1. A lighting unit, comprising:a light source;
a driver for driving the light source which provides an output current between first and second terminals;
a first current sensing element in series with the light source between the first and second terminals;
an auxiliary circuit for powering by the driver, which auxiliary circuit comprises a buffer capacitor;
a second current sensing element and a control switch in series with the buffer capacitor between the first and second terminals; and
a controller adapted to control the control switch in dependence on the currents sensed by the first and second current sensing elements thereby to control the current flowing to the buffer capacitor.