I really like the LM1117-ADJ part. It's an "MDO" regulator, cheap, lowish dropout but it still has an emitter follower output. The adjust pin current is 60 uA typ, so if you really need a dump resistor, 10K would work.
I use those a fair bit too--they're great except for the 12V maximum input voltage. It's a different situation, though, because all those adjustable regulators need a minimum output current, and since you need the resistors anyway, there's no point in stacking them.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
I'm almost sure 78xx need something like 5mA minimum or the output voltage will be off
-Lasse
That is not a true statement. It entirely depends on the FPGA. There are lower power FPGAs which do *not* draw significant quiescent current and even full power may only draw a small current unless heavily tasked.
-- Rick
The guaranteed range is 5 mA to 1 A or something, but in practice they're okay. LM317s go _way_ off if you violate their minimum output current (10 mA max over temperature, iirc). We're probably all been bitten by that one at some stage.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Den mandag den 7. december 2015 kl. 18.31.56 UTC+1 skrev John Larkin:
it specs a minimum load of 5mA, that's why the top resistor to set the voltage is usually 240R, 1.25V/240R ~= 5mA
-Lasse
What is the voltage at the input to the 78015?
-- Rick
A 500-ball FPGA and a uP and a couple of XOs will easily provide that load, so I seldom need to add more. Of course we consider that in design reviews; that's one thing on the list.
No, they're fine. There's an internal divider for the feedback voltage that provides a small load (something like 0.4mA IIRC) and the current used by the regulator flows out the GND pin (maybe 10x that, and fairly constant).
It is possible that at extremely elevated temperatures and no load the leakage through the pass transistor might overpower the divider resistance and cause the output voltage to rise out of spec, but I don't think that will happen within the recommended operating range (maybe if the device is forced into thermal shutdown).
--sp
-- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8 Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48
we were indeed
_________ Vin | |
0--------| 7805 |------ |_________| | +------------ ___|_____ | | o--------| 7905 |------ |_________|NT
Yikes. I wouldn't use that one on a bet, because the ground pins can't sink any current.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
-- Geez, John, you've gotten yourself almost convinced that you're the best thing that ever came down the pike to play this game, and you've become high-maintenance in that you want everyone to fall in lock-step behind you and prove you right, no matter what. impress, rather than to edify. John Fields
ALl I can say is, you didn't understand what I said..
oh well
Jamie
And your expectations were different ?>:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Not at all. No non-trivial FPGA is going to let its core voltage be pulled up by the minimum load requirement or adjust pin current of an of an LM1117. If it is, then worry about it.
Most modern products have XOs and uPs and FPGAs. Everybody does that. There's nothing unusual or impressive about that.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Both of you are living in the 555 age. Most of us design more complex stuff. In most modern products, there's tons of startup current load, more than enough to exceed a regulator's min load spec.
And as I said, if the load is below the reg minimum, fix it.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
-- Not really, but a worm _can_ turn. John Fields
If that's true, then your calling them into play without pedigree was superfluous and disingenuous since all you seemed to be touting was your ability to route a ball-grid array and a few peripherals.
And not even routing it yourself, at that.
John Fields
No, current imbalance has to be handled by power zeners, and thus voltage t olerances have to be watched with care, and the load's ability to deal with small voltage shifts. Big caps are used to handle most of the momentary im balance. Keeping a careful eye on all tolerances before the zeners is of co urse vital. I've used it once, and yes it works fine as expected. With so m any conditions to watch it's seldom useful.
NT
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