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Old 11th December 2002, 09:19 AM   #1
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Question chip reg with ext voltage ref...?

it has just come to my attention that it is possible to use an external voltage reference with a chip regulator - 3x7 or 108x types....
i have never seen that before, how does it work? is it better?

thanks,
troels


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Old 11th December 2002, 01:04 PM   #2
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I believe that the '723 used an external reference.
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Old 11th December 2002, 01:15 PM   #3
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The '723 has a built-in reference but you have to wire it externally. So you are free to leave this output open and use another reference if desired.

Regards

Charles
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Old 11th December 2002, 01:22 PM   #4
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Default chip reg

The 723 is very, very old and tired. It has a mediocre, 741-type error opamp with very low bandwidth, and the reference isn't up to modern standards either. This would not be a good chip for your high performance regulator.

The others as mentioned have internal references and I don't know of any way to disable them. Even using a 317 with separate error (feedback) opamp driving the adjust terminal will still have the internal ref voltage in series with the opamp output.

I think you have only the choice between a fully integrated regulator or a discrete approach with ref, error amp and pass transistor. This last option undoubtedly gives the best performance. Anything in between is, well, in between.

Jan Didden
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Old 11th December 2002, 02:06 PM   #5
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I only point out the '723 as it readily came to mind as having an external reference.

For now I just use LM317LZ's with outboard current maintenance. Perhaps I got a really good batch, but the noise is de minimus.

In my HV regulator circuit I am using a Maxim 6250 with the Maxim 5250 DAC -- a little inconvenient as it's an 8 pin DIP, but they seem "made for each other".
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Old 11th December 2002, 02:29 PM   #6
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Please correct me if I am wrong, but my understanding is that
any three-terminal regulator can be used with the reference
pin tied to an external reference, since the regulator adjusts
the output voltage relative to the reference pin voltage. For
a 317 we get Vout = Vref + 1.25V for instance. The reference
voltage source must be able to sink some current (about 150uA,
I think, for a 317) and it may be wise to include a protection
diode from ref. pin to output. Basically, this is the same
principle as can be found in application notes where a zener
diode is connected between ref. pin and ground to get
higher input and output voltages than usually allowed.
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Old 11th December 2002, 03:23 PM   #7
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Default chip reg

Christer,

Yes, that will work, but observe that the total ref voltage is the external one + the internal 1.25 - your own formula. Therefor, the result can never be better than the internal ref voltage in terms of stability and noise.

Jan Didden
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Old 11th December 2002, 06:13 PM   #8
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Jan,

yes, I completely agree, but the original posting didn't say
anything about the context of use, or why an external reference
was to be used. I admit, though, that in most cases there will
be no reason to use an external reference. I can think of two
reasons, either we want to use the regulator for higher input/
output voltages than it is rated for, or we vant to set the
output voltage with a voltage rather than with a potentiometer,
for some reason.
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Old 11th December 2002, 06:39 PM   #9
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i thank you all....(quote FM rip)

i didn't get it either.
somebody told me it would lower the noise.....i guess not..!

best regards,
troels
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