7805capacitorceramicelectrolytic-capacitor
I have some questions about using capacitors with an L7805CV
According to the datasheet, on page 4, it shows I need:
I bought a few sets some time ago with 'common' values and what I have (amongst others) are 100 nF ceramic capacitors.
Questions:
Within reason, too large is not a problem. Also usually this kind of regulator will actually work fine with no capacitors (advertised as such on some datasheets) under some conditions, but I don't recommend it (maybe if you're just powering an LED). Other parts (eg. LM7905, many LDOs) are about guaranteed to oscillate without the proper output cap. Most commercial designs will use a somewhat higher values than the datasheet example (not really 'recommended') because there is no cost penalty and 0.33uF is not very common. I don't think I've ever seen the 330nF outside of hobby/internet hacks. Similarly 1uF is perhaps more common on the output these days- there is little cost penalty and it's a bit better. On the other hand, these parts are mostly used at relatively low current these days because switching parts are better suited once you get into the hundreds of mA or higher, so 0.1uF has as much effect as 1uF at much higher current.
Too big a capacitor on the output might affect startup because the regulator can only supply ~1.5A to charge the capacitor, otherwise larger is somewhat better all other things being equal. If the ESR is larger a large output cap may not be that much help. Low ESR is not a problem. A 1uF ceramic (or 0.1uF) works well and is almost free. Here's a plot of output Z vs frequency- at a few hundred kHz the output cap starts to have an effect. Below that, no capacitor is typically as good!
The 3300uF is probably there as a mains filter, not anything to do with the regulator directly. If the input voltage drops too low the regulator cannot regulate so the input reservoir capacitor has to be adequate for the lowest input mains voltage, highest load and end of life capacitance value. The input ripple also gets passed to the output to some degree (-54 to -80dB - see datasheet). For the regulator's stability purposes, a small ceramic or electrolytic capacitor close to the regulator (within a couple cm) is fine. A 47uF/50V 105°C electrolytic for through hole or another 1uF ceramic works fine. If the input voltage gets too close to the point where it drops out then performance will tend to suffer even before it drops out of regulation- 3V is good.
Low ESR on the output caps does not make the regulator more immune to HF oscillation. It will generally improve the transient response to high frequency load changes. It's not unusual to have several uF of low-ESR ceramic caps distributed around a board near various chips. Several dollars? $2.61 will buy 1,000 0.1uF 10V X5R capacitors at Digikey if you don't mind compact 0402 parts.
Yes, both some negative and most LDO regulators are sensitive to ESR range- too high or too low and some will or may oscillate under some conditions. The LM780x is not an LDO regulator- it's actually quite difficult to convince it to oscillate. The LM790x, despite the similar name and the fact it's not an LDO is very easy to make oscillate (mostly because the output is taken from the collector of an NPN transistor rather than the emitter of a PNP as it would be if it was a true complementary part).
After some digging around on Digikey I think I found my answer. These caps appear to be the Z5U type. I looked at data sheets from various manufacturers and have found that for this type the dissipation factor can be as high as 4.5% which gives me theoretical ESR values very close to what I actually measured.
Best Answer
That datasheet is pretty poor compared to others, which will usually explain the range of capacitance and ESR acceptable for the device. Sometimes using a 100 nF ceramic capacitor can be a problem because the ESR is so low, that the regulator is not stable. It is possible to add a small resistor in series to the cap to increase the ESR to get into the stable range instead of going for an electrolytic one.
There are two footnotes hidden on page 27, which will give you a bit more information:
So basically if you place it close enough to your power source (low inductance path) you could get away with no capacitors at all.
On the other hand I also interpret that, that the value isn't hard set. So you could go for a higher input capacitor. I usually place a 1 µF and 100 nF ceramic capacitor on the input and something like 10 µF and 100 nF on the output. Some regulators want a 100 pF capacitor very close to the leads to reduce noise.
Place the capacitor with the lower capacitance closer to the device for better noise / EMI performance.
And because it is buried even farther (page 35), make sure to comply to this note as well:
So if you place a too large capacitor (how large is too large? I don't know, could not find a value) or you go above 6 V (not applicable for you), you have to place a diode across the regulator to protect it.