The UC3854A/B products are pin compatible
enhanced versions of the UC3854. Like the
UC3854, 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 UC3854A/B 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 amplifierwith soft-start, line voltage
feedforward (VRMS squarer), input supply voltage
clamp, and over current comparator round out the lilst of feataures.
Available in the 16-pin N (PDIP), DW (SOIC Wide),
and J (CDIP) and 20-pin Q (PLCC)
package. See Ordering Information table for availability by temperature range.
The UC3854A/B products improve upon the UC3854 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 amethod 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 conditions.
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