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