LR745 is a high input voltage SMPS start-up circuit. The LR745 is ideally suited for use with industry standard low voltage PWM ICs having start thresholds of 13.9 to 18.0V. It allows the PWM ICs to be operated from rectified 120 or 240VAC lines, and eliminates the use of power resistors often used for this purpose. The internal circuitry of the LR745 allows the PWM ICs to operate at a VCC voltage below their start threshold voltage after start-up. The auxiliary voltage can be less than the start threshold voltage, which allows for improved efficiency.Current from the high voltage line is drawn only during the start-up period. After start-up, the internal high voltage line is disconnected from the IC, thereby reducing the continuous power dissipation to a minimum.
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