converter IC may recommend capacitor and inductor values, so that is another source of information if you don't want to do calculations.

Usually in a switching regulator, you select the operating frequency first. Then, working with the input and output voltage and output current and your desired ripple current, you choose an inductor value, then the output capacitor value. Input capacitor value may be a bit more flexible. It is not strictly required for stability usually. But it can help prevent switching noise from propagating to the DC input voltage rail.

If the datasheet requires aluminum electrolytic caps for stability, but you don't want to use aluminum electrolytics, you can use a ceramic cap with a resistor in series. But given the questions you are asking, it may be kind of difficult for you to figure out what size resistor to use.

Answer 2

In general, you can find some tables in the datasheet of the dc-dc controller for quick design.

For example:

Table 9-3

https://www.ti.com/lit/ds/symlink/lm2596.pdf?ts=1623306793065&ref_url=https%253A%252F%252Fwww.google.com%252F