Filtering cap selectionfor switching regulators

Hi All,

I am looking to use a switching regulator as a pre-regulator to power a a battery monitor that will be monitoring a 12 v battery. I have found lots of great information on the forums, which has taken me in the direction of using a switching regulator to reduce wasted power, which is important to me for my application (http://arduino.cc/forum/index.php/topic,108249.0.html, http://arduino.cc/forum/index.php/topic,110954.0.html). So I know that there is a lot of information out there on this subject and I hope I am not being overly repetitive with my post (please point me to anything I may have missed). Alas, I am still a bit puzzled.

A schematic is attached.I have a prototype of this setup working on my bench, sans switching supply.

For my final build, though, I plan to use a Pro Mini board power from the 12v battery using the switching reg, and will be trying to achieve similar precision from the ADC using the 5v reference on the board. So it seems to me quite important to be feeding it (and maintaining) clean power. Also, the final project will be functioning in my camper trailer, and loads on the battery include a DC water pump, fans, and several electric ignitiors for propane appliances...which my reading and instinct tells me are likely to be “noisy”.

From the forums, it sounds like my problems should be solved by adding a fair amount of capacitance at the power supply input, and also spread throughout the circuit. I have tried to do the latter in this schematic to a degree, but I am a bit leery of going beyond the recommended capacitance on either side of the switching regulator, as I am still puzzled by ESR and its relationship to switching regulators/supplies. It sounded simple until I started looking at datasheets. From various datasheets, some switching regs seem to require low ESR at the inputs, some want low at the outputs, many have max load capacitance listed. The part I found recommends a low ESR ceramic across the input, but just a normal old electrolytic at the output, with no mention of ESR. Is this filtering, along with the additional regulation step in the Pro Mini and filtering I have added likely to suffice for me? Any comments or direction to additional information is greatly appreciated!

Mike

You don't need any more decoupling at the supply input than the datasheet suggests normally. You can increase the output decoupling to the maximum the datasheet allows for better supply quality, but there will be residual "hash" (switching noise)

If the battery voltage is actually going to drop substantially during load switching (called crowbarring), then you should probably add a proper filtering circuit to condition the power - either an RC or LC filter. If you just have a capacitor it will simply discharge on the heavy transients, using a resistor or inductor allows it to stay charged across the crowbar event and keep the input to the DC-DC converter within spec. You need to calculate the values of components from your supply's current drain, and the length of time the transient is likely to last.

If the transients never take the battery voltage below the minimum input voltage requirement of the switching converter there shouldn't be any great problem.

Thanks very much MarkT--that's very helpful and I also appreciate your explanation of crowbarring--a new concept for me.

I don't anticipate this load (or any on the battery) ever pulling the supply voltage below the input spec (7v) of the converter, so I'm thinking I should be okay in that regard. Alas, it again reminds me that I need to read up on RC and LC filters...one of these days when I can find the time!

Great info, and thanks again for your reply.

Mike