I wasn't trying to argue semantics. I was just trying to point out that it was the voltage divider ratio of the two resistors, not the current that determines the output voltage. Thus you can vary either R1 or R2 to vary the output voltage. But certainly with a fixed value of R1, then the current through the divider is constant for any value of R2.
Carl,
Well, I have always proposed the idea that the chicken preceded the egg in some fashion; an acknowledgment to the physicists Ohm and Kirchhoff.
Vref begets Iref, and Iref determines the divider based on the physical laws determined by those 19th century minds and the designers of the device being discussed. I see your point clearly, but just do not feel it covers the broader implications of design criteria overall. A current analysis is necessary to consider the impacts of Iref, Iadj and other criteria if deviations from the recommended configuration and limits are made. Also, I find it much simpler in all cases since critical currents can be compared to those calculated.
As to adjusting either resistor in the divider, sure it can be done, but at the cost of predictability of the outcome and very limited range owing to changes in Iref under all load conditions to say nothing of % of regulation with varying loads.
I finally got some time today to get back to this and look up an LT device that closely matches the LM317. The only differences I noted, in a brief review, were in the necessary overhead before drop out (load dependent), and the minimum load requirement (slight). It's the LT1086. I would have preferred having the spice model for the LM317, but I cannot find it anywhere.
I ran multiple sims and the results matched very closely my numbers for the LM317 in standard configuration including Iadj. Adjusting R1 and varying Iref yielded very poor results as expected under zero/light static load conditions. I also ran sims for your suggestion of connecting only the wiper and a 10k to ground to the Adj pin. The results were of the same general sort as varying R1 as its value varies by the value of R2 north of the wiper and not being shorted by the wiper connection to the top of R2.
Vout diverges rapidly from the calculated output after R1 reaches the area of 500Ω, in normal configuration, with a ratio of 3 for Vout=5V, by stepping R1 & R2 and maintaining the ratio. With R1 at 700Ω and R2 at 2.1k, a ratio of 3, Vout=12V; proof that the ratio alone is not the controlling agent. This is due to Iref being less than the minimum load current as I suggested in an earlier post. Adding a 1k load resistor brought this regulation error down to about 1.6%, somewhat greater than the 0.1% advertised, and stayed at that error level with a 100Ω load. This lesser error can be mostly attributed to the influence of Iadj adding to the lowered Iref value through R2, which would normally swamp that impact of Iadj if Iref were in the correct range indicated by the minimum load current in the datasheet.
I could be wrong, as I just downloaded LTSpice about six weeks ago so you might want to check the results for yourself. I sure find it much simpler to use than PSpice, which I loath, in my Orcad package, except for the limited library of parts. If you decide to run a sim with the LT1086, could you let me know if your results differ?
Merv