LTC3623 in a current source mode. Output current is floating.

Dear Sirs,

Our customer has some issues with LTC3623 while using it in a current source mode. (in a voltage source mode it works fine)

Please review the attached schematic.

The problem is that there is no current stabilization while load resistance changing on the connector XS3 (resistive load).

ASET is changing in the range from 0.3 V to 0.25 V. When measuring a voltage on the lead 3 of DA10 (LTC2063) then it differs by several tens of millivolts from the voltage on the inverting input 4. It seems that the VT1 transistor is skewing (the potential at the source is more than at the drain)

The value of the current from the ISET pin (LTC3623) changes when the XS3 contacts are short-circuited, but should remain stable.

Probably LTC3623 can’t work with voltages near zero, that’s why we see this.

The situation is better when you connect some kind of load to the XS3 pins. The current stabilizes, but if you change the supply voltage, then the current also floats, although it should not. Current source of 50μA is stabilized.

Can anybody help with the problem?

Best regards,

Evgeny