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LT3083 output not zero when set pin grounded.

BCook
on Jun 12, 2020

I am using an LT3083 as a 2-terminal current source, per the last figure in the data sheet. The LT Spice simulation works as expected, but the actual circuit has a residual voltage of over 2V when the set pin is grounded. I realize that there is a minimum 1mA current, but this represents 10mA of current. I see nothing in the data sheet that explains this.
I am able to make the circuit work in my application by using a resistor divider for the load, then shorting across the bottom load resistor with a MOSFET at the time I pull the set pin low with another MOSFET. 

It would be easy to ignore this and move on, but I don't like the potential for latent failures due to something I don't understand.

I can send or attach the spice file if that helps.

Brett

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    •  Analog Employees 
    on Jun 15, 2020 2:12 PM

    It is possible to violate the Abs. Max. Rating of the LT3083 by shorting SET and in that case the LT3083 may behave strangely as you report.  I assume you also have a capacitor from the input of your current source to the OUT pin of the LT3083.  The capacitor may make the situation worse by holding up OUT while SET is pulled down.

    I mention it is important that OUT and SET are always close to the same voltage in other posts like this one titled "LT3080 SET TO OUT ABS. MAX. IN OVERLOAD", this one titled "Using LT3092 with a DAC", this one titled "LT3092: Problem with supporting BJT for higher current", this one titled "LT3081 Output voltage using a DAC",  this one titled "LT3080 SET Pin Voltage", this one titled "LT3083 application", and this one titled "LT3090 output voltage controlled by DAC".

  • 0 on Jun 15, 2020 8:08 PM in reply to ARad

    I got the idea for shorting the Set pin to gnd from the Typical Applications "Adding Shutdown" example given:

    I see in one of the links you provided, that the LT3080 Data Sheet shows a 1N4148 diode between the Set and Out pins, which is not shown in the LT3083 DS. Should I add one? 

    Currently, I have a resistor (249 ohms) between the Out pin and drain of Q2, with a 10K resistor from the drain of Q2 back to the Set pin (replaces R1 in a current-source configuration).

  • 0 on Jun 17, 2020 5:19 PM in reply to BCook

    According to the LT3083 DS, the Max set pin voltage relative to Vo is:
    SET Pin Voltage (Relative to OUT).......................... ±10V

    So, this may explain why a diode is not shown between the Out pin and the Set pin.
    It also makes me believe that the ~2V I see on the Out pin is OK.

    Brett