Robertson Worldwide, Inc.

David J Moore

Joseph M Allison

A resonant fluorescent lamp ballast includes an IC driver for a half-bridge arrangement that supplies AC current to a lamp. The IC creates a frequency sweep from a pre-heat frequency, through a resonant frequency, to a still lower operating frequency. A pre-heat pin in the IC triggers a re-start or the frequency sweep when a first signal exceeds a first threshold level. A shut-down pin in the IC associated with an internal shut-down latch shuts down the driver when a second signal exceeds a second threshold level. Pre-heat trigger circuitry detects a current spike through half-bridge switches when the lamp has not yet started, supplying the pre-heat pin with a first signal exceeding the first threshold level. End-of-life circuitry provides to the shut-down pin a second signal exceeding the second threshold level if lamp current fails to reach a substantial portion of its normal level within a predetermined time. A DC current-supply path is provided from a DC current supply, through at least one filament of each lamp in the load circuit, to a power-supply pin for the IC. The end-of-life circuitry cooperates with the pre-heat trigger circuitry by limiting the number of the frequency sweeps to no more than occur during the predetermined time set by the end-of-life circuitry. The DC path cooperates with the end-of-life circuitry and the internal shut-down latch to reset the latch when the DC path is broken due to absence of the at least one filament in the path.

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St. John, IN, United States of America

David J Moore