Cold-cathode fluorescent lamp multiple lamp current matching circuit

Abstract

A method of matching the brightness of a plurality of lamps driven by an AC drive current may comprise the steps of: selecting a first one of said lamps having a lowest brightness; and reducing the AC drive current in a second lamp periodically so as to reduce the brightness of the second lamp to match that of the first lamp. According to another embodiment, a reference brightness may be selected, or optionally a reference AC current level, and the method may reduce the drive current periodically so as to set the lamp brightness in relation to the reference brightness or optionally the reference AC current level. A circuit for driving a lamp with controllable brightness may comprise: a drive circuit for supplying an AC drive current to the lamp; and a reducing circuit, advantageously comprised in an IC, for reducing the AC drive current in the lamp periodically to set the AC drive current in the lamp to match a reference. The reference may correspond to the brightness of another lamp driven by the AC drive circuit, or to a reference AC current level. The reducing circuit may comprise a switch in series with the lamp, or a parallel connection of a resistance and a switching device connecting the lamp to ground. The AC drive current may be reduced by a predetermined number of half-cycles, preferably by one half-cycle, of said AC drive current. The AC drive current may be reduced by supplying periodic OFF pulses to the reducing circuit, which may be spaced apart by a selected number of half-cycles of the AC drive current, for example by a frequency-dividing circuit which receives and divides the frequency of the AC drive current.

Dependencies are inferred from the claim text.