Ceramic or electrolytic capacitors for a switching buck regulator?

Question

I'm using the LM2734Z (step-down DC/DC converter), which operates at 3 MHz. I'm using it to step down 4.8 V - 20 V down to 3.3 V +/-5%. Is it better to use ceramic capacitors or electrolytic capacitors in this circuit?

They seem to show ceramic capacitors in the datasheet, but would electrolytic capacitors be smaller and better at filtering ripple and handling load transients? Size is critical for this product, and cost is a minor issue. I would like the operational temperature range to be -40 °C to +85 °C, if not that then -20 °C to +70 °C.

Answer 1

With chips like that it's best to follow the manufacturer's design closely, unless you really know what you are doing. Ceramics are generally preferred in that sort of application, they are smaller and more reliable than electrolytics, handle high temperatures better, and often have a lower ESR.

Answer 2

At 3 MHz, most electrolytics are useless due to ESL. (Some ceramics, too. Check the datasheets carefully!) They're OK on the input side, but stick with ceramic on the output side.

Answer 3

If size really is critical, you're going to have to go with ceramic. 5-10 years ago, this wasn't even possible with the values you might need (100uF) but it is today. I think you've got it figure out about the ESR, the resistance with the capacitance will change the frequency response of the circuit, so that's the only thing to watch for. I usually use LTSpice to simulate similar switching type circuits, as you can start playing with the detailed specs on the capacitors and see how the circuit responds (just choose one of the similar LT 1A buck switchers).

Answer 4

One big advantage of high-frequency switching regulators is that they allow you to use ceramic capacitors instead of low-ESR tantalum (because you don't need such large capacitance values as you would in a lower-frequency regulator).

Tantalum is a comparatively rare material, not mined in that many places, so tantalum capacitors are perpetually expensive and difficult to buy in large quantities. Large electronics companies have at times suffered delays in meeting high demand for their products due to shortages of tantalum capacitors.

Ceramic capacitors tend to be much cheaper and more available.

Electrolytic capacitors have a high ESR in comparison to tantalum and ceramic, as well as being quite a bit larger.