FEATURES

Provides Control of PFC and PWM Power

Stages In One Device

Trailing-Edge PFC, Trailing-Edge PWM

Modulation

Built-In Sequencing of PFC and PWM

Turn-On

2-A Source and 3-A Sink Gate Drive for Both

PFC and PWM Stages

Typical 16-ns Rise Time and 7-ns Fall Time

into 1-nF Loads

PFC Features

− Average-Current-Mode Control for

Continuous Conduction Mode Operation

− Highly-Linear Multiplier for Near-Unity

Power Factor

− Input Voltage Feedforward Implementation

− Improved Load Transient Response

− Accurate Power Limiting

− Zero Power Detect

PWM Features

− Peak-Current-Mode Control Operation

− 1:1 PFC:PWM Frequency Option

− Programmable maximum Duty Cycle Limit

Up to 90%

− Programmable Soft-Start

APPLICATIONS

High-Efficiency Server and DesktopSupplies

High-Efficiency Telecom AC-DC Converter

DESCRIPTION

The UCC28521 and UCC28528 combination

PFC/PWM controllers provide complete control

functionality for any off-line power system

requiring compliance with the IEC1000−3−2

harmonic reduction requirements. By combining

the control and drive signals for the PFC and the

PWM stages into a single device, significant

performance and cost benefits are gained.

Based on the UCC28511, the new devices employ

an average current mode control architecture with

input voltage feedforward. The major difference of

the UCC28521/28 is the trailing-edge /

trailing-edge (TEM/TEM) modulation scheme for

the PFC/PWM stages. The UCC28528 differs

from the UCC28521 as its PWM stage does not

turn off with the falling bulk voltage. The

UCC28528 PWM was designed for low power

auxiliary supplies.

These devices offer performance advantages

over earlier generation combination controllers. A

key PWM feature is programmable maximum duty

cycle. For the PFC stage, the devices feature an

improved multiplier and the use of a

transconductance amplifier for enhanced

transient response.

The core of the PFC section is in a three-input

multiplier that generates the reference signal for

the line current. The UCC28521/28 features a

highly linearized multiplier circuit capable of

producing a low distortion reference for the line

current over the full range of line and load

conditions. A low-offset, high-bandwidth current

error amplifier ensures that the actual inductor

current (sensed through a resistor in the return

path) follows the multiplier output command

signal. The output voltage error is processed

through a transconductance voltage amplifier.