Help with noise measurement and stability of regulator

I couldn't find any reference to it in data sheets but it may be that the 7905 does not like the 1uF ceramic output cap. Lots of older design monolithic regulators need some esr in the output cap to work.  Try a 10uF aluminium electrolytic output cap instead of the 1uF ceramic.

MK

Thanks Michael.  I tried with a 22uF on the output but in parallel with the 1uF and further from the regulator (no effect). I’ll remove the 1uF and solder in a 10uF in the morning and see how that goes. 

Hi Andrew,

According to note 2 in the data sheet the Tantalum Cap is important and at least a 10 X value of Electrolytic is needed to substitute. Also if the pig tail on your scope is earth grounded this may have some effect if you are using it to tie to the -5 volt rail.

John

Hi Andrew,

For the fun of it I decided to see if I could duplicate your problem. I fed a small adjustable DC switch mode converter with 15 volts from my power supply and regulated it down to 10 Volts. This was to simulate your switch mode chip on your board. I then fed the 10 volts from the convertor to a standard 7905 linear regulator. I also loaded the circuit with a 270 Ohm resistor. The regulator data sheet called for a 2.2 uF Tantalum on the input and a 1 uF Tantalum on the output. Here is my set up:

When I set up the regulator with only the input capacitor installed I found that there was a quite large 848 millivolt PP 3 MHz ripple on the -5V output of the 7905. The polarity of my oscilloscope probes did not matter.

As soon as I put a 1 uF Tantalum capacitor between the 5 volt output and the ground the ripple totally disappeared.

Your problem may be as simple as using the correct capacitor on the output of the 7905. Unfortunately in my case the Tantalum was not a magic bullet. A 1 uF electrolytic did just as well at eliminating the oscillation. Let me know if you want me to try any other permutations.

John

Thanks John, that's really helpful.  I tried a similar setup on a breadboard but could only use a TI LM7905 which seemed to have different behaviour as it worked with a 1uF ceramic as well as a 10uF and 22uF electrolytic.

michaelkellett you were right about the output capacitor and the problem lied with me leaving the 1uF ceramic in place and using a 10uF/22uF after that.  Removing the 1uF and tieing a 10uF electrolytic across the output and gnd terminals stabilised it:

I now have a <4mV ripple on the output which corresponds to the ripple from the charge pump output.

Thanks both for your help.  That's something else I've learned about these regulators.

Could I ask you guys a follow up question.

Given a choice between an input/ouput capacitor of tantalum or electrolytic, which would be preferable?  I have a feeling it may be down

I have stability and good noise management with an input cap 22uF electrolytic and output cap 10uF electrolytic.  The 22uF caps I have used have low ESR - I measured one that I have to hand at 0.6Ohms @ 100kHz so they're all likely to be of that order, including the one in-circuit.

The datasheet has this to say (I couldn't find the references you made in the datasheet, John jw0752)

If I look for 0.33uF tantalum caps, I can't find any with that low an ESR, most are in the 20s Ohms (actually, I've found one with an ESR of 6 Ohms).  "High-frequency" is a rather relative phrase as well!  Part of the reason for asking is curiosity - I've read somewhere that each capacitor material is more suitable for different frequencies and I've seen plenty of attenuation charts that show this.  One answer could be "don't worry about it if your electrolytics work" but I was thinking of re-working the PCB and providing space for these caps.  The pads for typical tantalum cases are different to pads for electrolytics (SMD rather than TH.)

Hi Andrew,

I am not sure if it was a typo but the highlighted section refers to a 0.33 mF cap which is a 330 uF cap and you are talking about a 0.33 uF cap in your comments.

The Data Sheet I quoted from is probably from a different company.

If the caps you are using are working that is the best measure of functionality.

John

It's a typo from pasting which I will fix - the uF was changed to a mF because in the datasheet its the actual Greek letter!  It is a 0.33uF tantalum that is required.  I picked up the change of 0.7 [Omega] to 0.7W on the paste though!!

Tantalum Polymer caps start at 1uF but are the only ones capable of getting an ESR < 0.7 Ohms.  Ditto Aluminium Polymer caps.  Tantalum at 0.33uF have way, way larger ESRs than 0.7Ohms - I can go higher to say 3.3uF or larger and find caps with a low enough ESR and larger is ok;  Alu electrolytics may just make it.  

It would seem that the output capacitor needs ESR so as long as I avoid MLCC and polymer it should be ok.

Given whatever I do I have to buy them in and I have to change the PCB, I'm still curious about the advantages of using tantalum over electrolytics. 

In the good old days (1970s) tantalum caps were cheap and common. Since then tantalum supplies have become more of an issue, ceramic caps have become available in high values and many new dielectrics, and electrolytic caps have improved in leaps and bounds. And surface mounting.

Most of the data sheets for 78xx and 79xx regulators have not been revised for decades (literally) and their references to capacitor types are totally out of date.

Now, as a first base, I would use a ceramic or aluminium electrolytic for both input and output capacitor on a 78xx or 78xx regulator.

For other (and modern) regulators I would check the data sheet, some are happy with ceramic input and output caps, some are fussy.

I might well ignore the recommendations and use aluminium polymer caps instead of large value ceramics.

I would only use tantalum or niobium caps as a very last resort - they are expensive, often hard to get and only very rarely a functional improvement on other types.

When using large ceramic caps be very careful about voltage ratings, dielectrics and values. Many of the high K dielectrics have terrible voltage coefficients - check the data sheets, some caps are down to well under 50% of nominal capacity at rated voltage !

MK

Very useful info, thanks Michael.  I have been looking at some different components but a lot seem to be old and refer to the use of tantalum caps.  The LT1175 whilst referring to the 'optimum capacitor being tantalum' at least acknowledges the use of other types and has been designed for an output capacitor with a wide range of size and 0Ohm to 10Ohm ESR - the datasheet is the most informative when it comes to describing input/output caps.