LM317 Adjustable Regulator Question

OK The KEY word in your explanation is "basically". In that sense, it's true. If you use an op-amp in a stable configuration with a resistor across the leds, it will work. But in the real world, with an actual digital logic gate, actual behavior may not comply with your desires. You're more likely to get yellow as the system oscillates. But it may be too dim to see. Depends on the strays.

It's become a moot point as the OP stated OFF is not required. The input will always be high or low. Digital gate works fine in that mode.

You can make a high current rail splitter from a LM317/LM337 pair that easily handles +/- 1A. The circuit below is about as low a parts count as you can do using cheap parts and no tweaking: Please view in a fixed-width font such as Courier.

. . . . LM317 . ------- . 5V>--+--[Rcs]-+------+-------+---|I O|----+---+->2.5V . | | | | | A | | | . | | [R1] --- ------- [100] | . | >| | --- | | | . | PN3906 |----+ 0.1u| +--------' | . | /| | | | | . | | [R2] | [100] | . | | | | | | . | | | | | | . GND>--|--------|------+-------+-------+--------+---|-->

. | | | | | | . | | [Rc] | --- | . | | | | 100u--- | . | | Cc --- | +| | . | | --- | '---+ . | | | | | . '--[100]-+------+-------|---------+----[100]-+ . | | | . | ------- | . | | A | | . '-----|I O|------+ . ------- . LM337 . . . . .

sink

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D

LTSpice files are lingua franca around here.

This (only the portion starting with "Version 4" and ending with "startup") should be saved as a .asc file and loaded into LTSpice.

?-)

This will work much better- slight problem with placement of Rcs in the first one:

Please view in a fixed-width font such as Courier.

. . . . LM317 . ------- . 5V>--+-------+-[Rcs]-+-------+---|I O|----+---+->2.5V . | | | | | A | | | . | | [R1] --- ------- [100] | . | >| | --- | | | . | PN3906 |-----+ 0.1u| +--------' | . | /| | | | | . | | [R2] | [100] | . | | | | | | . | | | | | | . GND>--|-------| ------+-------+-------+--------+---|-->

. | | | | | | . | | [Rc] | --- | . | | | | 100u--- | . | | Cc --- | +| | . | | --- | '---+ . | | | | | . '-[100]-+-------+-------|---------+----[100]-+ . | | | . | ------- | . | | A | | . '-----|I O|------+ . ------- . LM337 . . . . .

OK. But it may oscillate instead and produce yellow.

?-)

14 components? Blahh, try something like this...

. +5V . o . | TC4421, . '1G04 | etc. . |\ |\ 20uH . +---| >O---+------| >O-----+---)))---+----o 2.5V . | |/ | |/ | | . \ \ | | === 10uF . / 10k / 10k _|_ | | . \ \ GND | _|_ . | | | GND . +----------+-------||------+ . Ct

Size Ct and the inductor so the operating frequency is high (100-300kHz) and the current ripple is small (a few 100mA tops).

A CD4047 instead of the gate-and-three-resistors oscillator arguably has one fewer component, but the oscillator is SO-14. This is smaller. You could also use a TimerBlox, but then your cost triples...

Tim

-- Deep Friar: a very philos>> Wonder if anyone here can offer some insight on this issue. Googling

TC4421 is 3x-6x the price of a 317/337 and I just don't see that having an initial accuracy of 5% without adjustment like the linear circuit. It does save a heat sink though, and that's probably the biggest component.

The only problem is getting something that generates a square wave that = can=20 source and sink 1 amp. But you can use a MOSFET gate driver. They are = only=20 about $1-$2.

Paul=20

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2.5

side

or

volt

ate

irst one:

=A0 =A0 LM317

=A0 =A0-------

V

=A0 A =A0 | =A0 =A0| =A0 |

- =A0 [100] |

=A0 =A0| =A0 =A0 =A0 =A0| =A0 |

' =A0 |

=A0 | =A0 =A0 =A0 =A0 =A0 =A0|

100] =A0 =A0 =A0 =A0 =A0|

=A0 | =A0 =A0 =A0 =A0 =A0 =A0|

=A0 | =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 =A0 =A0| =A0 |

=A0 =A0 =A0 =A0 =A0 --- =A0|

=A0 =A0 =A0 100u--- =A0|

=A0 =A0 =A0 =A0 +| =A0 |

=A0 =A0 =A0 =A0 =A0 =A0'---+

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0------- =A0 =A0 =A0 |

=A0 =A0 | =A0 A =A0 | =A0 =A0 =A0|

---|I =A0 =A0 O|------+

=A0 =A0 =A0-------

=A0 =A0 =A0 LM337

That's neat! If I may show my ignorance and ask a question. I don't quite get the current sense bit. Enough current through Rcs turns on the 3906 which lowers the output voltage from the LM337 (If I'm reading it right.) So what's this section for?

Thanks

George H.

one:

Of course you could just use a TCA0372 1.3 amp op amp and two resistors, for about 40 cents.

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
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

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ny

first one:

=A0 =A0 =A0 LM317

=A0 =A0 =A0-------

.5V

=A0 A =A0 | =A0 =A0| =A0 |

--- =A0 [100] |

=A0 =A0| =A0 =A0 =A0 =A0| =A0 |

--' =A0 |

=A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0|

[100] =A0 =A0 =A0 =A0 =A0|

=A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 |

=A0 =A0 =A0 =A0 =A0 --- =A0|

=A0 =A0 =A0 =A0 100u--- =A0|

=A0 =A0 =A0 =A0 =A0 +| =A0 |

=A0 =A0 =A0 =A0 =A0 =A0 =A0'---+

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0------- =A0 =A0 =A0 |

=A0 =A0 | =A0 A =A0 | =A0 =A0 =A0|

-----|I =A0 =A0 O|------+

=A0 =A0 =A0 =A0-------

=A0 =A0 =A0 =A0 LM337

The Rcs part is about limiting the quiescent current circulating between th= e 317 and 337 due to mismatch in their respective Vrefs, while not signific= antly degrading the input-to-output headroom of the 317 at maximum output c= urrent in excess of 1A. The PNP conduction will raise the LM337 output to r= educe the current.

Might be simpler to just use something like

say a parallel combination of 3x- buffers driving the 8x LEDs on a single '595, which makes 1x 'AC04 per 2x '595. Then just distribute the low-level 5V square wave around the board to the inverters.

didn't

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r 2.5

r side

0" or

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.5 volt

erate

ny

e first one:

=A0 =A0 =A0 LM317

=A0 =A0 =A0-------

2.5V

| =A0 A =A0 | =A0 =A0| =A0 |

---- =A0 [100] |

=A0 =A0 =A0| =A0 |

---' =A0 |

=A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0|

=A0 [100] =A0 =A0 =A0 =A0 =A0|

=A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 |

=A0 =A0 =A0 =A0 =A0 =A0 --- =A0|

=A0 =A0 =A0 =A0 100u--- =A0|

=A0 =A0 =A0 =A0 =A0 +| =A0 |

=A0 =A0 =A0 =A0 =A0 =A0 =A0'---+

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|

| =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0|

| =A0 =A0 =A0------- =A0 =A0 =A0 |

| =A0 =A0 | =A0 A =A0 | =A0 =A0 =A0|

'-----|I =A0 =A0 O|------+

=A0 =A0 =A0 =A0-------

=A0 =A0 =A0 =A0 LM337

't

A three part 40 cent solution... where's the fun in that? :^)

I'll have to order a few of those opamps. I've never used one so never think about 'em. (And they come in 8 pin dips.) Looks like they might dissipate a watt or so?

George H.

t -

g didn't

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for 2.5

her side

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and

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operate

Any

he first one:

=A0 =A0 =A0 LM317

=A0 =A0 =A0-------

| =A0 A =A0 | =A0 =A0| =A0 |

----- =A0 [100] |

=A0 =A0 =A0| =A0 =A0 =A0 =A0| =A0 |

----' =A0 |

=A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0|

=A0 [100] =A0 =A0 =A0 =A0 =A0|

=A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 |

=A0 =A0 =A0 =A0 =A0 =A0 --- =A0|

=A0 =A0 =A0 =A0 100u--- =A0|

=A0 =A0 =A0 =A0 =A0 +| =A0 |

=A0 =A0 =A0 =A0 =A0 =A0 =A0'---+

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|

| =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0|

| =A0 =A0 =A0------- =A0 =A0 =A0 |

| =A0 =A0 | =A0 A =A0 | =A0 =A0 =A0|

'-----|I =A0 =A0 O|------+

=A0 =A0 =A0 =A0-------

=A0 =A0 =A0 =A0 LM337

't

the 317 and 337 due to mismatch in their respective Vrefs, while not signif= icantly degrading the input-to-output headroom of the 317 at maximum output= current in excess of 1A. "The PNP conduction will raise the LM337 output to reduce the current."- Oh, I got that backwards somehow, Thanks.

George H.

Hide quoted text -

...should be saved as a text file with .asc as the file extension...

Just for clarification. I apologize for the intrusion.

an

volt

first one:

Theta-JC is specified as 12 K/W for the wide SO package, so you can probably do a few watts if you have a nice big copper pour.

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
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

didn't

of an

2.5

side

or

volt

operate

first one:

The datasheet stops characterizing that part at +/-10V supplies. I seriously doubt it will have the advertized current drive at +/- 2.5V. Have you used it in a 5V app?

'595, which makes 1x 'AC04 per 2x '595. Then just distribute the low-level 5V square wave around the board to the inverters.

A 555 with complementary emitter followers oughta work just fine.