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.