Driver circuit for supplying a constant current to an LED arrangement

A driver circuit for supplying a constant current to an LED arrangement, comprising:

• a step-down converter formed by a first switching element, which is connected between a supply voltage and an output capacitor (Cout), and a coil connected in series therewith, wherein the connecting point of the first switching element and the coil and a reference potential point forming the connecting point of the supply voltage (Vin) and the output capacitor are connected via a rectifying element, and a current measuring resistor that measures the current through the coil is arranged in series with the coil,
• a two-point control circuit that switches the first switching element on or off depending on the current through the coil measured by the current measuring resistor in order to keep the average current through the LED arrangement (LED1. . . . LEDN) constant, wherein the two-point control circuit is formed by a window comparator to which a first and a second threshold voltage source supply a high and a low threshold voltage which determine the values of the current flowing through the coil, upon reaching which values the first switching element is switched on and off again, wherein the window comparator has a bistable trigger circuit connected downstream thereof, the first output terminal of which, which has a signal level that indicates that the low threshold value is reached, is connected to the control terminal of the first switching element,
• wherein
• a current amplifier circuit is provided, which is connected to the current measuring resistor to detect the current flowing through it and which, by superimposing an offset voltage applied to one of the inputs thereof by an offset voltage source, can measure both a positive and a negative current, and the output terminal of which is connected to the input terminals of the window comparator that are not supplied with threshold values,
• the offset voltage is also superimposed on the high and the low threshold voltage, and
• a starting pulse circuit is provided, which is connected via a diode to the control input of the first switching element.