The 1117 is the opposite, a small PNP driving a big NPN follower. That saves a junction drop. I call that an MDO, medium dropout regulator. Its load ESR requirements are similar to a 317 type, I usually hang a
33 uF tantalum on an 1117, after which it's happy driving a bunch of ceramic bypasses.There are pfet-output LDOs.
Everything is going low-voltage, low loss, and switchers. And the 317 types work fine, except that there could be a version compensated for ceramic caps.
That's there to let me snoop the current. I find it hard to aim the LT Spice current probe at device pins, so I'll stick in a resistor here and there to make current snooping easy.
I usually use 1m, but I wanted an accurate sim of regulator ringing and 1m might have affected that a little.
I suppose I could use 0 ohms... tries it... no, LT Spice will not probe the current in a 0 ohm resistor. Pity.
Looks like I should be OK here:
In real life, PCB traces can be many milliohms. I have rarely used them as current shunts.
A current snoop resistor could be on the input side too.
317 needs no such ESR compensation. The ringing looks suspiciously like exc itation of the SRF of an output capacitor. Did your model give it any ESL? And your solution merely reduces the shunt resistance by a factor of 20x wh ich probably has more to do with damping than anything else.
The output has an inductive characteristic at higher frequencies. It's stable as you can see, but that doesn't mean the impedance is great.
Tim
The data sheet says it does.
The frequency is low, and is different on the rising and falling edges of the load current pulse. It's the chip pseudo-inductance resonating, not the cap's ESL. If the ringing were local to the caps, my damping on ADJ wouldn't fix that.
Cap series L makes a different waveform than paralleled inductance.
With a big cap from ADJ to ground, it rings badly, too. It has to be the *right* capacitor to damp the ringing.
I tried this with two different LM317 models; the ringing is somewhat different (the LT317 is better), but the damping idea is the same.
It's amazing that LT ever made a 317. I think they did that early on, when they needed some revenue. They want $4 for it! I'm paying less than a tenth of that for TI.
Don't jump so far. Try an order of magnitude at a time, and also adjust read values according, of course.
Real current shunts are nice too. Especially for breadboard work.
Yes, it's cheaper than an LM395 and works pretty well for that. The compensation can be a bit squirrelly if the op amp is too fast or the range of load currents is too wide.
Probably also a bit load sensitive, no?
Voltage references also vary widely in stability--the ON Semi version of the TLV431 is unstable over most of the range you'd want to use it in--check out Figure 18 at
compared with the graph on P7 of
.Not exactly plug-in replacements!
Cheers
Phil Hobbs
See e.g. the Errol Dietz (*) article at
Cheers
Phil Hobbs
(*) Errol Dietz rose from being Bob Pease's technician to National's CTO. Not bad.
Right, any single pole rolloff has an effective inductance as output impedance, but the Q is generally too low (
I doubt you're going to see this energetic resonance on anything other than the LT part.
A Sziklai output as I'm thinking of would have a PNP in line with the output and an NPN driving the base; it would be the functional equivalent of just a regular NPN emitter follower, "in disguise."
Your 40 years out of date, probably.
Decent, high current, very fast complementary bipolar processes are available.
The X-Fab XTO18 SOI is a pretty good 0.18u BiCMOS process. It has good low noise mosfets and fully isolated, fast npn/pnp, and is very affordable.
-- Kevin Aylward
The "correct" way to probe currents in Spice if the data is not otherwise available, is to use a zero voltage dc source.
In fact, in basic Spice3 AC currents are not available for say, bipolars, so you have to do that if you want that data.
In SuperSpice to get transparent current probing in .subckts, I automatically add 0V sources into the pins.
-- Kevin Aylward
OK, big round 0 ohm resistors!
I was going to ask what in the world is so special about a 317 that one would need a gilded version of it
Only that LT Spice had a model already. I tried an independent LM317 model (can't recall where I got it) and the overall situation was similar... lots of ringing with ceramic caps, and the right RC helped a lot.
I'm borderline over using a switcher to get from +48 to +30. Not many switchers will handle 48 in. We have LM2576HV in stock, an ancient 50 KHz SimpleSwitcher, which has its own issues, so I'll go linear and dump the heat.
Right, so what does that tell you about the output impedance of the model in question? :-)
(To go much farther, this thread needs waveforms from a real part. Or an R+C and being done with it. ;-) )
Tim
Why would the Q be low? Active filters have Qs in the hundreds.
3T regulators are notorious for oscillating into low-ESR output caps. Opamps, too. A minority of opamps are c-load stable, and that's usually not determinable from the data sheets. Many opamps are stable with small capacitive loads and with giant ones, unstable between.My LT317 sim, without the fix, is starting to look decently damped with 300,000 uF of zero-ESR caps. The internal pole must be a few 10s of Hz.
LOL- 300,000u ??? Hahaha- what does that tell you? The simulation is not credible.
It tells me that the 317 internal pole is at a few 10s of Hz. Most opamps start to roll off at similar frequencies.
How do you get 10Hz, 10^-2 sec. from 300uF? (where's the ~30 ohms)
I like your circuit, I've never had any issue's with the '317, but I think I've always used it with a tant or Al cap.
George H.
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