Voltage regulator questions - capacitor on the input, difference between different regulators

Question

Hello everyone,

I have some questions about voltage regulators (it's the first time I'm dealing with them, so the question are *really* basic - but I wasn't able to find answers anywhere, so I've figured I'll ask).

I have a 6 AA battery pack that I want to get 5 volts out of. First I got this voltage regulator and it seems to be working: https://www.adafruit.com/products/2164

However, it's dropdown can be as high as 2.5V, meaning that once the batteries go below 1.25 V each (which as far as I know is not the end of their lifetime) it may stop working (if I understand correctly. Or will this one do fine?).

Anyway, I've decided to get a regulator with a lower dropdown. I got these two (I expect the output current to be around 1A, so I got ones that support >= 1.5A just in case)

MIC29150-5.0WT - MICREL SEMICONDUCTOR - LDO VOLTAGE REGULATOR, 5V, 1.5A, TO-220 | Newark element14

MIC29300-5.0WT - MICREL SEMICONDUCTOR - Fixed LDO Voltage Regulator, 2.5V to 26V, 370mV Dropout, 5Vout, 3Aout, TO-220-3 …

Now basically my questions are:

what capacitor should I use on the input? The spec doesn't really specify that. It only marks it as "C In". Does the value of that really matter? Will 10uF work fine (so I'd have 10uF on both input & output)?
just out of curiosity - what really is the difference between them? Seems like the second one supports higher current and has slightly higher dropdown. Is that really the only thing? Will they pretty much operate the same, or will the second one like get more warm or something? Basically what I'm trying ot ask is - why would anyone want to use the first one over the second one?

Thanks,

Tom

shabaz · Accepted Answer

Hi Tom, The 100nF and 10uF are used together, the reason is that they perform differently due to the material used in their construction (ceramic vs electrolytic). Both are advised. Figure 1 in this PDF highlights the difference http://rohmfs.rohm.com/en/products/databook/applinote/ic/power/switching_regulator/cera_cap_appli-e.pdf

shabaz · Answer

Hi Tom, The spec does specify that you may need a 100nF cap on the input (see page 20), you can additionally add the 10uF if you find you need it (it depends on what the impedance of your source is; there is no harm in adding a small amount of capacitance such as 10uF and the 100nF. To know why you're best picking up a book, because there is some theory including Ohm's Law and reactance that would need to be covered to do it justice - see here I Want to Do Electronics! Where do I start? But in summary you can use 10uF on the input (and 100nF) and 10uF on the output with that particular device because that is what page 20 text and the diagram on page 21 show. If they start mentioning ESR and capacitor type then that is important too. Regarding your second question, the 3A part is a similar part in the same family, rated for higher current. It may cost slightly more. As for if it will get hot - this is all down to the Power equation and Ohm's Law. You'll find that the amount of power dissipated as heat depends on your source voltage (e.g. 9V), and the output voltage (e.g. 5V) and the current through the voltage regulator and current through your load. You can calculate the power being dissipated as heat using Ohm's Law and the Power Equation, i.e. V=IR and P=IV. If your circuit is consuming 1A then the power dissipated by the voltage regulator as heat is approximately (first level approximation): V=9-5 = 4 P=IV = 1*4 = 4W So your regulator would dissipate 4W and would be very hot and would need a heat sink. That answer is the same for both of the regulators. I strongly recommend you checking out that link, because it will help with these types of questions.

jw0752 · Answer

Here is a link to Recom's DC to DC Book of Knowledge just in case you want more information on using switchers for converting voltages and some other really neat things.

http://www.recom-power.com/de/americas/downloads/book-of-knowledge.html

John

jw0752 · Answer

Hi Tomasz, If you can get by with one Amp you may want to check out these Recom Convetors. Since they are switching regulators they do not waste as much energy in IR loss. Further there is only a 1.5 volt drop out and probably no need for heat sinking. With efficiency up to 94% more of your battery power will go to your application. http://www.newark.com/recom-power/r-785-0-1-0/dc-dc-converter-5w-5v-1a-sip/dp/62X9542 Incidently if you need more than one amp these devices can be paralleled. John

jw0752 · Answer

Hi Tomasz, The Data sheets are not real clear on this I agree. The problem seems to be that the need is very application dependent. I would recommend a 4.7 uF to a 10 uF on the input of the Converter. While they say it is optional I would also put a 47 uF on the output as well as a 0.1 uF ceramic. This is in line with the recommendations from shabaz . The diode is also optional but if you have one why not afford protection if it is available. John

shabaz · Answer

Hi Tom, Now that there is a bit more information!! beware that you can't assume 1A is sufficient if you're using the Pi 3. According to this link the current consumption (I expect with nothing attached) during a reboot reaches 720mA. If you make the assumption 1A is enough for the Pi 3 I think this is just waiting to fail once your parts arrive. If your LCD screen really does need 500mA as you say, then you definitely need a higher power DC-DC converter Anyway it is unclear what Pi you're using. Also, there are LiPo USB power bricks which would power the Pi and display (save on needing 6xAA batteries).

jw0752 · Answer

Hi Tomasz, For the fun of it I put 8 used AA batteries in a pack and powered a 5 V Recom Converter. Here is a picture of the set up. The little voltmeters each draw 20 mA. I did not put any capacitors on the input or output in this case. John

Robert Peter Oakes · Answer

The larger the capacitor the lower the frequency it tends to filter efficiently Many people question why most schematics show a 10 or 100uF capacitor in parallel with 10 to 100nF capacitors, they think simplistically about capacitors in parallel and series (Much like resistors in parallel and series) but they do not consider the parasitic impedance / reactance etc at different frequencies For instance, a Large electrolytic at high frequencies can have a very high reactance (Think Resistance) due to the inductance of the capacitor (Its made up of a coil of metal after all) and is therefore useless at filtering the noise from all the digital switching that can be into the Mhz or even higher. Putting a smaller nF or pF capacitors in parallel to the electrolytic solves this problem, so they compliment each other so looking at this view you can see how this happens, the frequency increases, the capacitor impedance lowers but the internal inductor impedance increases, at a certain point the impedance from the inductor becomes significantly more than that of the capacitor In a power regulator like the one we're talking about in this thread, the larger capacitor is for smoothing of the typical mains voltages and to reduce low frequency oscillations, the lower value caps reduce the high frequency noise from the load and other sources.

Voltage regulator questions - capacitor on the input, difference between different regulators

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