Generic snubber values: fixed-/variable-voltage regs

[hollowman]

I'm seeking generic snubber values for common 3-pin regs:

Fixed-voltage (e.g. 7805, 7812, LM340-12, etc.):
Some one once suggested: 33uF normal electrolytic from output pin to ground. Parallel to the 33uF: 0.47R + 10uF electrolytic (use a good-quality cap here).
(suggest more here)

Variable-voltage (e.g. 317):
(suggest here)

Thx,
-hm

P.S. I don't have a lot of experience with snubbers nor do I have access to a 'scope. I think both are probably necessary to fine-tune snubber values. This is why I'm asking for "generic" suggestions.

[gootee]

Originally posted by hollowman
I'm seeking generic snubber values for common 3-pin regs:

Fixed-voltage (e.g. 7805, 7812, LM340-12, etc.):
Some one once suggested: 33uF normal electrolytic from output pin to ground. Parallel to the 33uF: 0.47R + 10uF electrolytic (use a good-quality cap here).
(suggest more here)

Variable-voltage (e.g. 317):
(suggest here)

Thx,
-hm

P.S. I don't have a lot of experience with snubbers nor do I have access to a 'scope. I think both are probably necessary to fine-tune snubber values. This is why I'm asking for "generic" suggestions.

I think you should definitely start with the input and output capacitors that are recommended by National Semiconductor, in the datasheets for those devices, and in any related Application Notes. They are available free from http://www.national.com .

There is a very good thread about what you are wanting, here:

paralleling film caps with electrolytic caps

If you can find the ESL and ESR specs (AT a frequency of interest) for an electrolytic that you want to bypass with a smaller cap, you can look at post #17 in the thread referenced above.

If you do need or want to use an actual snubber network (usually to try to cancel some excess inductance, possibly that of a large electrolytic, or, maybe more likely, a wire or PCB trace, or, of course, a transformer winding), you might be able to estimate the parameters needed to design the snubber, and then use the papers on snubber design, from the links below:

There is a good paper about snubber design on the Cornell Dubilier website, at http://www.cde.com , in the technical papers section:

http://www.cornell-dubilier.com/design.pdf

There are some other good ones on line, too:

http://www.hagtech.com/pdf/snubber.pdf

http://www.ridleyengineering.com/snubber.htm

http://www.maxim-ic.com/appnotes.cfm...te_number/3835

http://archive.chipcenter.com/circui.../c1100rp58.htm

It might be difficult, without an oscilloscope. But you can often get fairly comparable views and results with a circuit simulator, such as LTspice, free from http://www.linear.com , especially if you also model the parasitics of components and traces/wires.

[Eva]

These regulators are stable without any snubber, although output capacitor ESR may not be too low.

Check this out: Neat (FLASH) Regulator Tutorial from ST

The purpose of RC and RLC snubber networks is to exhibit a low and resistive impedance across a particular frequency band. This comes very handy either to reduce the HF gain of an amplifier stage (whose output is in the form of current) to a known value, or to damp parasitistic RLC resonances like the ones you get in the power paths of SMPS and class D amplifiers.

[KSTR]

Eva's point is important, those regs (and let alone low-drop types) quite often dislike an OSCON or a 10uF ceramic or other bigger ultra-low ESR stuff directly at their output. Such should be isolated with a small R, say 0.22R or 0.47R. The small increase in load-dependency of output voltage due to that series R is normally of no concern.

Decent layout and circuit design practices (local series LC/RC decoupling), cascading two reguators, reasonable quiescent ballast (>=20mA) and some other "tweaks" (especially for the 317/337) will typically give more improvements that snubber vs. no snubber.

And today's electrolytics are good enough not to worry about it, btw.

- Klaus

[hollowman]

Thx for everyone's useful feedback on this topic.

Originally posted by KSTR ...
Decent layout and circuit design practices (local series LC/RC decoupling), cascading two reguators, reasonable quiescent ballast (>=20mA) and some other "tweaks" (especially for the 317/337) will typically give more improvements that snubber vs. no snubber.

Definitely agree about the importance of layout. Can you elaborate a bit more on:

- cascading two reguators (e.g., perhaps something like this for DACs?)
- reasonable quiescent ballast (>=20mA)
- other "tweaks" (especially for the 317/337)

(Links to important, topical threads/posts on these issues would be fine).

Thx again!
-hm

[gootee]

For adjustable regulators, bypassing the adjust pin with, for example, a small electrolytic to ground can dramatically lower the output ripple + noise.

[hollowman]

Originally posted by gootee
For adjustable regulators, bypassing the adjust pin with, for example, a small electrolytic to ground can dramatically lower the output ripple + noise.

Thx for that tip! Just so my concepts and nomenclature correct -- and for the sake of correctly searching the web for more info on this topic -- is what you suggest "snubbering", "bypassing", and/or some mixture?

[KSTR]

Hi,

Originally posted by hollowman
- cascading two reguators (e.g., perhaps something like this for DACs?)
- reasonable quiescent ballast (>=20mA)
- other "tweaks" (especially for the 317/337)

Cascading is simple, just two complete regulators one after the other, say 7815-->7812. There are more elaborate schemes, see datasheets/AppNotes for these parts (and the LM317), like "floating pre-regulators" (only useful for circuits with variable output voltage). Care must be taken that both regulators always have enough voltage to work upon (the voltage "headroom"), usually 2V-3V for the common types is recommendend.

Also, the ballast can be as simple as a resistor, or combine it with a Zener and LED for a "PWR GOOD" display. If your circuit has enough guaranteed minimum quiescent current then you don't need then ballast, of course. I found that at least 50mA standing current greatly improves the regulation, as does a headroom of 3Volts minimum.

A good and simple tweak, besides the ADJ-pin bypass for 3x7, see here:
http://www.diyaudio.com/forums/showt...154#post356154

Another similar method, also usable for "fixed" regs (which aren't fixed, only two pre-configured resistors are built in), is to elevate the REF/GND-pin with a reference voltage, obtained from a LM329 or TL431 or similar. For example, a 7805+LM329 gives a far better 12V-reg as a plain 7812 (because the internal node, where the two resistors meet, cannot be bypassed and therefore loopgain gets less with increased output voltage). The advantage of elevation vs. bypass is that elevation has no backpower issues. The principle is the same: don't divide down the error voltage (at least for AC).

See LM317/LM317HV datasheets and AppNotes on this:
http://www.national.com/JS/searchDoc...extfield=LM317

- Klaus

[hollowman]

Originally posted by gootee
For adjustable regulators, bypassing the adjust pin with, for example, a small electrolytic to ground can dramatically lower the output ripple + noise.

Are you referring to the 10uF electro cap from the last schema from Datasheet pg. 15? This schema also suggests the use of a diode (D1).

Also --and addition to the suggestion above -- how about a 1uF (Vout to gnd), as shown here:

[martin clark]

There are more elaborate schemes, see datasheets/AppNotes for these parts (and the LM317), like "floating pre-regulators" (only useful for circuits with variable output voltage).

Actually the tracking preregulator as shown in the old LM117 / 317 datasheet works really well where higher performance is required even for fixed voltage outputs.

It separates the issues of line and load regulation - like a cascode; and performs better than the same two regs just connected in series.