An LM350, can I have both constant current, and specific voltage?

Question

I have a regulator such as in this setup to provide 1.2 Amps to a load that requires constant current:

R1 is set to 1 Ohm, which nicely brings the current within range (1.25A according to a small calculator below the image), but what I do not understand is that it speaks of a 1.25V reference. Does it output 1.25V only, or does it output the input voltage (minus) voltage drop (vdrop maybe 3V) and the 1.25V is just specific to ADJ?

Can I output voltage to, say, 2V as well as 1.2A constant current with just one LM350 device?

The datasheet for the LM350 I will be using is the following: http://www.ti.com/litv/pdf/snvs772a

Answer 1

In constant current mode, it will output whatever voltage is necessary to push (in your case) 1.2A through the load.
This will be limited by the supply voltage (minus a bit for the regulator drop) as obviously it can't output a higher voltage than goes in. So you can't expect it to put 1.2A through an e.g. \$ 1 M\Omega \$ resistor unless you have a supply of 1.2 Megavolts handy :-)

For example, if you have a 1 ohm resistor as the load, the voltage at the load will be:
1.2V (\$1.2V \div1 \Omega\$ = 1.2A)
If you have a 5 ohm resistor as the load, the voltage at the top of the load will be 6V (\$ 6V\div5\Omega\$ = 1.2A)

You can't set it up for constant current and constant voltage, as to keep one constant requires varying the other.
However, with a static load in constant current mode you can set it to drop 2V - e.g. \$2V\div 1.2A = 1.6666\Omega \$ resistor needed. So if you have a load of \$ 1.6666 \Omega \$ on the output of a constant current of 1.2A, the voltage will be 2V.

Answer 2

It's a 1.25 V reference because the circuit will operate to keep the ADJ pin 1.25 V below the OUT pin. The OUT pin will be driven to whatever it takes (up to Vin - 3 or so) to force the ADJ pin to be 1.25 V below Vout.

To answer the question the way you phrased it in the title, you can't get the power supply circuit to force both a fixed current and fixed voltage at the same time. That wouldn't leave any freedom for the load to affect the circuit. For example, if you have a 1 Ohm load, you need 1 V across it to get 1 A. If you have a 2 Ohm load, you need 2 V across it to get 1 A. If the supply circuit tried to force both at the same time, you'd have a logical contradiction on your hands.

Note that to get 1 A output you will need to be sure to pick the right package type for your LM317 and also you will probably want to use an external heat sink (or a good thermal connection to your PCB) to avoid overheating the part.