FEATURES
Combines PFC and Downstream Converter
Controls
Controls Boost Preregulator to Near-Unity
Power Factor
Accurate Power Limiting
Improved Feedforward Line Regulation
Peak Current-Mode Control in Second Stage
Programmable Oscillator
Leading-Edge/Trailing-Edge Modulation for
Reduced Output Ripple
Low Start-up Supply Current
Synchronized Second Stage Start-Up, with
Programmable Soft-start
Programmable Second Stage Shutdown

DESCRIPTION
The UCC2850x family provides all of the control
functions necessary for an active power-factorcorrected
preregulator and a second-stage dc-to- dc
converter. The controller achieves near-unity power
factor by shaping the ac input line current waveform to
correspond to the ac input-line voltage using average
current-mode control. The dc-to-dc converter uses
peak current-mode control to perform the step-down
power conversion.
The PFC stage is leading-edge modulated while the
second stage is trailing-edge synchronized to allow for
minimum overlap between the boost and PWM
switches. This reduces ripple current in the bulk-output
capacitor.In order to operate with over three-to-one
range of input-line voltages, a line feedforward (VFF) is
used to keep input power constant with varying input
voltage. Generation of VFF is accomplished using IAC in
conjunction with an external single-pole filter. This not
only reduces external parts count, but also avoids the
use of high-voltage components, offering a lower-cost
solution. The multiplier then divides the line current by
the square of VFF.
The UCC2850x PFC section incorporates a low
offset-voltage amplifier with 7.5-V reference, a
highly-linear multiplier capable of a wide current range,
a high-bandwidth, low offset-current amplifier, with a
novel noise-attenuation configuration, PWM
comparator and latch, and a high-current output driver.
Additional PFC features include over-voltage
protection, zero-power detection to turn off the output
when VAOUT is below 0.33 V and peak current and
power limiting.
The dc-to-dc section relies on an error signal generated
on the secondary-side and processes it by performing
peak current mode control. The dc-to-dc section also
features current limiting, a controlled soft-start, preset
operating range with selectable options, and 50%
maximum duty cycle.
The UCC28500 and UCC28502 have a wide UVLO
threshold (16.5 V/10 V) for bootstrap bias supply
operation. The UCC28501 and UCC28503 are
designed with a narrow UVLO range (10.5 V/10 V) more
suitable for fixed bias operation. The UCC28500 and
UCC28501 have a narrow UVLO threshold for PWM
stage (to allow operation down to 75% of nominal bulk
voltage), while the UCC28502 and UCC38503 are
configured for a much wider operation range for the
PWM stage (down to 50% of bulk nominal voltage).
Available in 20-pin N and DW packages.