off-label use of adjustable shunt regulator

[Michael Robinson]

I was going to use a TL431-type regulator like this
(view in Courier or fixed font)


7 volts
|
,----+---------,
| | pnpx2 |
| \e e/
1k |--+--|
| /c | c\
| | | |
'----+----' |
| |
| |
_|_/ |
/ ^ +--Vout
/ \ |
Vin---/ \ |
------- 100k
| |
| |
'----+----'
|
///


or this

7 volts
|
+---------,
| pnp |
| e/
1k ,--|
| | c\
| | |
+----' |
| |
| |
_|_/ |
/ ^ +--Vout
/ \ |
Vin---/ \ |
------- 100k
| |
| |
'----+----'
|
///

I just wanted to figure out whether I should mirror the cathode current like
in the first drawing,
or could the circuit use some gain.
So I started looking at datasheets for three-terminal shunt references, and
the information in them
is all about how they behave with the cathode tied to the reference pin,
whether directly or by voltage divider.
Nothing that would help me.
So I thought I would simulate it. LTSpice has a model for the LT1009, which
is a conventional 2.5 volt adjustable
shunt regulator.
Now, I know you can use a three-terminal shunt regulator like an op amp,
because I have.
But LTSpice's simulation was really screwy. In the sim, the cathode voltage
doesn't float. I'm pretty sure that's wrong.

Version 4
SHEET 1 880 680
WIRE 64 32 -176 32
WIRE -16 112 -96 112
WIRE -176 144 -176 32
WIRE -96 144 -96 112
WIRE 64 160 64 112
WIRE 32 192 -16 192
WIRE -96 224 -176 224
WIRE -80 224 -96 224
WIRE 64 224 -80 224
WIRE -80 256 -80 224
FLAG -80 256 0
SYMBOL References\\LT1009 64 192 M0
SYMATTR InstName U1
SYMBOL res 48 16 R0
SYMATTR InstName R1
SYMATTR Value 1k
SYMBOL voltage -176 128 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 5
SYMBOL voltage -96 128 R0
WINDOW 3 42 121 Left 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value PULSE(2 3 0 1)
SYMBOL res -32 96 R0
SYMATTR InstName R2
SYMATTR Value 10k
TEXT -114 280 Left 2 !.tran 1

Maybe the moral of the story is you can't trust simulations. But it would
be nice to get some real info about
how these chips behave in more than the limited way the datasheets address,
or a faulty sim.
Is there some literature out there, or maybe a functioning model?

Here is the datasheet for the LT1009:
http://cds.linear.com/docs/Datasheet/1009ff.pdf
the datasheet for the regulator I actually want to use:
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00005105.pdf

 

[Jasen Betts]

I was going to use a TL431-type regulator like this
(view in Courier or fixed font)
[...]

| |
_|_/ |
/ ^ +--Vout
/ \ |
Vin---/ \ |
------- 100k
| |
| |
'----+----'
|
///

you need to use a TL431 in a closed loop (with feedback) unless you want a digital
output. It has too much gain to be used upen loop.

So I thought I would simulate it. LTSpice has a model for the LT1009, which
is a conventional 2.5 volt adjustable
shunt regulator.

no, it's a 2.5v reference it's nothing like a TL341
the TL341 has lots of negative gain, the LTC1009 as less than unity
positive gain.

There's a TL341 model out there which can be installed in ltspice.

 

[Michael Robinson]

I was going to use a TL431-type regulator like this
(view in Courier or fixed font)
[...]

| |
_|_/ |
/ ^ +--Vout
/ \ |
Vin---/ \ |
------- 100k
| |
| |
'----+----'
|
///

you need to use a TL431 in a closed loop (with feedback) unless you want a
digital
output. It has too much gain to be used upen loop.

There must be something about the way I post that makes people assume the
worst.
No fear, it is indeed part of a much larger circuit with a feedback loop.

no, it's a 2.5v reference it's nothing like a TL341
the TL341 has lots of negative gain, the LTC1009 as less than unity
positive gain.

The datasheet for the LT1009 clearly shows a three-terminal adjustable
shunt.

There's a TL341 model out there which can be installed in ltspice.

I guess I already knew I'd have to do that. Oh, bother.