I want to use the LT3086 as a voltage controlled current source. Based on https://www.analog.com/en/design-center/reference-designs/circuit-collections/lt3086-adjustable-voltage-controlled-current-source.html#cc-overview I realized the following circuit:
Imon/Ilim is connected to a 16k resistor (R67, R66 and R68 are not used), thus limiting the current to 50mA. TRACK-Pin is connected to the output of a MCP4725 DAC in order to be able to adjust the current via I2C. An on board INA226 measures the resulting current and voltage. Its output values have been validated via external measurements. The load is a light emitting diode.
To test the behaviour, I slightly sweep the TRACK-Voltage and measure the resulting constant load current. Additionally I simulated the circuit using LTSpice. The measured and the simulated curve read as follows:
As can be clearly seen, there is a slight shift between the simulated and the measured curve. If VTRACK goes below 0.12V, the LED starts flickering and finally just turns off. So, current below 15mA can not be reached, although simulation suggests so. Unfortunatly, the current region between 3mA and 15mA is my region of interest.
What might cause this shift in constant current and how could current lower than 15mA be realized?