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.