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SLUS329E – MONTH 2003 – REVISED JANUARY 2008
Voltage Amplifier
The UC3854A/B voltage amplifier design is essentially similar to the UC3854 with two exceptions. The first is
with the internal connection. The lower voltage reduces the amount of charge on the compensation capacitors,
which provides improved recovery form large signal events, such as line dropouts, or power interruption. It also
minimizes the dc current flowing through the feedback. The output of the voltage amplifier is also changed. In
addition to a 6.0-V temperature compensated clamp, the output short circuit current has been lowered to 2 mA
typical, and an active pull down has replaced the passive pull down of the UC3854.
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 assures 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. AV
CC
supply voltage clamp at
20 V allows the controller to be current fed if desired. The lower startup supply current (250
µA
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 assures 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.
Copyright © 2003–2008, Texas Instruments Incorporated
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