A circuit arrangement (1), a detection circuit (50, 70) and an LED driver circuit (100, 110) is disclosed for operating at least one lighting unit, such as an LED unit, with a phase-cut operating voltage. The circuit arrangement (1) comprising at least an input (6) for receiving a phase-cut operating voltage from said power supply and/or an output (7) for connection to said at least one lighting unit and a pulse injection circuit, configured to determine a phase-cut operation of said power supply and to draw a current pulse from said power supply within a delay time between 200-700 μs after said phase-cut operation to provide a stable operation of said LED unit with the phase-cut power supply. The detection circuit (50,70) comprises at least an input (6) for receiving a phase-cut operating voltage from said power supply and a lamp compatibility detector (52, 72), configured to determine the presence of a parallel lamp (57), connected in parallel with the detection circuit (50, 70) to said phase-cut power supply during operation and to provide a compatibility signal to a LED driver circuit (100, 110) corresponding to the determination of said parallel lamp (57), so that said driver circuit (100, 110) is set between a normal operating mode and a compatibility mode in dependence of the presence of said parallel lamp (57).