LDO problem

I don't see much of a difference between #1 and #2. I'd certainly tie the op-amp and the pass element to the same rail, just for simplicity, with perhaps a R-R opamp to lower the headroom.

Did you draw that a year ago or are you having an automatic reflex to keep using 2010 for a while yet?

First mistake.

Second mistake.

Where are the 12V logic specs?

What is that "DTTL"?

Wrong approach.

I take it that the resistors will limit the current under all conditions (including faults) so you don't really need the thermal cutout and current limiting incorporated in an LDO.. in which case I kind of like the first one.. it has a high-Z input for the PWM filter if you go that way, though the 17V low current supply adds a tiny bit of complexity (maybe just drop an isolated 5V out DC-DC converter module in there). You might not need all those compensation parts..

Best regards, Spehro Pefhany

John,

Take a look at Linear Tech's LT3080. Feed the SET pin with the Vout that you want.

Bob

Posiive feedback in 2?

#1 uses a n-fet follower, so needs a +17 supply for the opamp. I really prefer this one for its nice dynamics and low parts count, but I'd have to create the +17 supply from the available +12. I'd only need 10 mA or so, so it wouldn't be a big deal. I could use one of those $4 dc/dc converters, +12 to +5, and bootstrap the +5 output on top of +12. Or build a real-life boost converter.

John

Features are not only appealing on their own, they allow a user to write a purchase spec that only we meet.

John

I don't like the potential dissipation. If programmed to 5V and 100mA is drawn then that's 0.7W. You could use an NCP3063 and opamp as voltage controlled buck.

Why would that feature be a mistake?

There are no firm logic specs. A programmable pullup could not only drive most any positive logic, it could drive mosfet gates, LEDs, SSRs, all sorts of things.

Features like this are fairly easy to add, especially on a low density board like this. If they increase sales by 5%, they're worth it.

I think we'll do programmable input debounce, too. A premium version could do event logging, output pattern sequencing, PWM, stepper control, stuff like that.

How about you do some real electronics, like commenting on the circuits, instead just bitching? There are things to say about all of them. Better yet, come up with one of your own.

John

What's the rush?

John

Nice, but dropout voltage is 1.2 typ at 100 mA. I thought it would be nice to go all the way to +12, or at least very close.

John

Why not a simple NPN follower? Your current requirements are certainly low enough so Hfe(min) isn't an issue.

Way too messy, for no gain.

No, the fet inverts and the opamp doesn't.

I just simulated this one and it works pretty well. It rings a little on transients, and a resistor in series with the comp cap fixes it, but it's probably not worth adding another part. The dynamics are interesting, because going into the + input of the opamp adds a "proportional" gain term that keeps the thing from being a double integrator, which would ring big-time.

John

I'm going to be driving eight maybe 1K pullup resistors, so worst-case fet dissipation is at +6 out, 48 mA, under 300 mW. That's not bad. VME has lots of air! Eight switchers would work, but sounds like overkill.

John

I'd lose about 0.7 volts of pullup. I thought it would be nice to go all the way to +12. If I changed the spec to +10 max pullup, I could use a r-r opamp and an NPN. That would sure be simple.

John

You can run the follower a *little* into saturation, but you're losing a lot more than 700mV with your FET follower. There is nothing magic about a 12V supply. Simple is good.

If there is software somewhere in the path (to correct for the 1.25V offset), an LM317 makes a pretty good pass element, too. I'm sure there is a simple way to do the same with a buck regulator, if saving power was worth a little complexity.

The NPN follower sure has nice dynamics. It's pretty stiff open-loop, so the opamp doesn't have to thrash very hard to get really nice regulation. I suppose there's nothing special about 12 volts.

The issue with using an LM317 (or better yet, an 1117) is the dropout and the fact that I'd like to go to zero volts on the low end.

Beats yardwork.

John