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From: email@example.com (Dale Chisholm)
Subject: Re: three-terminal regulator question
Date: 9 Jan 2003 07:00:36 -0800
References: <firstname.lastname@example.org> <3E1DFF1C.CDD61760@rica.net>
NNTP-Posting-Date: 9 Jan 2003 15:00:36 GMT
John Popelish wrote in message news:<3E1DFF1C.CDD61760@rica.net>...
> Ernst Baumgartner wrote:
> > Hello,
> > is it possible to connect a few (say four) three-terminal regulator ic's
> > (such as 78xx) in series in order to obtain a higher output voltage?
> > Thank you!
> > Ernst
> These regulators produce an output voltage that is some fixed voltage
> above their reference pin (that is normally grounded), as long as the
> output is a couple volts less than the voltage on the input pin. If
> you use one regulator to produce a voltage higher than zero to act as
> the reference, then the regulator connected to that elevated reference
> will produce an elevated output (assuming there is sufficient supply
> voltage). However, this sort of operation may defeat the over
> temperature and over current protection functions, which expect the
> output current ot go to zero as the output voltage approaches the
> reference voltage and this in not the case when the reference is fixed
> and elevated.
John, I like your answer. I'm not sure you'd lose the overtemp &
overcurrent features, though - they'll still be active on the topmost
unit in this totem pole, which is the only one where it really
matters. (The lower units are - like somebody else said - simply
providing a reference voltage. The design issues are satisfying their
MINIMUM output current requirements, and providing a path for the
quiescent bias currents flowing out of the "ground" pins of the upper
members of the totem pole.)
Maybe the original poster will come back and give us a better idea of
what he's trying to accomplish.
Nobody has mentioned using ANY 78XX-type as an adjustable regulator,
though. It works the same as for an LM317, except your calculations
for the resistive divider use the regulator's rated output voltage
(5V, 12V or whatever) in place of the 317's 1.2V. I think there's a
slight penalty in transient response. This trick is sometimes
justified under the premise that it allows purchasing efficiencies,
reduces the number of line-items in the manufacturing and logistics
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