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UC2854BJ Datasheet(PDF) 8 Page - Texas Instruments |
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UC2854BJ Datasheet(HTML) 8 Page - Texas Instruments |
8 / 16 page UC1854B UC2854A, UC2854B UC3854A, UC3854B SLUS329B − JUNE 1998 − REVISED FEBRUARY 2005 8 www.ti.com Current Amplifier The current amplifier for an average current PFC controller needs a low offset voltage in order to minimize AC line current distortion. With this in mind, the UC3854A/B current amplifier has improved the input offset voltage from ±4 mV to 0 V to ±3 mV. The negative offset of the UC3854A/B guarantees that the PWM circuit will not drive the MOSFET is the current command is zero (both current amplifier inputs zero.) Previous designs required an external offset cancellation network to implement this key feature. The bandwidth of the current amplifier has been improved as well to 5 MHz typical. While this is not generally an issue at 50 Hz or 60 Hz inputs, it is essential for 400 Hz input avionics applications Miscellaneous Several other important enhancements have been implemented in the UC3854A/B. A VCC supply voltage clamp at 20 V allows the controller to be current fed if desired. The lower startup supply current (250 mA typical), substantially reduces the power requirements of an offline startup resistor. The 10.5 V/10 V UVLO option (UC3854B) enables the controller to be powered off of an auxiliary 12 V supply. The VREF GOOD comparator guarantees that the MOSFET driver output remains low if the supply of the 7.5 V reference are not yet up. This improvement eliminates the need for external Schottky diodes on the PKLMT and Mult Out pins that some UC3854 designs require. The propagation delay of the disable feature has been improved to 300 ns typical. This delay was proportional to the size of the VREF capacitor on the UC3854, and is typically several orders of magnitude slower. |
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