DESCRIPTION

The UTC UC3854 provide all the functions necessary for active

power factor corrected preregulators. The controller achieves near

unity power factor by shaping the AC input line current waveform to

correspond to that of AC input line voltage. Average current mode

control maintains stable, low distortion sinusoidal line current

without the need for slope compensation, unlike peak current mode

control.

Designed in UTC’s Bipolar process, the UTC UC3854 offers

new features such as wide bandwidth, low offset Current Amplifier,

a faster responding and improved accuracy enable comparator, a

VREF good comparator, UVLO threshold options (16/10V for

offline, 10.5/10V for startup from an auxiliary 12V regulator), lower

startup supply current, and an enhanced multiply/divide circuit.

New features like the improved amplifier current sinking capability,

amplifier output clamps, and low offset VAC pin reduce the external

component count while improving performance. Improved common

mode input range of the Multiplier output/Current Amp input allow

the designer greater flexibility in choosing a method for current

sensing. Unlike its predecessor, the maximum multiplier output

current is now clamped to a maximum of 2 * IAC at all times.

A 1% 7.5V reference, fixed frequency oscillator, PWM, Voltage

Amplifier with softstart, line voltage feedforward (VRMS squarer),

input supply voltage clamp, and over current comparator round out

the list of features.

? FEATURES

* Controls Boost Preregulator to Near Unity Power Factor

* Limits Line Distortion

* World-Wide Line Operation

* Accurate Power Limiting

* Fixed Frequency Average Current Mode Control

* High Bandwidth (5MHz), Low Offset Current Amplifier

* Integrated Current and Voltage Amp Output Clamps

* Multiplier Improvements: Linearity, 500mV VAC Offset (eliminates

external resistor), 0-5V Multout Common Mode Range

* VREF GOOD Comparator

* Faster and Improved Accuracy ENABLE Comparator

* 300uA Startup Supply Current