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I would like to use a LT3080 as a current source (see datasheet page 17 in Typical Applications Section). However, I need to be able to shut it off. Since $$I_{\text{out}}=10\mu\text{A} \times R_{\text{set}}/R_{\text{out}}$$ it seems just a matter of shorting \$R_{\text{set}}\$ with a PMOS. The problem is that for all PMOS devices I've reviewed the best I've found for zero gate leakage current is 1uA, and at high temperatures 10uA, so depending on the temperature it will suck all the current from the 10uA current source, thereby killing the output current when it is not supposed to.

I thought of using a PNP but the problem is that \$V_{\text{ce(sat)}}\$ will be comparable to the 0.5V-1V I intend to drop accross \$R_{\text{set}}\$, so it will not shut off \$I_{\text{out}}\$ effectively. The only solution I have at the moment is to shut off the input voltage. Any ideas?

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Depending on your load you may be able to just force the SET pin to GND.

If you pull the SET pin to GND the LT3080 will force the OUT pin to 0V. The internal schematic shows that the SET pin connects to the positive input of the control amplifier and the OUT pin connects to the control amplifier output (through a transistor buffer). The OUT pin follows the voltage on the SET pin.

Use an N channel FET to short the SET pin to ground.

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