LM317 compensation

LM317's like some ESR in their output capacitors. I don't want any electrolytic or tantalum caps in my new thing, just ceramics, and the sim sure rings:

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But this fixes it:

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This ain't rocket science, but I haven't seen it done before.

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John Larkin         Highland Technology, Inc 

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John Larkin
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You can put resistors in series with ceramics...

Tim

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Seven Transistor Labs, LLC 
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Reply to
Tim Williams

Sure, but that raises the bus impedance, and I expect load current pulses. Putting resistors in series with just some of the caps still rings.

This takes less parts. Actually, it's pretty good without R6.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Well, your way raises the bus impedance rather more significantly.

You can't do "some", you need to do >2/3rds of total capacitance. Example:

3uF worth of 0.1's scattered around the board, 10uF + (0.1R or whatever works best) "bulk" cap.

If you need low bus impedance, you have to do both!

Tim

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Reply to
Tim Williams

Den fredag den 16. februar 2018 kl. 02.51.57 UTC+1 skrev John Larkin:

from the data sheet:

The adjustment terminal can be bypassed to ground on the LM317-N to improve ripple rejection. This bypass capacitor prevents ripple from being amplified as the output voltage is inc reased. With a 10-?F bypass capacitor,

80-dB ripple rejection is obtainable at any output level. Increases over 10 ?F do not appreciably improve the ripple rejection at frequencies above 120 Hz. If the bypass capacitor is us ed, it is sometimes necessary to include protection diodes to prevent the capacitor from discharging through internal low current paths and damaging the device.
Reply to
Lasse Langwadt Christensen

I do that reasonably routinely with LDOs and higher-current cap multipliers--a zero-ohm jumper in series with the output cap to make it look like we did it on purpose. ;)

With cap multipliers I usually put the resistor in series with the output, but with LDOs I usually put it in series with the cap so that it mimics a small ESR.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

Straight Darlington output emitter follower voltage regulators are meh, maybe relics of when you couldn't make a decent high beta PNP on a chip

Reply to
bitrex

apart from the dropout, whats meh about them?

Reply to
Lasse Langwadt Christensen

All things being equal the quasi-complimentary/Sziklai topology is better in just about all respects than the straight Darlington; the complex output impedance of a straight Darlington has an inductive component that increases rapidly with frequency (hence stability problems), the complimentary less so

Reply to
bitrex

Sure, everybody knows that. If you use a big cap from ADJ to ground, and use low ESR output caps, it still rings wildly on a load step.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Are you referring to the 1u resistor? That's just there to conveniently snoop the current in the sim. It's the RC from ADJ to ground that kills the ringing.

I expect big, fast current spikes in my load, so I don't want to add ESR to my output bypass caps. Aluminums would have too much ESR anyhow, but they would be too tall too.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

I'm regulating +48 down to to +30, which few LDOs will do, and I have the 317's in stock.

I thought this was a cute trick, but everybody's a critic.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

The emitter follower is basically low output impedance. The PNP starts with high output impedance. Sziklai types, like the LM1117, have plenty of ESR constraints too. There are few regs around that are stable down to zero ESR, and most are low-voltage parts.

The 1117 is a nice part, but limited to 15 volts in:out. (They typically die around 60.)

Anyway, my thing seems to work. If you don't approve, don't use it.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

I have never heard of a 317 ringing in a properly designed real application. Anyone seen it?

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Reply to
David Eather

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."

Unfortunately for high power regulators that's also a non-starter because IIRC you can't make a high power monolithic PNP to pass the current, either. It works pretty well with discretes.

As far as I can tell design work on high power non-LDO voltage regulators ceased around 1980 they pretty much all seem to be variants on the LMxxx LMxxxx topologies.

Easy John Wayne, the LM317 is meatloaf not filet mignon but sometimes meatloaf works just fine

Reply to
bitrex

How would you know if one did? Do all LM317s communicate with u telepathically

Reply to
bitrex

Normally a 317 running at high CL gain is pretty stable IME, but I can imagine that a bit of lead/lag could help the load transient response.

I use the lead network a bit like that in buck switchers with cap multipliers--take the DC feedback from the cap mult and AC from the switcher output.

Cheers

Phil Hobbs

Reply to
pcdhobbs

Urhhhh...... the 1 uohm resister is finger nails down a blackboard for me.

For a regular spice engine on regular doubles, numbers can't span more than

12 digits and have it solve correctly. So, 1 uohm means 1 Meg max.

Don't use smaller than 1m ohm

er... This ain't rocket science.

And alternatively, ... you could put a low value resister in the ic output prior to the feedback, and use a feedback cap direct from the ic output to its input. Effectively, if configured correctly, the output resister does the job of an out of loop esr, but without the dc error.

-- Kevin Aylward

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Reply to
Kevin Aylward

One way to get a stiff programmable power rail is to drive the ADJ pin of a 3T regulator with an opamp, open-loop. Maybe that could benefit from some sort of damping too.

For a stiff bipolar rail, the TCA0372 opamp (dual, 1 amp per, 50 cents) is great, but it sure doesn't like low-ESR caps on its output. A tenth of an ohm in series with the output fixes that.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

that aussie guy had some fun with that recently

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tldw: A new batch of boards with virtual ground made with an opamp oscillates at certain loads, even though it has 270R resistor on the output. Turns out the opamp with the same part number from a different manufacturer wasn't quite identical

Reply to
Lasse Langwadt Christensen

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