How to select proper capacitors and capacitance for LM78xx voltage regulators?

Question

I want to use LM78xx voltage regulators for various projects. I want to know in general, how should I choose a correct value for input and output capacitors of LM78xx family when I want to change DC to DC (without a need to think about the reservoir cap in AC to DC applications)?

I have done a quick research and I found that various vendors and people recommend different values:

	C1(uF)	C2(uF)
Fairchild	0.33	0.1
KEC	0.33	0.1
National	0.22	0.1
Texas	0.33	0.1
ST	0.33	0.1
MCC	0.33	0.1
Web1	10.0	1.0
Web2	100	10.0
Web3	100	0.1
Web4	10.0	10

1. What are the consequences of choosing large input and output capacitors?

2. I have seen some people say too big a capacitor on output has detrimental effect on transient response. But how and when?

3. I have also seen that people say too big an input is not able to damp oscillation if high frequency impedance of cap being too high. But What if I use a big (3300uf) low or ultra low ESR electrolyte parallel to a metalized film capacitor like 473j100? Isn't the metalized film capacitor with ultra low ESR taking care of high frequency oscillation? While the big 3300uf cap providing a reservoir?

4. If it is ok to use more than 100nf for output caps, and 330nf for input caps do you you think adding several low ESR metalized disk caps in parallel which lowers the ESR value further is going to make it more robust against high frequency oscillation? Assuming I don't mind wasting several bucks for caps!

5. I have heard that Low Dropout regulators are sensitive to too low or too high ESR. Is LM78xx considered an LDO? I think it is not. But I have seen some websites are listing them as LDOs.

Please bear in mind that I don't mind about the high losses, heat problems and other sort of problems. I only want to know about the caps value.

I'd be thankful if you could elaborate on these questions as much as you could.

Thank you in advance.

Answer 1

1. 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.

2. 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!

3. 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.

4. 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.

5. 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).

Answer 2

My experience is that the choice of these capacitors is non-critical in 99 out of 100 cases. Unless you have very specific demands (and if you had, you would not ask this question !), just follow the 7805 datasheet's recommendations. The exact brand and model capacitor does not matter very much. did you know most manufacturers just use the cheapest available (from a respective brand of course) ?!

You can focus on all the details involving 7805 regulator decoupling but it will probably bring you nothing ! What will this 7805 circuit feed ? Some micro controller perhaps ? I bet you will never notice any difference in transient response.

Low ESR capacitors: you only need these for switched-mode regulators. The 7805 is a linear regulator so any cheap-ass capacitor will work just fine.

No the 7805 is not an LDO, it needs too much voltage difference between input and output for that (2 Volt out of my head). A proper LDO can work with 0.2 V or thereabouts.

So don't worry about this too much, just get some cheap capacitors of the values suggested in the datasheet and that'll be allright, no worries :-)

Oh and if you open any consumer device containing some LM78XX series regulator, what will be the decoupling capacitors ? Yes, just get some cheap capacitors of the values suggested in the datasheet !

Answer 3

Clearly this depends upon the load characteristics and the stability of the supply. Hooked up to a 12v battery and supplying a constant resistive load you might dispense with these entirely. This is more to do with ripple (upstream) and reactivity (downstream).
This is what oscilloscopes are for. I agree with frequency analysis if concerned.

C on the supply side should not be large enough to trip protection on startup. But you could go to 1F with a choke and diode if there is a long run to the regulator circuit.