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I see many references to input and output caps helping with "stability" and some outright references to preventing oscillation in both answers on this site, and even in some data sheets. I also have seen references saying that the 7805 is "inherently stable" in both answers on this site and even in some data sheets and app notes. Are the datasheets/people using the term "stability" confused or are there versions of the 7805 that actually can become unstable? If so, I would like to see an example of a circuit in which this would happen.

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  • \$\begingroup\$ Have you tried things like googling for "unstable 7805" for inspiration? \$\endgroup\$ – PlasmaHH Feb 13 '16 at 22:49
  • \$\begingroup\$ Yes, I've been neck deep in Google, various textbooks (reread my entire copy of FOE by Razavi), my copy of AOE3 (ch 1-4 an 9), etc... =) If there is a particular Google result you feel is inspiring please share! \$\endgroup\$ – mhz Feb 13 '16 at 22:52
  • \$\begingroup\$ To clarify, I interpret the word "unstable" to mean "begin oscillating" i.e. Barkhausen criterion have been violated. \$\endgroup\$ – mhz Feb 13 '16 at 23:12
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    \$\begingroup\$ 79xx regulator is more sensitive for instability. Check the datasheet. If the datasheet says the device may be unstable then you should not rely on it without proper counter-measures. The datasheet may vary from manufacturer to manufacturer. \$\endgroup\$ – jippie Feb 13 '16 at 23:38
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It's quite challenging to make them misbehave to the point of oscillation in practice, especially with light loads. Try heavy loading, no input capacitance at all and some source inductance, but I offer no guarantees. You can tell as oscillation approaches you'll see reduced phase margin, meaning overshoot/undershoot on load or line changes.

Based on simulation, it looks like something like 500uH with a 0.5A load will be close to oscillation typically. This is a pretty pathological arrangement. The plot below is with 400uH and a 0.55A load which decreases* to 0.5A at t=100usec

enter image description here

* the decrease is done with a tanh function over a period of about 1usec, so not quite an ideal switch opening.

This comment does not extend to other types of regulators (especially LDOs) which are easy to make oscillate. And, of course, in real circuits we prefer to live on the side of "guaranteed to be stable" rather than "guaranteed to oscillate", at least for voltage regulators. The opposite would be true of things that are intended to oscillate- as the old saw says "amplifiers oscillate, oscillators don't".

Edit: I tried a couple quick tests- with a relatively large input capacitor at the regulator (1uF) it shows low level (2.5mV p-p) oscillation at about 8kHz. Much below that it lowers the frequency of oscillation but magnitude remains high. Adding only a capacitor to the output with the input inductor present reduces the stability- 5-10nF is enough to make it oscillate with 400uH inductance on the input and a 0.5A load.

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  • \$\begingroup\$ Thanks for the reply. I'm guessing you have a 7805 model you've used for this sim? Just to make sure I understand, is the 500uH in series with the input of the 7805? Is the application of the input or output cap the solution to this oscillation? Would that be a case of compensation (early rolling off the gain in a feedback loop) or is this something else entirely? \$\endgroup\$ – mhz Feb 13 '16 at 23:29
  • \$\begingroup\$ Yes, an input cap should prevent it- there are graphs on some datasheets showing the gain vs. frequency of the 7805. I have not attempted to analyze this in detail- the fact is that they are extremely stable in any reasonable arrangement. \$\endgroup\$ – Spehro Pefhany Feb 14 '16 at 4:15
  • \$\begingroup\$ So basically, the above is an inductance forming an underdamped series LC with the capacitance seen at the input of the regulator? Wouldn't adding the input cap just change the ringing frequency? I agree this is maybe an unreasonable arrangement but it's an interesting answer! \$\endgroup\$ – mhz Feb 14 '16 at 12:37
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I had a 7805 in an application oscillate. However is was not quite in the way you are thinking here. I had a 7805 that was loaded in such way that the part was getting rather warm. It had a rather measly little heat sink but it was not enough to keep the parts temperature below the critical thermal cutout level of the regulator. As such the part would get really hot and cutout causing the output to drop to zero volts. As soon as the load was removed the part would start to cool off and eventually come back on again. It would oscillate like this at a couple of seconds rate. I also found I could change the oscillation frequency by placing a tool on the small heatsink!!

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While a Vreg can be "inherently stable," it's still beneficial to add 'stabilizing' components externally, to prevent the possibility of load instabilities outweighing the internal stability & 'forcing' the regulator into unsta le operation.

To use a more obvious analogy, imagine you're feeding rope to someone as they rappel down a cliff.

  • As long as the climber maintains a fairly predictable/steady pace, you (being inherently helpful) can maintain an even, stable tension on the rope.
  • Now, imagine the same climber stops for a few minutes, trying to find a new route; then they suddenly slip off the face & drop; then catch themselves on the face again; climb up a foot or two; then fall yet again.

In this instance; while you may have a stable footing, be very good at maintaining stable tension on the rope, etc. Wouldn't it be incredibly useful to have a rope-stabilizing pulley/drag system to keep the rope from getting into unstable movements?

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  • \$\begingroup\$ Thanks for the reply. Can you point to an example of a circuit I can study in which this happens? \$\endgroup\$ – mhz Feb 13 '16 at 22:59
  • \$\begingroup\$ Take a look at the ti.com/lit/ds/symlink/lm1117.pdf - see section 8.2.2.1.3 - An output ESR outside of the stated range will yield the regulator unstable. \$\endgroup\$ – Peter Smith Feb 14 '16 at 16:46
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    \$\begingroup\$ @PeterSmith that's not applicable. LM1117 is an LDO. 7805 is not. The subtext of my question is basically: "Do people think that standard, old school linear regulators can oscillate only because LDOs can and they're confusing the two topologies?". All LDOs are linear regulators but not all linear regulators are LDOs. \$\endgroup\$ – mhz Feb 22 '16 at 19:46

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