diagnosing an Intel Series II MDS monitor failure

tony duell ard at p850ug1.demon.co.uk
Mon Aug 3 08:33:23 CDT 2015

> >
> > Was the failed IC marked '723' or some house number?
> >
> > -tony
>  Hi Tony As I recall, it was marked LAS723-2.5Luckily, before I simply replaced it, I checked the 
> feedbackresistor values. They didn't make sense for 7V.My guess is that it made using the 723 
> for 5V supplies morepractical, although it could be wired up.The data sheet seems to indicate 

The error amplifier of the 723 has both inputs brought out, the reference is not connected to it [1].
So for 5V (or voltages under the reference voltage) you can link the sense wire directly to the +ve
input of the error amplifier and a divided down refence to the -ve input. 

The only practcal advantage of the lower reference is that you can run the chip with a lower supply 
voltage I guess

[1] It is not at all uncommon in systems with multiple 723s to use the reference of only one of them, feeding
the error amplifiers of all the ICs. All PSU outputs should then track together. Or in some cases to
ignore the reference altogether and, for example tie one input of the error amplifier to ground and the
other to a potential divider between the output of that section (configured as a -ve regulator) and a 
positive supply rail. This then gives you supplies balanced about the ground rail. HP98x0 machines are
one place this turns up, there are many others.

> a better tolerance.The 2.5V references are usually bandgap and are tightertolerance. As I recall,
>  in the supply I was working on, it was a low current 12V line.Dwight

Possibly, although for most applications this would not be important.


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