LT3092: Problem with supporting BJT for higher current

Dear Analog-Team,

I am having troubles using the LT3092 current source with a BJT as current amplification device. 

It behaves very strange.

My goal is this: I want to controll current from V2 from 0 to about 2Amps by setting V1 (in reality it is an output of an opamp). To extend the range of the LT3092 a BJT is used.

When simulating everything works as it should: with v1 increasing at the SET Pin, the same voltage is seen at the OUT Pin, thus controlling the current.

Reality: the current rises uncontrolled until V_IN-V_OUT at the pins of the LT3092 is at about 1.18V. More than 10uA come out of the SET Pin.

Why is there this discrepancy between Spice Sim and reality and why is the LT not keeping SET and OUT voltages exactly in sync but rather uncorrelated?

Any advice is welcome, how to debug it.

Thanks,

Jo

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  • 0
    •  Analog Employees 
    on Jun 21, 2019 3:55 PM

    Thanks for the post.  I don't have time to test your circuit using hardware at the moment, but I will mention that the LT3092 and other similar LDOs will act strangely if SET is driven too far away from OUT.  See the link below to a post that discusses OUT and SET.

    https://ez.analog.com/power/f/q-a/111002/lt3081-output-voltage-using-a-dac

    Keep OUT and SET the same as I mention, and then make sure the circuit is stable.  To check stability, try to find a condition where your hardware will regulate and then  just do a fast line transient of a few volts and make sure the current does not ring.  I will update this post later if I get a chance to try your circuit with hardware.  Usually LTspice is accurate in terms of stability predictions for these LDOs.  Keep in mind the minimum load for the LT3092 is 500uA. 

Reply
  • 0
    •  Analog Employees 
    on Jun 21, 2019 3:55 PM

    Thanks for the post.  I don't have time to test your circuit using hardware at the moment, but I will mention that the LT3092 and other similar LDOs will act strangely if SET is driven too far away from OUT.  See the link below to a post that discusses OUT and SET.

    https://ez.analog.com/power/f/q-a/111002/lt3081-output-voltage-using-a-dac

    Keep OUT and SET the same as I mention, and then make sure the circuit is stable.  To check stability, try to find a condition where your hardware will regulate and then  just do a fast line transient of a few volts and make sure the current does not ring.  I will update this post later if I get a chance to try your circuit with hardware.  Usually LTspice is accurate in terms of stability predictions for these LDOs.  Keep in mind the minimum load for the LT3092 is 500uA. 

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