analogconstant-currentvoltage-regulator
I am trying to use LM317T as a current regulator and the setup is from its data sheet. I couldn't get it to work.
I want a constant output current of 10uA when input voltage is changed from 3V to 2V. Since the internal reference voltage between output and adj is 1.25V, I put a resistors of 125kohm from adj to output.
I am getting very high currents like 3mA and I can't figure out why. I appreciate all your help.
simulate this circuit – Schematic created using CircuitLab
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 :-)
Foe 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.
Using an op-amp is probably the best way to deal with this requirement but you might be better with an emitter follower circuit: -
The op-amp will drive the NPN so that its emitter (Voltage across the load) is the same voltage as the +Vin pin on the op-amp.
I borrowed "The Photon's" drawing because although it was wrong, it was easily amended to show an emitter follower. If you did use a FET (N channel with source to load) this would work but you might find that Vin needs to be higher for it to work. BJT base drive only needs to be 0.6V above the emitter - I'd still recommend using an op-amp with rail-to-rail output capability so that Vin can be a bit lower.
I'm also assuming the op-amp is powered from Vin and GND.
Best Answer
Datasheet check!
IADJ: Adjustment pin current
So... it will not work.
For an accurate 10µA you will need to use a different topology.
Choice depends on factors you will need to mention, like how much parasitic capacitance you can tolerate at the output of your current source, voltage compliance, available supply voltage, etc.