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
😉
i have never seen that before, how does it work? is it better?
thanks,
troels
😉
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
Regards
Charles
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
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
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".
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".
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.
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.
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
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
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.
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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