The UCC2850x family provides all of the control functions necessary for an active power-factor-corrected
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
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 PFC-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.
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