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.