FEATURES
• Controls Boost PWM to Near Unity
Power Factor
• Fixed Frequency Average Current
Mode Control Minimizes Line Current
Distortion
• Built-in Active Snubber (ZVT) allows
Operation to 500kHz, improved EMI
and Efficiency
• Inductor Current Synthesizer allows
Single Current Transformer Current
Sense for Improved Efficiency and
Noise Margin
• Accurate Analog Multiplier with Line
Compensator allows for Universal
Input Voltage Operation
• High Bandwidth (5MHz), Low Offset
Current Amplifier
• Overvoltage and Overcurrent
protection
• Two UVLO Threshold Options
• 150μA Startup Supply Current Typical
• Precision 1% 7.5V Reference

DESCRIPTION
The UC3855A/B provides all the control features necessary for high
power, high frequency PFC boost converters. The average current mode
control method allows for stable, low distortion AC line current programming
without the need for slope compensation. In addition, the UC3855
utilizes an active snubbing or ZVT (Zero Voltage Transition technique) to
dramatically reduce diode recovery and MOSFET turn-on losses, resulting
in lower EMI emissions and higher efficiency. Boost converter switching
frequencies up to 500kHz are now realizable, requiring only an
additional small MOSFET, diode, and inductor to resonantly soft switch
the boost diode and switch. Average current sensing can be employed using
a simple resistive shunt or a current sense transformer. Using the current
sense transformer method, the internal current synthesizer circuit
buffers the inductor current during the switch on-time, and reconstructs the
inductor current during the switch off-time. Improved signal to noise ratio
and negligible current sensing losses make this an attractive solution for
higher power applications.
The UC3855A/B also features a single quadrant multiplier, squarer, and
divider circuit which provides the programming signal for the current loop.
The internal multiplier current limit reduces output power during low line
conditions. An overvoltage protection circuit disables both controller outputs
in the event of a boost output OV condition.
Low startup supply current, UVLO with hysteresis, a 1% 7.5V reference,
voltage amplifier with softstart, input supply voltage clamp, enable comparator,
and overcurrent comparator complete the list of features. Available
packages include: 20 pin N, DW, Q, J, and L.