The UC2854B products are pin compatible enhanced versions of the UC2854. Like the UC2854,
these products provide all of the functions necessary for active power factor corrected
preregulators. The controller achieves near unity power factor by shaping the ac-input line current
waveform to correspond to the ac-input line voltage. To do this the UC2854B uses average current
mode control. Average current mode control maintains stable, low distortion sinusoidal line current
without the need for slope compensation, unlike peak current mode control.
A 1% 7.5-V reference, fixed frequency oscillator, PWM, voltage amplifier with soft-start,
line voltage feedforward (VRMS squarer), input supply voltage clamp, and over current comparator
round out the list of features.
The UC2854B is available in a DW (SOIC-wide) package.
The UC2854A/B products improve upon the UC2854 by offering a wide bandwidth, low offset
current amplifier, a faster responding and improved accuracy enable comparator, a VREF GOOD
comparator, UVLO threshold options (16 V/10 V for offline, 10.5 V/10 V for startup from an
auxiliary 12-V regulator), lower startup supply current, and an enhanced multiply/divide circuit.
New features like the amplifier output clamps, improved amplifier current sinking
capability, and low offset VAC pin reduce the external component count while improving performance.
Improved common mode input range of the multiplier output/current amplifier input allow the
designer greater flexibility in choosing a method for current sensing. Unlike its predecessor, RSET
controls only oscillator charging current and has no effect on clamping the maximum multiplier
output current. This current is now clamped to a maximum of 2 × IAC at all times which simplifies
the design process and provides foldback power limiting during brownout and extreme low line
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