Patent ID: 6366032
Filing Date: 2002-04-02
Classification: H05B

Abstract:
A fluorescent lamp ballast, comprising:a) a load circuit for at least one lamp, including a resonant inductance and a resonant capacitance for setting a resonant frequency of the circuit; b) a half-bridge switching arrangement for supplying AC current to the load, c) an integrated circuit comprising: i) a driver for the half-bridge arrangement including control means to create a frequency sweep from a pre-heat frequency, through a substantially lower, resonant frequency, to a still lower operating frequency; ii) a pre-heat pin for triggering the control means to re-start a frequency sweep in response to a first signal exceeding a first threshold level, iii) a shut-down pin associated with an internal shut-down latch for shutting down the driver in response to a second signal exceeding a second threshold level; and iv) a power-supply pin for power the integrated circuit; and d) pre-heat trigger circuitry to detect a current spike through switches of the half-bridge switching arrangement when the lamp has not yet started and, in response, to supply the pre-heat pin with a first signs exceeding the first threshold level; e) end-of-life circuitry for providing 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 period of time; and f) a DC current-supply path from a DC current supply, through at least one filament of each lamp in the load circuit, to the power-supply pin; g) the end-of-life circuitry cooperating with the pre-heat trigger circuitry by limiting the number of the frequency sweeps to no more than occur during the predetermined period of time set by the end-of-life circuitry; and h) the DC current-supply path cooperating with the end-of-life circuitry and the internal shut-down latch to reset the latch when the DC current-supply path is broken due to absence of the at least one filament in the path.