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Help with noise measurement and stability of regulator

Andrew J

I'm looking for some help with resolving a noise and what I believe is stability problem of a -5V LDO regulator.  Here is the circuit:

image

and in case it's useful, the relevant part of the PCB:

image

 

The circuit section I'm having difficulty with is the top right (yellow ICs).  I have a 285Ohm resistor soldered across terminals of TP9 and it draws 17.5mA.  R22 and C34 are not populated.

 

The TC7662TC7662 DC-DC charge pump, is fed from a regulated 12V and outputs -11.8V to feed into a -5V LDO regulatorMC7905CDMC7905CD which is outputting -5.011V.

 

When I take measurements (AC coupled, BW 20MHz, 1xprobe with the pigtail rather than flying lead) of the charge pump output (at TP12) and -5V output (at TP9) I get:

image

CH3: Charge Pump output with HF reject on (matches what is expected in the datasheet).  I was expecting that the frequency of the output would be 35kHZ due to way I have pins 1 and 8 connected together.

 

image

CH4: -5 LDO output with LF reject on.

 

I think the LDO output looks like it is oscillating.  Looking at the circuit the input capacitor, C18, is too small and should be a 0.33uF tantalum for stability, according to the datasheet but I have used a 0.33 ceramic capacitor as I don't have any useable tantalums and my thinking was the ceramic should have a low ESR.  I've actually tried with 22uF and then a 68uF electrolytic capacitors, in parallel, but both have zero effect on the LDO output which remains steadfastly at 1.32V.  I've tried the circuit on a breadboard but unfortunately I only have a TI LM7905 - this seems to work ok even with the original capacitor value and doesn't oscillate.  I think the TI part behaves differently so it's not a great comparison.  Its output is slightly improved with a 22uF input capacitor.

 

I really thought that adding an electrolytic capacitor at the input would help (the ESR of the 22uF was around 2.2Ohm) but it makes no difference at all.  I'm out of ideas short of "it really needs a tantalum capacitor on the input", so I'm putting this out there to see if anyone can give me pointers or things to try.

Parents

  • 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 )

     

    In many low current applications, compensation capacitors are not required. However, it is recommended that the regulator input be bypassed with a capacitor if the regulator is connected to the power supply filter with long wire lengths, or if the output load capacitance is large. An input bypass capacitor should be selected to provide good high−frequency characteristics to insure stable operation under all load conditions. A 0.33 uF or larger tantalum, mylar, or other capacitor having low internal impedance at high frequencies should be chosen. The capacitor chosen should have an equivalent series resistance of less than 0.7 Ohms

     

    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.)

     

    Thanks

  • in reply to Andrew J

    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

  • in reply to michaelkellett

    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.

  • in reply to Andrew J

    I'm a bit late to this party, but this might be of interest. This is what Bob Pease [a National Semicondutor designer] wrote back in the early nineties. Sounds like the negative regulators were much more touchy than the positive ones, though personally I don't remember being aware of that. I can remember some LDOs being difficult.

     

    You'll have problems with regulators when you don't provide the required, specified output bypassing. Most negative regulators and some other types, such as low-dropout regulators, require an electrolytic bypass capacitor to ground. If you insert a tantalum capacitor, you may be able to get away with a value of 1 or 2uF; if you use an aluminum electrolytic capacitor, you can get away with 20 to 100uF, or whatever the datasheet spells out. But in all cases, on all the parts I know, an electrolytic will work, and a film or ceramic won't work - its series resistance is just too small. Now, if you put a 1 ohm resistor in series with a 1uF ceramic capacitor, the filtering will probably be adequate at room temperature; the loss factor is then similar to a tantalum capacitor. But if you take it to -40 or +100C, the ceramic capacitor's value will shrink badly and the regulator will be unhappy again. It may start oscillating, or it might just start ringing really badly.

     

    Robert A Pease

     

    Troubleshooting Analog Circuits, 1991

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  • in reply to Andrew J

    I'm a bit late to this party, but this might be of interest. This is what Bob Pease [a National Semicondutor designer] wrote back in the early nineties. Sounds like the negative regulators were much more touchy than the positive ones, though personally I don't remember being aware of that. I can remember some LDOs being difficult.

     

    You'll have problems with regulators when you don't provide the required, specified output bypassing. Most negative regulators and some other types, such as low-dropout regulators, require an electrolytic bypass capacitor to ground. If you insert a tantalum capacitor, you may be able to get away with a value of 1 or 2uF; if you use an aluminum electrolytic capacitor, you can get away with 20 to 100uF, or whatever the datasheet spells out. But in all cases, on all the parts I know, an electrolytic will work, and a film or ceramic won't work - its series resistance is just too small. Now, if you put a 1 ohm resistor in series with a 1uF ceramic capacitor, the filtering will probably be adequate at room temperature; the loss factor is then similar to a tantalum capacitor. But if you take it to -40 or +100C, the ceramic capacitor's value will shrink badly and the regulator will be unhappy again. It may start oscillating, or it might just start ringing really badly.

     

    Robert A Pease

     

    Troubleshooting Analog Circuits, 1991

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  • in reply to jc2048

    It's been my experience here too!  I had tried with a 22uF on the output but in parallel to the 1uF ceramic and approx. 3 cm away.  It didn't like that and that's the point I became stuck as I didn't know what else to try.  Once I removed the 1uF it started working so there's something else to bear in mind.